Last Updated: August 23, 2020
The supratentorial arteries include the supraclinoid portion of the internal carotid artery and its anterior and middle cerebral, ophthalmic, posterior communicating, and anterior choroidal branches, the components of the circle of Willis, which in the posterior midline includes the basilar apex, and finally, the posterior cerebral artery. The origin of all of these arteries is located deep under the center of the cerebrum and their proximal trunks are relatively inaccessible because they course in deep clefts like the sylvian or interhemispheric fissure or in the basal cisterns between the brainstem and temporal lobe (Fig. 2.1). Only the smaller terminal branches are accessible on lateral convexity and even there, these branches are often hidden in cortical sulci rather than coursing on the gyral surfaces. No single operative approach will access all of the branches of the three major cerebral arteries because of their long courses. Thus, each operative approach must be carefully tailored based on the relationships of the arterial segment involved. The relationship of these arteries to the common aneurysm sites and their operative exposure is reviewed in Chapter 3.
Supraclinoidal Portion of the Internal Carotid Artery
The supraclinoidal portion of the internal carotid artery (ICA) is a common site of intracranial aneurysms, and its branches are frequently stretched, displaced, or encased by cranial base tumors. The ICA and its major and perforating branches are frequently exposed during operations on aneurysms of the circle of Willis and tumors of the sphenoid ridge, anterior and middle cranial fossae, and suprasellar region. Agenesis or aplasia of the internal carotid artery is rare.
Segments of the Internal Carotid Artery
The ICA is divided into four parts: the C1 or cervical portion extends from its junction with the common carotid artery to the external orifice of the carotid canal; the C2 or petrous portion courses within the carotid canal and ends where the artery enters the cavernous sinus; the C3 or cavernous portion courses within the cavernous sinus and ends where the artery passes through the dura mater forming the roof of the cavernous sinus; and the C4 or supraclinoid portion begins where the artery enters the subarachnoid space and terminates at the bifurcation into the anterior (ACA) and middle cerebral arteries (MCA) (Fig. 2.2) (25, 36).
The C4 begins where the artery emerges from the dura mater, forming the roof of the cavernous sinus. It enters the cranial cavity by passing along the medial side of the anterior clinoid process and below the optic nerve. It courses posterior, superior, and slightly lateral to reach the lateral side of the optic chiasm and bifurcates below the anterior perforated substance at the medial end of the sylvian fissure to give rise to the ACA and MCA. The C4 segment is defined as including the crotch from which the MCA and ACA arise, and the branches originating from the apex of the wall between the origin of the ACA and MCA are considered to be branches of the ICA, just as aneurysms arising at this apex are considered to be aneurysms of the bifurcation of the ICA. When viewed from laterally, the cavernous (C3) and intracranial (C4) portions have several curves that form an S shape, and together these portions are called the carotid siphon. The lower half of the S, formed predominantly by the intracavernous portion, is convex anteriorly, and the upper half, formed by the supraclinoid portion, is convex posteriorly. The junction of the anteriorly and posteriorly convex segments passes along the medial side of the anterior clinoid process. The prebifurcation branches of the C4 are the ophthalmic, anterior choroidal (AChA), posterior communicating arteries (PComA), perforating, and superior hypophyseal arteries.
The intradural exposure of the C4 and the anterior portion of the circle of Willis is directed along the ipsilateral sphenoid ridge or orbital roof to the anterior clinoid process. In exposing the ICA, the approach is usually from proximal to distal, beginning with the ophthalmic segment and working distally toward the bifurcation. The ophthalmic artery is difficult to expose because of its short intradural length and its location under the optic nerve.
In exposing the C4 beyond the origin of the ophthalmic artery, the surgeon often sees the AChA before the PComA, although the AChA arises distal to the PComA (Figs. 2.1 and 2.3). This occurs because of three sets of anatomic circumstances. First, the C4 passes upward in a posterolateral direction, placing the origin of the AChA further lateral to the midline than the origin of the PComA. Second, the AChA commonly arises further laterally on the posterior wall of the C4 portion than the PComA. The site of origin of the AChA from the posterior wall of the C4 portion is lateral to the site of origin of the PComA in 94% of hemispheres (33). Third, the AChA pursues a more lateral course than the PComA; the former passes laterally around the cerebral peduncle and into the temporal horn, whereas the latter is most commonly directed in its initial course in a posteromedial direction above the oculomotor nerve toward the interpeduncular fossa.
Segments of the C4
The C4 is divided into three segments based on the site of origin of the ophthalmic, PComA, and AChA. The ophthalmic segment extends from the roof of the cavernous sinus and the origin of the ophthalmic artery to the origin of the PComA; the communicating segment extends from the origin of the PComA to the origin of the AChA; and the choroidal segment extends from the origin of the AChA to the terminal bifurcation of the ICA. The ophthalmic segment is the longest, and the communicating segment is the shortest (15).
C4 Perforating Branches
Each of the three C4 segments gives off a series of perforating branches with a relatively constant site of termination. An average of 8 (range, 3–12) perforating arteries (excluding the ophthalmic, PComA, and AChA) arise from the C4 (Figs. 2.4–2.6).
An average of four (range, one to seven) perforating arteries arise from the ophthalmic segment. Most arise from the posterior or medial aspect of the artery. These branches are most commonly distributed to the infundibulum (stalk) of the pituitary gland, the optic chiasm, and less commonly, in descending order of frequency, to the optic nerve, premamillary portion of the floor of the third ventricle, and the optic tract. A few vessels terminate in the dura mater covering the anterior clinoid process, sella turcica, and tuberculum sellae. The arteries that arise from this segment and pass to the infundibulum of the pituitary gland are called the superior hypophyseal arteries (13, 15).
No perforating branches arise from the communicating segment in more than half of hemispheres, and if present, only one to three are found. They arise from the posterior half of the wall and terminate, in descending order of frequency, in the optic tract, premamillary part of the floor of the third ventricle, the optic chiasm, and infundibulum, and infrequently, enter the anterior or posterior perforated substance. The branches are often stretched around the neck of posterior communicating aneurysms.
An average of four (range, one to nine) branches arise from the choroidal segment. Most branches arise from the posterior half of the arterial wall and terminate, in descending order of frequency, in the anterior perforated substance, optic tract, and uncus.
Superior Hypophyseal and Infundibular Arteries
The superior hypophyseal arteries are a group of one to five (average, two) small branches that arise from the C4’s ophthalmic segment and terminate on the pituitary stalk and gland, but also send branches to the optic nerves and chiasm and the floor of the third ventricle (Figs. 2.4–2.6). The largest of the branches is often referred to as the superior hypophyseal artery. Most branches arise from the posteromedial, medial, or the posterior aspects of the artery. The infundibular arteries are a group of arteries that originate from the PComA and are distributed to the infundibulum. There are fewer infundibular arteries than superior hypophyseal arteries. One-quarter of hemispheres have one or two infundibular arteries and the remainder have none.
The superior hypophyseal and infundibular arteries pass medially below the chiasm to reach the tuber cinereum. They intermingle and form a fine anastomotic plexus around the pituitary stalk called the circuminfundibular anastomosis. These arteries and the circuminfundibular plexus are distributed to the pituitary stalk and anterior lobe. The inferior hypophyseal branch of the meningohypophyseal trunk of the intracavernous carotid perfuses the posterior lobe. The capsular arteries also arise from the intracavernous carotid and supply the capsule of the pituitary gland (16).
This circuminfundibular plexus gives rise to ascending and descending arteries. The descending arteries include shortstalk and superficial arteries. The short-stalk arteries penetrate the infundibulum and form capillaries that lead into sinusoids running down the stalk. The superficial arteries course inferiorly on the outside of the stalk in the subarachnoid space and penetrate the anterior lobe. The ascending arteries supply the tuber cinereum, median eminence, and the inferior surface of the optic chiasm. The superior hypophyseal arteries also send branches to the chiasm and proximal portions of the optic nerves.
The ophthalmic artery is the first branch of the C4. Most ophthalmic arteries arise below the optic nerve in the supraclinoid area above the dural roof of the cavernous sinus and pass anterolaterally below the optic nerve to enter the optic canal and orbit. The distal course is reviewed in Chapter 7. Eight percent of ophthalmic arteries originate within the cavernous sinus. The ophthalmic artery may rarely arise from the clinoid segment of the ICA located on the medial side of the anterior clinoid process or from the middle meningeal artery (16, 20, 29). It is rarely absent. The ophthalmic arteries uncommonly give rise to intracranial perforating branches and, if present, these branches run posteriorly and are distributed to the ventral aspect of the optic nerve and chiasm and the pituitary stalk.
The ophthalmic artery usually arises from the medial third of the superior surface of the C4 immediately distal to the cavernous sinus in the area below the optic nerve. In our earlier study, it arose above the medial third of the superior surface of the C4 in 78% of hemispheres and above the middle third of the superior surface in 22% of cases (15). None arise from the lateral third of the superior surface. It may kink laterally, infrequently presenting a short segment lateral to the optic nerve before entering the optic canal. The origin varies from as far as 5 mm anterior to 7 mm posterior to the tip of the anterior clinoid process and from 2 to 10 mm medial to the clinoid process (16). Most ophthalmic arteries arise anterior to the tip of the anterior clinoid process, approximately 5 mm medial to the anterior clinoid.
The intracranial segment of the ophthalmic artery is usually very short. In a previous study from this laboratory, 14% of the segments were found to exit the ICA and immediately enter the optic canal; in the remaining 86%, the maximum length of the preforaminal segment was 7.0 mm, and the mean length was 3.0 mm (16). The intracranial segment usually arises from the medial third of the superior surface of the ophthalmic segment under the optic nerve and commonly enters the optic foramen within 1 to 2 mm of its origin. The exposure of the ophthalmic artery is facilitated by removing the anterior clinoid process and roof of the optic canal, and incising the falciform process, a thin fold of dura mater that extends medially from the anterior clinoid process and covers a 0.5- to 11-mm (average, 3.5 mm) segment of the optic nerve immediately proximal to the optic foramen (16).
Posterior Communicating Artery
The PComA, which forms the lateral boundary of the circle of Willis, arises from the posteromedial surface of the C4 approximately midway between the origin of the ophthalmic artery and the terminal bifurcation (Figs. 2.1, 2.3, and 2.6–2.8). It sweeps backward and medially below the tuber cinereum, above the sella turcica, and slightly above and medial to the oculomotor nerve to join the posterior cerebral artery (PCA). In the embryo, the PComA continues as the PCA, but in the adult, the latter artery is annexed by the basilar system. If the PComA remains the major origin of the PCA, the configuration
is termed fetal. If the PComA is of small or normal size, it courses posteromedially to join the PCA above and medial to the oculomotor nerve, but if it is of a fetal type, it courses further laterally above or lateral to the oculomotor nerve.
The PComA usually arises from the posteromedial or posterior aspect of the C4. The diameter at the carotid origin is slightly larger than at the junction with the PCA, but the difference is not usually more than 1 mm. Dilations of the origin of the PComA from the C4, known as functional dilatation or infundibular widening, are found in approximately 6% of hemispheres. Such dilation may be difficult to distinguish from an aneurysm. Some authors regard it as an early stage of aneurysm formation because the histological appearances are identical with those of aneurysms, but other authors, based on histological techniques, conclude that the junctional dilations are neither aneurysmal nor preaneurysmal (9, 17).
An average of 8 (range, 4–14) perforating branches arise from the PComA, mostly from the superior and lateral surfaces, and course superiorly to penetrate, in decreasing order of frequency, the tuber cinereum and premamillary part of the floor of the third ventricle, the posterior perforated substance and interpeduncular fossa, the optic tract, the pituitary stalk, and the optic chiasm, to reach the thalamus, hypothalamus, subthalamus, and internal capsule (37). Branch origins are distributed relatively evenly along the course of the artery, with the anterior half having slightly more branches than the posterior half.
The premamillary artery is the largest branch that arises from the PComA. It enters the floor of the third ventricle in front of or beside the mamillary body between the mamillary body and optic tract (Fig. 2.3). There are commonly two or three branches terminating in the premamillary area, but only the largest branch is referred to as the premamillary artery. The premamillary artery has also been referred to as the anterior thalamoperforating artery. The premamillary artery most commonly originates on the middle third of the communicating artery, but can also arise on the anterior or posterior third. It supplies the posterior hypothalamus, anterior thalamus, posterior limb of the internal capsule, and subthalamus. The anterior group of PComA perforating branches supplies the hypothalamus, ventral thalamus, anterior third of the optic tract, and posterior limb of the internal capsule; the posterior group reaches the posterior perforated substance and subthalamic nucleus. Occlusion of the branches to the subthalamic nucleus leads to contralateral hemiballism.
Anterior Choroidal Artery
The AChA usually arises from the C4 as a single artery, with the majority arising nearer the origin of the PComA than to the carotid bifurcation (Figs. 2.1, 2.9, and 2.10). It may infrequently arise from the C4 as two separate arteries or as a single artery that divides immediately into two trunks (47% of hemispheres) (33, 37). Infrequent origins, occurring in less than 1%, include the MCA and PComA. Its origin is similar in diameter to that of the ophthalmic artery, but smaller than tthose of the PComA, unless the PComA is small or hypoplastic. The origin of a fetal-type PComA may be more than twice the diameter of the AChA. The AChA is the first branch on the C4 distal to the PComA in two-thirds of hemispheres and the second, third, or even the fourth branch after one or more perforating branches, in descending order of frequency, in the remainder. The perforating branches arising between the PComA and AChA most commonly terminate in the optic tract, medial temporal lobe, and posterior perforated substance.
The initial segment of the AChA is directed posteromedial behind the internal carotid artery. On the anteroposterior angiogram, the initial segment of the AChA is seen medial to the internal carotid artery. The origin of the artery is lateral to the optic tract, but the initial segment crosses from the lateral to the medial side of the optic tract in many hemispheres, only infrequently remaining lateral to the optic tract throughout its course. It passes below or along the medial side of the optic tract to reach the lateral margin of the cerebral peduncle. The average length that the artery follows the optic tract is 12 mm (range, 5–25 mm) (33). At the anterior margin of the lateral geniculate body, the AChA again crosses the optic tract from medial to lateral and passes posterolateral through the crural cistern, located between the cerebral peduncle and uncus, to arrive superomedial to the uncus, where it passes through the choroidal fissure to enter the choroid plexus within the temporal horn. It courses along the medial border of the choroid plexus in close relation to the lateral posterior choroidal branches of the PCA. In some cases, it can pass dorsally along the medial border of the plexus, reaching the foramen of Monro.
The artery is divided into cisternal and plexal segments (33). The cisternal segment extends from the origin to the choroidal fissure and is divided at the anterior margin of the lateral geniculate body into a proximal and distal portion. The plexal segment is composed of one or more branches that pass through the choroidal fissure to branch and enter the choroid plexus of the temporal horn. The length from its origin to its passage through the choroidal fissure averages 2.4 cm (range, 20–34 mm). If there is a double artery, the distal branch usually terminates in the temporal lobe and the proximal branch nourishes the remaining anterior choroidal field.
The branches, which average 9 (range, 4–18), are divided on the basis of whether they arise from the cisternal or plexal segment. The branches from the cisternal segment penetrate, in decreasing order of frequency, the optic tract, uncus, cerebral peduncle, temporal horn, lateral geniculate body, hippocampus, dentate gyrus and fornix, and anterior perforated substance. These branches more commonly supply the optic tract, lateral part of the geniculate body, posterior two-thirds of the posterior limb of the internal capsule, most of the globus pallidus, the origin of the optic radiations, and the middle third of the cerebral peduncle. Less commonly supplied structures include part of the head of the caudate nucleus, pyriform cortex, the uncus, posteromedial part of the amygdaloid nucleus, substantia nigra, red nucleus, subthalamic nucleus, and the superficial aspect of the ventrolateral nucleus of the thalamus (1). None of these structures is always supplied by the artery, but, in approximately two-thirds of the hemispheres, it supplies the medial part of the globus pallidus, the posterior limb and retrolenticular part of the internal capsule, the optic tract and the lateral geniculate body. No structure other than the choroid plexus of the temporal horn received branches in every case. In approximately half of the hemispheres, it supplies the lateral part of the globus pallidus and the caudate tail; in one-third, it supplies the thalamus, hypothalamus, and subthalamus.
There is a marked interchangeability of the field of supply of the AChA and the nearby branches of the C4, PCA, PComA, and MCA. The C4 frequently gives rise to small arteries distributed to the areas commonly supplied by the proximal branches of the AChA. These arteries, as many as four, arising from the posterior wall of the carotid artery between the PComA and AChA, also frequently terminate, in decreasing order of frequency, in the optic tract, anterior perforated substance, uncus, hypothalamus, pituitary stalk, and cerebral peduncle (37).
Another example of the interchangeability of field occurs within the internal capsule. If the PComA is small, the anterior choroidal artery may take over its normal area of supply to the genu and the anterior third of the internal capsule, or if the AChA is small, the field of supply of the PComA may enlarge to supply the greater part of the posterior limb of the internal capsule (1). Such inverse relationships, in which one artery’s field of supply enlarges as the other’s contracts, occur between the PCA and AChA in the supply to the cerebral peduncle, substantia nigra, red nucleus, subthalamic nucleus, optic tract, and lateral geniculate body. A large AChA is usually associated with a small PComA on that side.
The plexal segment, in most cases, originates as a single branch of the AChA, which passes through the choroidal fissure. Additional smaller branches to the choroid plexus may arise proximal to the choroidal fissure. These plexal branches divide and enter the medial border of the choroid plexus of the temporal horn to course in close relation to and frequently anastomose with branches of the lateral posterior choroidal arteries. Some branches of the AChA pass posteriorly into the choroid plexus in the atrium and then forward above the thalamus to supply the choroid plexus of the body as far forward as the foramen of Monro.
Nearly half of hemispheres have anastomoses between the PCA and AChA. The richest anastomoses are those located on the surface of the choroid plexus with the lateral posterior choroidal branches of the PCA. Anastomoses between the AChA and PCA are also found on the lateral surface of the lateral geniculate body and on the temporal lobe near the uncus. These complex and variable anastomoses make it difficult to predict the effects of occlusion of a single AChA, but explain some of the inconsistent results of AChA occlusion.
The classic reported clinical features of occlusion of the AChA are contralateral hemiplegia, hemianesthesia, and hemianopsia (1, 11). The contralateral hemiplegia and hemianesthesia (to all sensory modalities) results from infarction in the posterior two-thirds of the posterior limb of the internal capsule and the middle third of the cerebral peduncle. The homonomous hemianopsia of varying degrees results from interruption of the supply to the origin of the optic radiations, the optic tract, and part of the lateral geniculate body. Infarction found in the globus pallidus seems to produce no symptoms.
Inconstant results, including absence of deficit, have followed surgical occlusion for the treatment of Parkinson’s disease (5, 28). In 1952, while performing a pedunculotomy on a patient incapacitated with Parkinsonism, Coopers tore and had to clip the AChA (4, 5). The operation was terminated without cutting the peduncle. Postoperatively, there was disappearance of tremor and rigidity from the involved extremities, with preservation of voluntary motor function. This beneficial effect was presumed to be caused by ischemic necrosis of the globus pallidus. This represented a case of known occlusion of the AChA with none of the classic symptoms. The sparing of motor function was presumed to be caused by anastomosis over the lateral geniculate body and in the choroid plexus, which provided a collateral source for the capsular branches.
Surgical occlusions were then made by Cooper and his associates in 50 patients with Parkinsonism (4, 5). Each artery was clipped twice: once at its origin and once 1.5 cm from the origin, just distal to the pallidal branches. This distal clip was applied to prevent retrograde filling into the pallidal branches through the anastomosis in the choroid plexus. This was
thought to isolate the pallidum and its efferent fiber tracts from their normal antegrade blood supply and from retrograde supply through anastomosis with the lateral posterior choroidal and other arteries. At the same time, it allowed the more distal structures, such as the internal capsule, the benefit of this retrograde collateral circulation. Cooper reported good relief of tremor and rigidity, a 20% morbidity, and 6% mortality in this group. Postoperative complications included a hemiplegia in three patients, a partial aphasia in one, and a homonymous quadrantanopsia in one. Twelve patients studied in detail had no visual defects. Several patients developed a memory loss and became confused, and it was not uncommon for the patients to remain somnolent for 1 to 10 days. Cooper assumed that collateral circulation spared the corticospinal fibers and the optic radiations, while failing to preserve the pallidum and / or its efferent fibers.
Rand et al. (28) later reported the results of occlusion of six arteries in five cases. Although finding no therapeutic value of AChA occlusion, these authors agreed that the artery could be occluded with little resultant damage. In four patients there was no effect on the Parkinsonism and no neurological deficit after the occlusion, but the fifth patient developed a contralateral
hemiparesis after occlusion of the artery. A homonymous visual field defect occurred in two patients. In two cases, in which the brain became available for pathological examination, small and inconstant lesions were found within the areas supplied by the artery. The inconstant symptoms and infarction after AChA occlusion are attributed to collateral circulation through anastomoses with adjacent arteries and variations in the area of supply of the artery.
Middle Cerebral Artery
The MCA is the largest and most complex of the cerebral arteries. Some of its branches are exposed in most operations in the supratentorial area, whether the approach is to the cerebral convexity, parasagittal region, or along the cranial base (Figs. 2.1, 2.3, and 2.7). In the past, surgical interest in the MCA has been directed at avoiding damage to its branches during operations within its territory, but micro-operative techniques have now made reconstruction of and bypass to the MCA an important method of preserving and restoring blood flow to the cerebrum.
The MCA arises as the larger of the two terminal branches of the internal carotid artery. The diameter of the MCA at its origin ranges from 2.4 to 4.6 mm (average, 3.9 mm), roughly twice that of the anterior cerebral artery. Its origin is at the medial end of the sylvian fissure, lateral to the optic chiasm, below the anterior perforated substance, and posterior to the division of the olfactory tract into the medial and lateral olfactory striae. From its origin, it courses laterally below the anterior perforated substance and parallel, but roughly 1 cm posterior, to the sphenoid ridge. As it passes below the anterior perforated substance, it gives rise to a series of perforating branches referred to as lenticulostriate arteries. It divides within the sylvian fissure and turns sharply posterosuperiorly at a curve, the genu, to reach the surface of the insula. At the periphery of the insula, the branches pass to the medial surface of the opercula of the frontal, temporal, and parietal lobes. Its branches pass around the opercula to reach the cortical surface and supply most of the lateral surface and some of the basal surface of the cerebral hemisphere.
The MCA is divided into four segments: Ml (sphenoidal), M2 (insular), M3 (opercular), and M4 (cortical) (Figs. 2.11– 2.14). The M1 begins at the origin of the MCA and extends laterally within the depths of the sylvian fissure. It courses laterally, roughly parallel to and approximately 1 cm (range, 4.3–19.5 mm) posterior to the sphenoid ridge in the sphenoidal compartment of the sylvian fissure. This segment terminates at the site of a 90-degree turn, the genu, located at the junction of the sphenoidal and operculoinsular compartments of the sylvian fissure. The M1 is subdivided into a prebifurcation and postbifurcation part. The prebifurcation segment is composed of a single main trunk that extends from the origin to the bifurcation. The postbifurcation trunks of the M1 segment run in a nearly parallel course, diverging only minimally before reaching the genu. This bifurcation occurs proximal to the genu in nearly 90% of hemispheres (14). The small cortical branches arising from the main trunk proximal to the bifurcation are referred to as early branches.
The M2 segment includes the trunks that lie on and supply the insula (Fig. 2.15). This segment begins at the genu where the MCA trunks passes over the limen insulae and terminates at the circular sulcus of the insula. The greatest branching of the MCA occurs distal to the genu as these trunks cross the anterior part of the insula. The branches passing to the anterior cortical areas have a shorter path across the insula than those reaching the posterior cortical areas. The branches to the anterior frontal and anterior temporal areas cross only the anterior part of the insula, but the branches supplying the posterior cortical areas course in a nearly parallel but diverging path across the length of the insula. The frontal branches cross only the short gyri before leaving the insular surface, whereas the branches supplying the posterior parietal or angular regions pass across the short gyri, the central sulcus, and the long gyri of the insula before leaving the insular surface.
The M3 segment begins at the circular sulcus of the insula and ends at the surface of the sylvian fissure. The branches forming the M3 segment closely adhere to and course over the surface of the frontoparietal and temporal opercula to reach the superficial part of the sylvian fissure. The branches directed to the brain above the sylvian fissure undergo two 180-degree turns. The first turn is located at the circular sulcus, where the vessels coursing upward over the insular surface turn 180 degrees and pass downward over the medial surface of the frontoparietal operculum. The second 180-degree turn is located at the external surface of the sylvian fissure, where the branches complete their passage around the inferior margin of the frontoparietal operculum and turn in a superior direction on the lateral surface of the frontal and parietal lobes. The arteries supplying the cortical areas below the sylvian fissure pursue a less tortuous course. These branches, on reaching the circular sulcus, run along its inferior circumference before turning upward and laterally on the medial surface of the temporal operculum, thus producing a less acute change in course at the inferior margin of the circular sulcus. On reaching the external surface of the sylvian fissure, these branches are directed downward and backward on the surface of the temporal lobe.
The M4 is composed of the branches to the lateral convexity. They begin at the surface of the sylvian fissure and extend over the cortical surface of the cerebral hemisphere. The more anterior branches turn sharply upward or downward after leaving the sylvian fissure. The intermediate branches follow a gradual posterior incline away from the fissure, and the posterior branches pass backward in nearly the same direction as the long axis of the fissure.
The perforating branches of the MCA enter the anterior perforated substance and are called the lenticulostriate arteries
(Fig. 2.16). There is an average of 10 (range, 1–21) lenticulostriate arteries per hemisphere (36). Lenticulostriate branches arise from the prebifurcation part of the M1 in every case and from the postbifurcation part of the M1 segment in half of the hemispheres. Of the total number of lenticulostriate branches, approximately 80% arise from the prebifurcation part of the M1. Most of the remainder arise from the postbifurcation part of the M1, but a few arise from the proximal part of the M2 near the genu. The earlier the bifurcation, the greater the number of postbifurcation branches. No branches to the anterior perforated substance arise from the postbifurcation trunks if the bifurcation is 2.5 cm or more from the origin of the middle cerebral artery.
The lenticulostriate arteries are divided into medial, intermediate, and lateral groups, each of which has a unique origin, composition, morphology, and characteristic distribution in the anterior perforated substance. The medial group is the least constant of the three groups and is present in only half of the hemispheres (36). When present, it consists of one to five branches that arise on the medial prebifurcation part of the M1 segment near the carotid bifurcation or an early branch, and pursue a relatively direct course to enter the anterior perforated substance just lateral to the C4 branches. Most arise from the posterior or superior aspect of the main trunk. Branching before entering the anterior perforated substance is less common than in the intermediate or lateral groups.
The intermediate lenticulostriate arteries form a complex array of branches before entering the anterior perforated substance between the medial and lateral lenticulostriate arteries. They are present in more than 90% of hemispheres. The most distinctive feature of the intermediate group is that it possesses at least one major artery, which furnishes a complex arborizing array of as many as 30 branches to the anterior perforated substance. The fewer perforating branches in this group (average, three) and the division yielding a great number of total branches entering the anterior perforated sub stance is evidence of this distinctive morphology. The intermediate lenticulostriate arteries arise almost exclusively on the M1 or its early branches. Most arise from the posterior, posterosuperior, or superior aspect of the MCA. They arise predominantly from the main or prebifurcation part of the M1 or an early branch.
The lateral lenticulostriate arteries are present in almost all hemispheres. They originate predominantly on the lateral part of the M1, pursue an S-shaped course, and enter the posterolateral part of the anterior perforated substance. An average of five lateral lenticulostriate arteries per hemisphere divide to yield as many as 20 branches before they enter the anterior perforated substance. They may also arise from the early branches of the M1 or from the M2. They can arise from the pre- or postbifurcation trunks of the M1. More branches arise from postbifurcation branches if there is an early bifurcation; they could arise from either the superior or inferior trunk distal to the bifurcation, but there is a strong predilection for the inferior trunk. They arise from either the posterior, superior, or posterosuperior aspect of the parent trunks, travel medially with the parent trunks, then loop sharply posteriorly, laterally, and superiorly, and finally, turn posteromedially just before penetrating the anterior perforated substance. The branches with a more medial origin arise at a less acute angle to the parent vessel and pursue a more direct posterior, superior, and medial route to the anterior perforated substance.
The lateral and intermediate groups of lenticulostriate arteries pass through the putamen and arch medially and posteriorly to supply almost the entire anterior-to-posterior length of the upper part of the internal capsule and the body and head of the caudate nucleus. The medial lenticulostriate arteries irrigate the area medial to and below that supplied by the lateral and intermediate lenticulostriate arteries; this area includes the lateral part of the globus pallidus, the superior part of the anterior limb of the internal capsule, and the anterosuperior part of the head of the caudate nucleus. The relationship of the lateral lenticulostriate arteries to the M1 bifurcation is important because the bifurcation is the site of most aneurysms arising from the middle cerebral artery. Nearly 30% of the lateral lenticulostriate arteries originate from the pre- or postbifurcation trunks 2.0 mm or less from the M1 bifurcation; and nearly 70% are positioned 5.0 mm or less from the bifurcation (36). Some branches arise directly on the bifurcation. Of the arteries originating near the bifurcation, there is a nearly even split between an origin on the pre- and postbifurcation trunks. The area of supply and clinical features are reviewed below, under the Anterior Perforating Arteries.
The cortical territory supplied by the MCA includes the majority of the lateral surface of the hemisphere, all of the insular and opercular surfaces, the lateral part of the orbital surface of the frontal lobe, the temporal pole, and the lateral part of the inferior surface of the temporal lobe. The MCA territory does not reach the occipital or frontal poles or the upper margin of the hemisphere, but it does extend around the lower margin of the cerebral hemisphere onto the inferior surfaces of the frontal and temporal lobes (Fig. 2.17).
The narrow peripheral strip on the lateral surface of the cerebral hemisphere, supplied by the ACA and PCA rather than the MCA, extends along the entire length of the superior margin of the hemisphere from the frontal to the occipital pole. It is broadest in the superior frontal region and narrowest in the superior parietal area. This strip continues around the occipital pole and onto the posterior part of the lateral surface of the temporal lobe and narrows and disappears anteriorly on the temporal lobe where the branches of the MCA extend around the lower border of the hemisphere onto the inferior surface of the temporal lobe and the orbital surface of the frontal lobe.
The cortical area supplied by the MCA is divided into 12 areas (Fig. 2.17):
- Orbitofrontal area. The orbital portion of the middle and inferior frontal gyri and the inferior part of the pars orbitalis.
- Prefrontal area. The superior part of the pars orbitalis, the pars triangularis, the anterior part of the pars opercularis, and most of the middle frontal gyrus.
- Precentral area. The posterior part of the pars opercularis and the middle frontal gyrus, and the inferior and middle portions of the precentral gyrus.
- Central area. The superior part of the precentral gyrus and the inferior half of the postcentral gyrus.
- Anterior parietal area. The superior part of the postcentral gyrus, and frequently, the upper part of the central sulcus, the anterior part of the inferior parietal lobule, and the anteroinferior part of the superior parietal lobule.
- Posterior parietal area. The posterior part of the superior and inferior parietal lobules, including the supramarginal gyrus.
- Angular area. The posterior part of the superior temporal gyrus, variable portions of the supramarginal and angular gyri, and the superior parts of the lateral occipital gyri (the artery to this area is considered the terminal branch of the MCA).
- Temporo-occipital area. The posterior half of the superior temporal gyrus, the posterior extreme of the middle and inferior temporal gyri, and the inferior parts of the lateral occipital gyri.
- Posterior temporal area. The middle and posterior part of the superior temporal gyrus, the posterior third of the middle temporal gyrus, and the posterior extreme of the inferior temporal gyrus.
- Middle temporal area. The superior temporal gyrus near the level of the pars triangularis and pars opercularis, the middle part of the middle temporal gyrus, and the middle and posterior part of the inferior temporal gyrus.
- Anterior temporal area. The anterior part of the superior, middle, and inferior temporal gyri.
- Temporopolar area. The anterior pole of the superior, middle, and inferior temporal gyri.
The main trunk of the MCA divides in one of three ways: bifurcation into superior and inferior trunks; trifurcation into superior, middle, and inferior trunks; or division into multiple (four or more) trunks (Figs. 2.18 and 2.19). In our study, 78% of the MCAs divided in a bifurcation, 12% divided in a trifurcation, and 10% divided by giving rise to multiple trunks (14). The distal division of the MCA also generally occurs in a series of bifurcations. The small arteries that arise proximal to the bifurcation or trifurcation and are distributed to the frontal or temporal pole are referred to as early branches.
The MCAs that bifurcate are divided into three groups, designated equal bifurcation, superior trunk dominant, and inferior trunk dominant, based on the diameter and the size of the cortical area of supply of their superior and inferior trunks. The equal bifurcation (18% of hemispheres) yields two trunks with nearly equal diameters and size of cortical area. The inferior trunk supplies the temporal, temporo-occipital, and angular areas, and the superior trunk supplies the frontal and parietal regions. The superior trunk usually supplies the orbitofrontal to the posterior parietal areas, and the inferior trunk usually supplies the angular to the temporopolar areas. The inferior trunk dominant type of bifurcation (32% of hemispheres) yields a larger inferior trunk that supplies the temporal and parietal lobes and a smaller superior trunk that supplies all or part of the frontal lobe. The maximal area perfused by the inferior trunk includes all of the territory between and including the precentral and temporopolar areas. The superior trunk dominant type of bifurcation (28% of hemispheres) yields a larger superior trunk that supplies the frontal and parietal regions and a smaller inferior trunk that supplies only the temporal lobe. The maximal area supplied by the dominant superior trunk includes the orbitofrontal to the temporo-occipital areas.
The stem arteries arise from the trunks and give rise to the individual cortical branches (Fig. 2.20). They arise from the main trunk and the two or more trunks formed by a bifurcation, trifurcation, or division into multiple trunks. There is considerable variation in the number and size of the area supplied by the stem arteries. The most common pattern is made up of 8 stem arteries per hemisphere (range, 6 to 11) (14).
The individual stem arteries give rise to one to five cortical arteries. The most common pattern is for one of the 12 cortical areas to be supplied by a stem artery supplying one or two adjacent areas. The cortical areas most commonly receiving a stem artery serving only that area are the temporo-occipital, angular, and central areas. Stem arteries supplying four or five of the cortical areas are most commonly directed to the area below the sylvian fissure. In our study, we also examined the stem arteries supplying each lobe (14). The frontal lobe is supplied by one to four stem arteries. The most common pattern, a two-stem pattern, had one stem giving rise to the orbitofrontal, prefrontal, and precentral arteries, and the other stem giving rise to the central artery. The parietal lobe and the adjoining part of the occipital lobe are supplied by one to three stem arteries. The most frequent pattern is for each of the three cortical areas to have its own stem. In the most frequent two-stem pattern, one stem gives rise to the anterior and posterior parietal arteries and the other stem gives rise to the angular artery. The temporal lobe, along with the adjoining part of the occipital lobe, is supplied by one to five stem arteries; the most common pattern is to have four stem arteries. This lobe has more stem arteries than the other lobes supplied by the MCA.
The cortical arteries arise from the stem arteries and supply the individual cortical areas. Generally, one, or less commonly, two cortical arteries (range, one to five) pass to each of the 12 cortical areas (Figs. 2.17 and 2.20). The smallest cortical arteries arise at the anterior end of the sylvian fissure and the largest arteries arise at the posterior limits of the fissure. The cortical branches to the frontal, anterior temporal, and anterior parietal areas are smaller than those supplying the posterior parietal, posterior temporal, temporo-occipital, and angular areas. The smallest arteries supply the orbitofrontal and temporopolar areas, and the largest ones supply the temporooccipital and the angular areas. There is an inverse relationship between the size and number of arteries supplying a cortical area. The temporo-occipital area has the smallest number of arteries, but they are the largest in size, and the prefrontal area has the largest number of arteries, but they are smaller.
The temporopolar, temporo-occipital, angular, and anterior, middle, and posterior temporal arteries usually arise from the inferior trunk; the orbitofrontal, prefrontal, precentral, and central arteries usually arise from the superior trunk. The anterior and posterior parietal arteries have an origin evenly divided between the two trunks and usually arise from the dominant trunk.
The cortical arteries arising from the main trunk proximal to the bifurcation or trifurcation are called early branches (Fig. 2.3). The early branches are distributed to the frontal or temporal lobes. Nearly half of MCAs send early branches to the temporal lobe, but less than 10% give early branches to the frontal lobe (14). The temporal branches usually supply the temporopolar and anterior temporal areas. The frontal branches terminate in the orbitofrontal and prefrontal areas. A few MCAs will give rise to early branches to both the frontal and temporal areas.
There is most commonly only one early branch, but a few hemispheres will give rise to two early branches. In our study, the distance between the bifurcation or trifurcation of the MCA and the origin of the early branches to the frontal lobe was 5.5 mm (range, 5.0–6.0 mm) and 11.2 mm (range, 3.5–30.0 mm) for the temporal lobe (14).
Anomalies of the MCA, consisting of either a duplicate or an accessory MCA, are infrequent and occur less often than anomalies of the other intracranial arteries (14). A duplicated MCA is a second artery that arises from the internal carotid artery and an accessory MCA is one that arises from the anterior cerebral artery. Both the duplicate and accessory MCAs send branches to the cortical areas usually supplied by the MCA. The accessory MCAs usually arise from the anterior cerebral artery near the origin of the anterior communicating artery (AComA). The accessory MCA is differentiated from a recurrent artery of Heubner by the fact that the recurrent artery, although arising from the same part of the anterior cerebral artery as an accessory MCA, enters the anterior perforated substance, but the accessory MCA, although sending branches to the anterior perforated substance, also courses lateral to this area and sends branches to cortical areas normally supplied by the MCA (Fig. 2.16H).
MCA Branches for Extracranial-Intracranial Bypass
Important factors in selecting a cortical artery for a bypass procedure are its diameter and the length of artery available on the cortical surface. The largest cortical artery is the temporo-occipital artery (14). Nearly two-thirds are 1.5 mm or more in diameter, and 90% are 1 mm or more in diameter. The smallest cortical artery is the orbitofrontal artery; approximately one quarter are 1 mm or more in diameter. The central sulcal artery is the largest branch to the frontal lobe, and the angular artery is the largest branch to the parietal lobe. The temporo-occipital and the posterior temporal arteries are the largest branches to the temporal lobe. The minimum length of a cortical artery needed to complete a bypass is 4 mm. The length of each of the cortical arteries on the cortical surface averages 11.8 mm or more. The angular, posterior parietal, and temporo-occipital arteries have the longest segments on the cortical surface, and the orbitofrontal and temporopolar arteries have the shortest cortical segment.
Chater et al. (3) undertook an analysis of the cortical branches of the MCA available in three circular cortical zones with a diameter of 4 cm. These three zones were centered over the convexity of the frontal lobe, the tip of the temporal lobe, and the region of the angular gyrus and were selected to be readily accessible by means of a small craniectomy. An external diameter of 1 mm was postulated to be the minimum required for long-term anastomosis patency. Chater et al. (3) found a cortical artery with a diameter of more than 1.4 mm in the angular zone in 100% of hemispheres. The arteries over the tip of the temporal lobe and the frontal lobe were considerably smaller. In the temporal zone, an artery with a diameter of more than 1.0 mm was present in 70% of hemispheres, and in the frontal zone, an arterial diameter of more than 1.0 mm was present in only 52%. These authors also noted that the vessels in the region of the angular gyrus had the advantage of being located so as to be accessible for anastomosis not only with the superficial temporal artery, but also with the occipital artery. They recommended that the craniotomy for exposing the cortical branches of the MCA be 4 cm in diameter, and that it be centered 6 cm above the external auditory canal.
Occlusion of the individual cortical branches of the MCA, depending on the area supplied, may cause the following deficits: motor weakness caused by involvement of the corticospinal tract in the central gyrus; sucking and grasping reflex caused by involvement of the premotor area; motor aphasia resulting from involvement of the posteroinferior surface of the frontal cortex of the dominant hemisphere; changes in mentation and personality caused by involvement of the prefrontal area; visual field defects caused by a disturbance of the geniculocalcarine tract in the temporal, parietal, and occipital lobes; impairment of discriminative sensations and neglect of space and body parts resulting from involvement of the parietal lobes; finger agnosia, right-left disorientation, acalculia, and agraphia (Gerstmann’s syndrome) caused by involvement of the functional area between the parietal and occipital lobes of the dominant hemisphere; or a receptive aphasia caused by disturbance of the dominant temporoparietal area.
Reports of specific clinical syndromes associated with occlusion of the individual cortical branches are rare. Occlusions of the individual cortical arteries are difficult to identify on angiograms, but, when detectable, they frequently correlate well with the neurological deficit (42). Embolism is a more frequent cause of occlusion of the MCA than thrombosis. In series of angiographically and autopsy-proven occlusions of the branches and trunks of the MCA, the ratio of embolic to thrombotic occlusions is approximately 13:1 to 16:1 (10).
Fisher (10) described the syndromes of obstructing the superior and inferior trunk of the MCA as follows: obstruction of the superior trunk causes a sensory-motor hemiplegia without receptive aphasia in the dominant hemisphere; obstruction of the inferior division causes a receptive aphasia in the absence of hemiplegia in the dominant side. Fisher’s syndromes would apply if the trunks were nearly equal in size, with the superior trunk supplying the frontal and parietal regions and the inferior trunk supplying the temporal and occipital lobes. However, we found marked variation in the size of the superior and inferior trunks and the area that they supply. In a few hemispheres, the inferior trunk supplied the temporal and parietal lobes and extended forward onto the precentral motor area, and, in another group of hemispheres, a large superior trunk supplied the frontal and parietal lobes and extended onto the speech centers on the posterior part of the temporal lobe.
The site of an MCA anastomosis for an MCA branch, trunk, or stem occlusion should be selected only after a careful review of the angiogram. If an early branch to the temporal lobe were used as a recipient vessel for a bypass operation, in cases of MCA stenosis or occlusion near the bifurcation, the new flow would frequently be channeled into the MCA proximal to the occlusion and none would have been delivered into the hypoperfused area distal to the occlusion. Some early branches, although arising proximal to the carotid bifurcation, may reach as far distally as the posterior temporal area. If one trunk of the MCA is stenotic or obstructed, an anastomosis to the other trunk will deliver blood to the proximal MCA and distally into the normal rather than into the ischemic area. Most surgeons use the angular, temporo-occipital, or posterior temporal branch of the MCA for a bypass, the three largest branches in this study (30).
Anterior Cerebral Artery
The ACA, the smaller of the two terminal branches of the internal carotid artery, arises at the medial end of the sylvian fissure, lateral to the optic chiasm and below the anterior perforated substance (Figs. 2.1 and 2.3). It courses anteromedially above the optic nerve or chiasm and below the medial olfactory striate to enter the interhemispheric fissure. Near its entrance into the fissure, it is joined to the opposite ACA by the AComA, and ascends in front of the lamina terminalis to pass into the longitudinal fissure between the cerebral hemispheres.
The arteries from each side are typically not side by side as they enter the interhemispheric fissure and ascend in front of the lamina terminalis (Figs. 2.1 and 2.21). Rather, one distal ACA lies in the concavity of the other. Above the lamina terminalis, the arteries make a smooth curve around the genu of the corpus callosum and then pass backward above the corpus callosum in the pericallosal cistern. In their midcourse, one or both ACAs frequently turns away from the corpus callosum only to dip sharply back toward it. After giving rise to the cortical branches, the ACA continues around the splenium of the corpus callosum as a fine vessel, often tortuous, and terminates in the choroid plexus in the roof of the third ventricle. The posterior extent of the ACA depends on the extent of supply of the PCA and its splenial branches. The ACA often has four convex curves as viewed laterally: the convexity is posterosuperior between its origin and the AComA, anteroventral as it turns into the interhemispheric fissure, posterosuperior at the junction of the rostrum and genu of the corpus callosum, and anterior as it courses around the genu of the corpus callosum (Fig. 2.22). Branches of the distal ACA are exposed in surgical approaches to the sellar and chiasmatic regions, third and lateral ventricles, falx and parasagittal areas, and even in approaches to the medial parieto-occipital and pineal regions.
The ACA is divided at the AComA into two parts, proximal (precommunicating) and distal (post-communicating) (Fig. 2.22). The proximal part, extending from the origin to the AComA, constitutes the A1 segment. The distal part is formed by the A2 (infracallosal), A3 (precallosal), A4 (supracallosal), and A5 (posterocallosal) segments. The relationships of the four distal segments are reviewed below, under Distal Part.
A1 Segment and the Anterior Communicating Arteries
The A1 courses above the optic chiasm or nerves to join the AComA. The junction of the AComA with the right and left A1 is usually above the chiasm (70% of brains) rather than above the optic nerves (30%) (Figs. 2.23 and 2.24) (26). Of those passing above the optic nerves, most journey above the nerve near the chiasm rather than distally. The shorter A1s are stretched tightly over the chiasm; the longer ones travel anteriorly over the optic nerves. The arteries with a more forward course are often tortuous and elongated, with some resting on the tuberculum sellae or planum sphenoidale. The A1 varies in length from 7.2 to 18.0 mm (average, 12.7 mm) (26). The length of the AComA is usually between 2 and 3 mm, but may vary from 0.3 to 7.0 mm (26). The longer AComAs are commonly curved, kinked, or tortuous.
A normal ACA-AComA complex is one in which an AComA connects A1s of nearly equal size, and both A1s and the AComA are of sufficient size to allow circulation between the two carotid arteries and through the anterior circle of Willis. The AComA diameter averages approximately 1 mm less than the average diameter of the A1. The AComA diameters are the same or larger than their smaller A1 in only 25% of the brains (26). Ten percent of the brains have an A1 of 1.5 mm or less in diameter and only 2% have an A1 with a diameter of 1.0 mm or less. The diameter of the AComA was 1.5 mm or smaller in 44% of brains and 1.0 mm or smaller in 16%.
The A1 is the favorite site on the circle of Willis for hypoplasia. A1 hypoplasia has a high rate of association with aneurysms; it is found with 85% of AComA aneurysms (Figs. 2.23 and 2.24) (38). It is the only anatomic variant that correlates with the location of cerebral aneurysm. The importance of this variant in aneurysm formation is reviewed in more detail in Chapter 3.
There is a direct correlation between the difference in size of the right and left A1s and the size of the AComA. As the difference in diameter between the A1s increases, so does the size of the AComA. Thus, a large AComA is often associated with a significant difference in diameter between the right and left A1. This is understandable from a functional point of view because, with a small or hypoplastic A1, more collateral circulation flows across the AComA to make up the deficit. A
difference in diameter of 0.5 mm or more between the right and left A1 is found in half of the brains and a difference of 1 mm or more in 12%. The average AComA diameter is 1.2 mm in the group of brains in which the difference in diameter between the right and left A1s is 0.5 mm or less and 2.5 mm if the difference is more than 0.5 mm. This correlation between the size of the A1s permits a rough estimate of the size of the AComA, even though the artery is not visualized, because it is the most difficult part of the circle of Willis to define on cerebral angiography.
Another difficulty in angiographically defining the AComA is that it is frequently not oriented in a strictly transverse plane. The length of the AComA is oriented in an oblique or straight anterior-posterior plane if one ACA passes between the hemispheres behind the other ACA. The ACAs are side by side as they pass between the cerebral hemispheres in approximately one in five hemispheres, and the left is anterior to the right more often than the right is anterior to the left. These variations may explain why angiography in the oblique position is often needed to define the AComA. The AComA usually has a round appearance, but it may seem flat because of a broad connection with both ACAs, or even triangular with a large base on one ACA and a threadlike connection on the other.
One AComA was present in 60%, two in 30%, and three in 10% of the brains we examined (Fig. 2.24) (26). Double AComAs can take a variety of forms; one is simply a hole in the middle of a broad or triangular artery separating arteries. The double or triple arteries can be approximately the same size or can vary markedly in diameter. A common pattern is for one to be large and the others relatively small. It is rare to find no connection between the two sides, but in some cases, the connection may be tiny—as small as 0.2 mm in diameter. An infrequent finding is duplication of a portion of the A1. Another infrequent anomaly consists of a third or median ACA arising from the AComA. The median artery courses upward and backward above the corpus callosum. It frequently divides opposite the paracentral lobule and gives branches to the paracentral lobules of both sides. In such cases, the ACAs proper are usually small and supply the anteromedial surfaces of the hemispheres.
The recurrent branch of the ACA, first described by Heubner in 1874, is unique among arteries in that it doubles back on its parent ACA and passes above the carotid bifurcation and MCA into the medial part of the sylvian fissure before entering the anterior perforated substance (Figs. 2.16, 2.23, and 2.24) (18). It pursues a long, redundant path to the anterior perforated substance, sometimes looping forward on the gyrus rectus and inferior surface of the frontal lobe. In its journey to the anterior perforated substance, it is often closely applied to the superior or posterior aspect of the A1. It may seem, falsely, to be issuing from the A1 until further dissection clarifies its site of origin at the level of the AComA. The recurrent arteries arising proximally on the A1 follow a more direct path to the anterior perforated substance than those arising distally.
The recurrent branch is the largest artery arising from the A1 or the proximal 0.5 mm of the A2 in the majority of hemispheres (26). It may infrequently be absent on one side or arise as several branches. In our study, there was a single recurrent artery in 28% of the hemispheres, two in 48%, and three or four in 24% (26). If there were two or more recurrent arteries, both or at least one arose at the level of the junction of the A1 and A2 (36). Rarely does more than a single recurrent artery arise from the A1. If there are two recurrent arteries and one arises on the A1, the second usually arises at the junction of the A1 and A2. A large basal perforating artery may infrequently arise from the A1 between the AComA and the recurrent artery. The recurrent artery diameter is usually less than half that of the A1, but it may infrequently be as large as or exceed the A1 diameter if the A1 is hypoplastic.
The recurrent branch usually arises from the distal A1 or from the proximal part of the ACA segment just distal to the AComA, referred to as the A2; however, it may emerge at any point along the A1. It most commonly originates from the A2. In our study, it originated from the A2 in 78%, from the A1 in 14%, and at the A1–A2 junction at the level of the AComA in 8% (26). In 52%, it arose within 2 mm of the AComA, in 80% within 3 mm, and in 95% within 4 mm. The recurrent arteries arising near the AComA usually arise from the lateral side of the junction of the A1 and A2 at a right angle to the parent vessel. They may originate either in common with or give rise to the frontopolar artery.
Most recurrent arteries course anterior to the A1 and are seen on elevating the frontal lobe before visualizing the A1, but they may also course superior to the A1, between it and the anterior perforated substance, or may loop posterior to A1. It courses above the internal carotid bifurcation and the proximal middle cerebral artery in its lateral course.
The recurrent artery may enter the anterior perforated substance as a single stem or divide into many branches (average, four). Of the total branches, approximately 40% terminate in the anterior perforated substance medial to the origin of the ACA, and 40% terminate lateral to the ACA origin. The remaining branches pass to the inferior surface of the frontal lobe adjacent the anterior perforated substance. The recurrent artery supplies the anterior part of the caudate nucleus, anterior third of the putamen, anterior part of the outer segment of the globus pallidus, anteroinferior portion of the anterior limb of the internal capsule, and the uncinate fasciculus, and, less commonly, the anterior hypothalamus. The hypothalamic supply is less than from the A1. In the treatment of anterior communicating aneurysms, great care must be taken to avoid unnecessary manipulation or occlusion of Heubner’s artery. Occlusion may cause hemiparesis with facial and brachial predominance because of compromise of that branch supplying the anterior limb of the internal capsule, and aphasia if the artery is on the dominant side.
Basal Perforating Branches
The A1 and A2 and the AComA give rise to numerous basal perforating arteries (Figs. 2.16 and 2.24). An average of 8 basal perforators (range, 2–15), exclusive of Heubner’s artery, arise from each A1 (26, 27). The lateral half of A1 is a richer source of branches than the medial half. The A1 branches terminate, in descending order of frequency, in the anterior perforated substance, the dorsal surface of the optic chiasm or the suprachiasmatic portion of the hypothalamus, the optic tract, dorsal surface of the optic nerve, and the sylvian fissure between the cerebral hemispheres and the lower surface of the frontal lobe. The striking difference in the termination of A1 branches as compared with those from the recurrent artery is the lack of recurrent artery branches to the upper surface of the optic nerves and chiasm and the anterior hypothalamus and the greater number of recurrent branches entering the sylvian fissure. Approximately 40% of both A1 and recurrent artery branches terminate in the anterior perforated substance medial to the A1 origin, but almost no Heubner’s branches enter the area around the optic chiasm and tract, although 40% of those from A1 terminated there. Approximately 40% of the recurrent artery branches enter the anterior perforated substance lateral to the carotid bifurcation.
The A1, excluding the recurrent artery and the A2, most consistently supplies the chiasm and anterior third ventricle and hypothalamic area, but only inconsistently supplies the caudate and globus pallidus. Heubner’s artery, by contrast, provides a rich supply to the caudate and adjacent internal capsule, but much less to the hypothalamus than the A1. Involvement of the hypothalamic branches that arise mainly from A1, without implication of the recurrent artery, may result in emotional changes, personality disorders, and intellectual deficits, including anxiety and fear, weak spells, and symptoms referable to disordered mentation, such as dizziness, agitation, and hypokinesis without paralysis or alterations of the conscious or waking state (6, 26). The frequent inclusion of recurrent artery ischemia when the A1 branches are involved adds a hemiparesis with brachial predominance to the deficit. This contrasts with the crural weakness of distal ACA occlusion.
The AComA also frequently gives rise to perforating arteries that terminate in the superior surface of the optic chiasm and above the chiasm in the anterior hypothalamus (Figs. 2.16, 2.23, and 2.24). The AComA is frequently the site of origin of one or two, but as many as four branches that terminate, in descending order of frequency, in the suprachiasmatic area, dorsal surface of the optic chiasm, anterior perforated substance, and frontal lobe, and perfuse the fornix, corpus callosum, septal region, and anterior cingulum (6, 8). Most arise from the superior or posterior surfaces of the AComA. The A2, to be discussed below, is also the site of origin of perforating branches terminating in the inferior frontal area, anterior perforated substance, dorsal optic chiasm, and the suprachiasmatic area.
The distal or postcommunicating part of the ACA begins at the AComA and extends around the corpus callosum to its termination (Figs. 2.22 and 2.25). The distal ACA is divided into four segments (A2 through A5). The A2 (infracallosal) segment begins at the AComA, passes anterior to the lamina terminalis, and terminates at the junction of the rostrum and genu of the corpus callosum. The A3 (precallosal) segment extends around the genu of the corpus callosum and terminates where the artery turns sharply posterior above the genu. The A4 (supracallosal) and A5 (postcallosal) segments are located above the corpus callosum and are separated into an anterior (A4) and posterior (A5) portion by a point bisected in the lateral view close behind the coronal suture. The A2 and A3 segments, together, and A4 and A5 have been referred to as the ascending and horizontal segments, respectively (27). In our discussion, the distal ACA is synonymous with the precallosal artery.
The Pericallosal Artery
The pericallosal artery is the portion of the ACA distal to the AComA around and on or near the corpus callosum (Figs. 2.22, 2.25, and 2.26). Some authors reserve that term for the artery formed by the bifurcation near the genu of the corpus callosum into the pericallosal and callosomarginal arteries (27). We refer to the segment distal to the AComA as the pericallosal artery because both the AComA and pericallosal artery are consistently present, but the callosomarginal artery is inconsistent; it is quite variable with regard to its site of origin and is absent in nearly 20% of hemispheres (27). If one assumes the pericallosal artery begins at the callosomarginal origin, the variability of origin of the callosomarginal artery could place the origin of the pericallosal artery at any point from near the AComA to the genu of the corpus callosum, and, in addition, if the callosomarginal artery is absent, some arbitrary point must be selected as the origin of the pericallosal artery. Thus, the term pericallosal artery refers to the portion of the ACA beginning at the AComA, which includes the A2 to A5 segments.
The Callosomarginal Artery
The callosomarginal artery, the largest branch of the pericallosal artery, is defined as the artery that courses in or near the cingulate sulcus and gives rise to two or more major cortical branches (Figs. 2.22, 2.25, and 2.26) (27). The callosomarginal artery is present in 80% of hemispheres. The callosomarginal artery cannot be defined in terms of a given group of vessels that arises from it because any of the usual branches of the callosomarginal artery may arise directly from the pericallosal artery. It follows a course roughly parallel to that of the pericallosal artery, coursing above the cingulate gyrus in or near the cingulate sulcus. Its origin varies from just distal to the AComA to the level of the genu of the corpus callosum. Its most frequent origin is from the A3, but it may also arise from the A2 or A4. Its branches ascend on the medial surface of the hemisphere and continue on to the lateral convexity for approximately 2 cm. Portions of the premotor, motor, and sensory areas are included in its area of perfusion.
The size of the pericallosal artery distal to the callosomarginal origin varies inversely with the size of the callosomarginal artery. Immediately past the origin of the callosomarginal artery, the pericallosal and callosomarginal arteries are equal in diameter in only 20% of hemispheres; the pericallosal is larger in 50%; and the callosomarginal is larger in 30% (27). The callosomarginal artery should not be mistaken for the pericallosal artery in lateral angiography, because the mistaken wider curvature may be falsely interpreted as representing hydrocephalus.
The anterior portion of the falx cerebri is consistently narrower than its posterior part, with the free margin of its anterior portion lying well above the genu of the corpus callosum, whereas the free margin of its posterior portion is more closely applied to the splenium (Fig. 2.22). The entire course of the pericallosal artery, except for the posterior portion, is below the free margin of the falx cerebri and is free to shift across the midline. The callosomarginal artery, on the other hand, has only the most anterior portion below the free margin of the falx; the remainder lies above the free edge, and its displacement across the midline is limited by the rigidity of the falx (Fig. 2.22, A–C).
Distal ACA Branches
The distal ACA gives origin to two types of branches: 1) basal perforating branches to basal structures including the optic chiasm, suprachiasmatic area, lamina terminalis, and anterior hypothalamus, structures located below the rostrum of the corpus callosum; and 2) cerebral branches divided into cortical branches to the cortex and adjacent white matter and subcortical branches to the deep white and gray matter and the corpus callosum.
Basal Perforating Branches
The A2 segment typically gives rise to 4 or 5 (range, 0–10) basal perforating branches that supply the anterior hypothalamus, septum pellucidum, medial portion of the anterior commissure, pillars of the fornix, and anteroinferior part of the striatum (Figs. 2.16, 2.23, and 2.24) (26, 27, 39). They commonly take a direct course from the A2 segment to the anterior diencephalon. In a few cases, the perforating branches may arise from a larger artery, referred to as the precallosal artery, that originates from A2 and passes upward between the A2 segment and the lamina terminalis toward the genu of the corpus callosum (Figs. 2.21 and 2.23). The recurrent artery may also arise from the A2, as described above.
The cortical branches supply the cortex and adjacent white matter of the medial surface from the frontal pole to the parietal lobe where they intermingle with branches of the PCA (Figs. 2.25–2.27). On the basal surface, the ACA supplies the medial part of the orbital gyri, the gyrus rectus, and the olfactory bulb and tract. On the lateral surface, the ACA supplies the area of the superior frontal gyrus and the superior parts of the precentral, central, and postcentral gyri. The band
of lateral cortex supplied by the ACA is wider anteriorly, often extending beyond the superior frontal sulcus, and narrows progressively posteriorly. The distal ACA on one side sends branches to the contralateral hemisphere in nearly two-thirds of brains.
Eight cortical branches are typically encountered (Figs. 2.26 and 2.27). They are orbitofrontal, frontopolar, internal frontal, paracentral, and the parietal arteries; the internal frontal group is divided into the anterior, middle, and posterior frontal arteries, and the parietal group is divided into superior and inferior parietal arteries. The smallest cortical branch is the orbitofrontal artery, and the largest is the posterior internal frontal artery. The frontopolar and orbitofrontal arteries are present in nearly all hemispheres; the least frequent branch is the inferior parietal artery, present in approximately two-thirds of hemispheres. The most frequent ACA segment of origin of the cortical branches is as follows: orbitofrontal and frontopolar arteries, A2; the anterior and middle internal frontal and callosomarginal arteries, A3; the paracentral artery, A4; and the superior and inferior parietal arteries, A5. The posterior internal frontal artery arises with approximately equal frequency from A3, A4, and the callosomarginal artery. All of the cortical branches arise from the pericallosal artery more frequently than they do from the callosomarginal. Of the major cortical branches, one of the internal frontal arteries or the paracentral artery arises most frequently from the callosomarginal. The cortical branch that arises most frequently from the callosomarginal artery is the middle internal frontal artery. Of the callosomarginal arteries present in our study, 50% gave rise to two major cortical branches, 32% gave rise to three, 16% gave rise to four, and, in one hemisphere (2%), five of the eight major cortical branches arose from the callosomarginal artery (27).
1. Orbitofrontal Artery
This artery, the first cortical branch of the distal ACA, is present in nearly all hemispheres. It commonly arises from the A2, but may also arise as a common trunk with the frontopolar artery. It may uncommonly arise from the A1 segment just proximal to the AComA. From its origin, it passes down and forward toward the floor of the anterior cranial fossa to reach the level of the planum sphenoidale. It supplies the gyrus rectus, olfactory bulb, and tract, and the medial part of the orbital surface of the frontal lobe.
2. Frontopolar Artery
The next cortical branch, the frontopolar artery, arises from the A2 segment of the pericallosal artery in 90% of hemispheres and from the callosomarginal artery in 10%. From its origin, it passes anteriorly along the medial surface of the hemisphere toward the frontal pole. It crosses the subfrontal sulcus and supplies portions of the medial and lateral surfaces of the frontal pole.
3. Internal Frontal Arteries
The internal frontal arteries supply the medial and lateral surfaces of the superior frontal gyrus as far posteriorly as the paracentral lobule (6). They most commonly arise from the A3 segment of the pericallosal artery or from the callosomarginal artery. Combinations of origins in which one or two internal frontal arteries have separate origins from the pericallosal artery, but the remaining artery or arteries arise from the callosomarginal, are common. The anterior internal frontal artery usually arises as a separate branch of the A2 or A3, but may also arise from the callosomarginal artery; it supplies the anterior portion of the superior frontal gyrus. The origin, whether from the pericallosal or callosomarginal artery, is most often at or inferior to the level of the genu of the corpus callosum. The middle internal frontal artery arises with nearly equal frequency from the pericallosal and the callosomarginal arteries and courses posteriorly in the cingulate sulcus a short distance before turning vertically to cross over the superior cortical margin im the middle portion of the superior frontal gyrus. It supplies the middle portion of the medial and lateral surfaces of the superior frontal gyrus. It is the cortical branch that arises most frequently from the callosomarginal artery. The posterior internal frontal artery arises with nearly equal frequency from the A3 and A4 and the callosomarginal artery and courses upward to the cingulate sulcus, then backward for a short distance before turning superiorly to terminate in the uppermost limit of the precentral fissure. It supplies the posterior third of the superior frontal gyrus and part of the cingulate gyrus. Its branches frequently reach the anterior portion of the paracentral lobule.
4. Paracentral Artery
This branch usually arises from the A4 or the callosomarginal artery approximately midway between the genu and splenium or the corpus callosum. It usually courses anterior to the marginal limb of the cingulate sulcus or in the paracentral sulcus before turning vertically to the superior portion of the paracentral lobule, where it supplies a portion of the premotor, motor, and somatic sensory areas. It may represent the terminal portion of the ACA.
5. Parietal Arteries
The parietal arteries, named the superior and inferior parietal arteries, supply the ACA distribution posterior to the paracentral lobule. The superior parietal artery arises from the A4 or A5 and from the callosomarginal artery and supplies the superior portion of the precuneus. It usually originates anterior to the splenium of the corpus callosum and courses in the marginal limb of the cingulate sulcus. If it courses posterior to the marginal limb, it often sends a branch to it. It is frequently the last cortical branch of the ACA. The inferior parietal artery most commonly arises from the A5 just before the latter courses around the splenium of the corpus callosum and supplies the posteroinferior part of the precuneus and adjacent portions of the cuneus. It is the least frequent cortical branch of the ACA (64% of hemispheres). An origin from the callosomarginal artery is uncommon.
There are large areas of the lateral cortical distribution of the ACA where there is a good chance of finding a vessel of sufficient diameter for a bypass anastomosis with a frontal branch of the superficial temporal artery. The area offering the best chance of finding an adequate ACA branch on the lateral surface was determined by drawing a circumferential line on the outer circumference of the hemisphere beginning at the sylvian fissure and continuing around the frontal pole and over the superior hemispheric margin toward the occipital pole. The minimum diameter needed for an anastomosis is usually considered to be 0.8 mm (27). An identical line was drawn 2 cm inside the circumferential line. The largest percentage of ACA branches crossing these lines was located on the anterior portion of the hemisphere between the 5-cm and 15-cm points on the circumferential line.
The ACA is the principal artery supplying the corpus callosum. The pericallosal artery sends branches into the rostrum, genu, body, and splenium and often passes inferiorly around the splenium. The terminal pericallosal branches are joined posteriorly by the splenial branches of the PCA. The corpus callosum is most commonly supplied by perforating branches, called short callosal arteries because they arise from the pericallosal artery and penetrate directly into the corpus callosum. As many as 20 short callosal branches (average, 7) may be found in one hemisphere (27). These branches not only supply the corpus callosum, but continue through it to supply the septum pellucidum, the anterior pillars of the fornix, and part of the anterior commissure.
In a few cases, well-formed longer branches, referred to as long callosal arteries, arise from the pericallosal artery and course parallel to the pericallosal artery, between it and the surface of the corpus callosum, to give origin to callosal perforating branches (Fig. 2.22). In addition to sending branches to the corpus callosum, they may supply adjacent cortex as well as the septal nuclei, septum pellucidum, and upper portions of the column of the fornix (27). The pericallosal artery frequently continues around the splenium of the corpus callosum, distal to the origin of the last cortical branch, and passes forward on the lower callosal surface, reaching the foramen of Monro in a few cases.
The precallosal artery, an infrequently occurring A2 or AComA branch, passes upward like a long callosal artery between the pericallosal artery and the lamina terminalis, sending branches into the anterior diencephalon and giving off multiple small branches to the rostrum and inferior part of the genu of the corpus callosum.
Anomalies of the distal ACA, including triplication of the postcommunical segment, failure of pairing of the distal ACA, and bihemispheric branches, are found in approximately 15% of brains (2). A bihemispheric branch is one that divides distal to the AComA and provides the major supply to the medial surface of both hemispheres. In the presence of such an anomaly, occlusion of one ACA distal to the AComA may produce bilateral cerebral injury similar to that produced by blocking both ACAs. The distal ACA on one side sends branches to the contralateral hemisphere in nearly two-thirds of brains (Fig. 2.28). However, most supply only a small area on the medial surface of the contralateral hemisphere. An infrequent anomaly is one in which the ACA distal to the A1 segment is unpaired and a single distal ACA divides to supply both hemispheres (26).
Anterior Perforating Arteries
The anterior perforating arteries are the group of arteries that enter the brain through the anterior perforated substance (Figs. 2.29–2.31). The interrelationships between the anterior perforated arteries from the different sources and the vital tracts and nuclei they supply in the central part of the cerebrum make them deserving of special attention. These arteries have an intimate relationship to aneurysms of the internal carotid and the middle and anterior cerebral arteries, and to tumors arising deep under the brain (Fig. 2.32) (31, 35, 36).
The anterior perforated substance is a rhomboid-shaped area buried deep in the sylvian fissure, bounded anteriorly by the lateral and medial olfactory striae, posteriorly by the optic tract and the temporal lobe, laterally by the limen insulae; medially, it extends above the optic chiasm to the interhemispheric fissure (Fig. 2.29). The arteries passing below and sending branches into the anterior perforated substance are the ICA, MCA, and ACA, and the AChA. The perforating arteries from each parent artery enter a specific mediolateral and anteroposterior territory of the anterior perforated substance. The site of penetration in the mediolateral direction is described in relation to a line passing posteriorly along the olfactory tract. This line, dividing the anterior perforated substance into medial and lateral territories, crosses the anterior perforated substance near its greatest anterior-posterior dimension and transects the optic tract as it passes around the cerebral peduncle. The medial territory extends above the optic chiasm to the interhemispheric fissure, and the lateral territory extends into the sylvian fissure to the limen insulae. The site of penetration of each group of arteries is also relatively constant in an anterior-posterior direction, based on subdivision of the anterior perforated substance into anterior, middle, and posterior zones extending across the full width of the anterior perforated substance, from the interhemispheric fissure to the limen insulae (Figs. 2.29–2.31).
Choroidal Segment of the C4
The C4 branches entering the anterior perforated substance arise from the choroidal segment (Fig. 2.30, A and B). The choroidal segment sends branches to the anterior perforated substance in nearly 100% of hemispheres (36). These branches tend to originate closer to the bifurcation than to the origin of the AChA. The branches arising at the bifurcation tend to be stouter than those arising below the bifurcation. Typically, these C4 branches follow a posterosuperior route to the posterior portion of the anterior perforated substance, near the optic tract. Approximately half of the branches penetrate the medial territory of the anterior perforated substance and half penetrate the lateral territory. Most enter the posterior or middle zones and very few enter the anterior zone.
Anterior Choroidal Artery
The AChA sends branches to the anterior perforated substance in 90% of hemispheres (13, 33, 36) (Fig. 2.30, C–F). The majority of the branches pursue a posterior, superior, and medial course, or a direct posterior and superior course to the anterior perforated substance. The branches arising at the origin of the AChA are somewhat stouter than those arising distally. These branches enter the posteromedial portion of the anterior perforated substance close to the optic tract and the line along the olfactory tract separating the medial and lateral territories. Approximately two-thirds of these branches enter the medial and one-third enter the lateral territory of the anterior perforated substance. Most enter the posterior zone or adjacent part of the middle zone of the anterior perforated substance.
Middle Cerebral Artery
The branches to the anterior perforated substance, called the lenticulostriate arteries, arise from the M1 and M2 and
occasionally from the early branches (Fig. 2.30, G–J). They arise from the prebifurcation part of the M1 in every case and from the postbifurcation part of the M1 segment in half of the hemispheres. The lenticulostriate arteries are divided into medial, intermediate, and lateral groups. The medial group, present in half of the hemispheres, pursues a relatively direct course to enter the anterior perforated substance just lateral to the C4 branches. Ninety percent of the medial lenticulostriate arteries enter the lateral territory of the anterior perforated substance, whereas only 10% enter the medial territory (36). The predominant pattern is for them to enter the middle and posterior zones of the anterior perforated substance. In the hemispheres in which the medial group of lenticulostriate arteries are absent, their territory in the anterior perforated substance is occupied by branches from the C4 and the ACA, AChA, and the intermediate lenticulostriate arteries.
The intermediate lenticulostriate arteries entering the anterior perforated substance between the medial and lateral lenticulostriate arteries are present in more than 90% of hemispheres. They enjoy a generous area of distribution in the lateral territory of the anterior perforated substance. Nearly 90% enter the middle or posterior zones of the anterior perforated substance between the territory of the medial and lateral lenticulostriate arteries, lateral to the branches from the C4, and posterior to the branches of the recurrent artery.
The lateral lenticulostriate arteries, present in almost all hemispheres, originate predominantly on the lateral part of the M1, but may also arise from the early branches of the M1 or from the M2. They pursue an S-shaped course to enter the posterolateral part of the anterior perforated substance. All of the lateral lenticulostriate arteries enter the lateral territory of the anterior perforated substance near the limen insulae, and nearly all enter the posterior zone of the lateral part of the anterior perforated substance.
Anterior Cerebral Artery
The branches of the anterior cerebral artery to the anterior perforated substance arise from two sources. First, the A1 gives rise to branches that pass directly to the anterior perforated substance. Second, the A1 and proximal part of the A2 give rise to the recurrent artery that sends branches to a broad extent of the anterior perforated substance (Fig. 2.30, M–P). Nearly all A1s send branches to the anterior perforated substance. Nearly 90% arise from the proximal half of the A1 and pursue a direct posterior and superior course to the anterior perforated substance. The ones with a more medial origin journey laterally to reach the anterior perforated substance. Most enter the medial territory of the anterior perforated substance near the optic chiasm and tract, and the remainder enter the lateral territory. Most enter the middle and posterior zones of the anterior perforated substance, predominantly posterior to the branches from the recurrent artery, anteromedial to those from the internal carotid and anterior choroidal arteries, and medial to those from the middle cerebral artery.
The recurrent artery is the largest and longest of the branches directed to the anterior perforated substance. It is present, sending branches to the anterior perforated substance, in all hemispheres. The recurrent branches enter the full mediolateral extent of the anterior perforated substance, yet have a limited representation in the anterior-posterior dimension. The territory penetrated by recurrent branches extends into the narrow part of the medial territory above the optic chiasm and into the lateral territory as far as the inner margin of the limen insulae. Their anteroposterior distribution is limited in contrast to their rich mediolateral representation, in that they are confined predominantly to the anterior half of the anterior perforated substance. The branches from recurrent arteries with a more lateral origin from the A1 have a greater tendency to enter the middle and posterior zones than those arising at the junction of the A1 and A2. By virtue of its long mediolateral extent, the recurrent artery borders on the territory of all the other groups entering the anterior perforated substance.
In summary, the ICA and AChA branches enter the posterior half of the central portion of the anterior perforated substance. The MCA enters the middle and posterior portions of the lateral half of the anterior perforated substance. The A1 gives rise to branches that enter the medial half of the anterior perforated substance above the optic nerve and chiasm. The recurrent artery sends branches into the anterior two-thirds of the full mediolateral extent of the anterior perforated substance. There are minimal anastomoses and limited overlap between the different groups at the level of the anterior perforated substance, making it most important that each of these groups be protected in operative approaches to the area. There is a reciprocal relationship between the intraparenchymal and anterior perforated substance territories of the ICA, AChA, ACA, and MCA such that the size of one artery’s territory increases or decreases the other artery’s territory in a reciprocal manner.
The deep cerebral structures located directly above the anterior perforated substance are the frontal horn and the anterior part of the caudate nucleus, putamen, and internal capsule (23). The anterior perforating arteries pass through the parts of the caudate nucleus, putamen, and internal capsule directly above the anterior perforated substance, and spread posteriorly to supply larger parts of these structures and the adjacent areas of the globus pallidus and thalamus (Fig. 2.32) (39, pp 30–33). The C4 branches penetrating the anterior perforated substance perfuse the genu of the internal capsule and the adjacent part of the globus pallidus, posterior limb of the internal capsule, and thalamus. The branches of the AChA entering the anterior perforated substance supply the medial two segments of the globus pallidus, the inferior part of the posterior limb of the internal capsule, and the anterior and ventrolateral nuclei of the thalamus. The lateral and intermediate groups of lenticulostriate arteries pass through the putamen and arch medially and posteriorly to supply almost the entire anterior-to-posterior length of the upper part of the internal capsule and the body and head of the caudate nucleus. The medial lenticulostriate arteries irrigate the area medial to and below that supplied by the lateral and intermediate lenticulostriate arteries; this area includes the lateral part of the globus pallidus, the superior part of the anterior limb of the internal capsule, and the anterosuperior part of the head of the caudate nucleus.
The A1 branches supply the area below the anteromedial part to the territory supplied by the lenticulostriate arteries. This region includes the area around the optic chiasm, the anterior commissure, the anterior hypothalamus, the genu of the internal capsule, and the anterior part of the globus pallidus. Its area of supply may less commonly extend to the contiguous part of the posterior limb of the internal capsule and to the anterior part of the thalamus (26). The recurrent artery supplies the most anterior and inferior parts of the head of the caudate nucleus and putamen, and the adjacent part of the anterior limb of the internal capsule (26).
The arteries entering the anterior perforated substance are intrinsically related to and commonly exposed in operations for aneurysms of the internal carotid, anterior communicating, and middle cerebral arteries. These relationships are reviewed in Chapter 3. The intradural exposure of the C4 and all of the arteries sending branches to the anterior perforated substance can be achieved using a small frontotemporal flap centered at the pterion. All of these aneurysms related to the anterior perforating arteries can be exposed by this approach along the ipsilateral sphenoid ridge, with opening of the sylvian fissure. Selected striatal arteriovenous malformations involving the arteries entering the anterior perforated substance have been treated by incision of the anterior perforated substance and occlusion of the feeding arteries without producing a deficit (Fig. 2.16I) (41). Operative treatment of these arteriovenous malformations is usually considered only if the lesion is located directly above the anterior perforated substance in the area anterior to the genu of the internal capsule, unless the genu and posterior limb of the internal capsule have already been damaged.
The Posterior Part of the Circle of Willis
The posterior part of the circle of Willis is formed by the proximal PCA and PComA and, together, in varying degrees, they provide the flow to the distal PCA (Figs. 2.8, 2.33, and 2.34). The posterior circle is one of the most difficult sites to approach surgically because of its location in the midline below the third ventricle, the complex series of perforating vessels surrounding and arising from it, and its intimate relationship to the extraocular nerves and upper brainstem. Its branches are exposed in surgical approaches to the basilar apex, tentorial notch, lateral and third ventricles, inferior temporal and medial parieto-occipital areas, and the pineal region— all relatively inaccessible areas.
A normal posterior circle, in which the proximal PCAs have a diameter larger than their PComAs and are not hypoplastic, is present in approximately half of the brains (Figs. 2.8 and 2.34). The other half harbor anomalies of the posterior circle, including either a hypoplastic PComA or a fetal configuration in which the proximal PCA is small and the PComA provides the major supply to the PCA and is larger than the P1 (24).
A hypoplastic arterial segment is defined as one having a diameter of 1 mm or less. In our study, PComA hypoplasia was found unilaterally in 26% and bilaterally in 6%, and a fetal configuration, in which the PCA arises predominantly from the carotid artery, was found unilaterally in 20% and bilaterally in 2% (37). Eight percent had a hypoplastic communicating artery on one side and a fetal complex on the other side. Absence of either the communicating artery or a P1 segment is very uncommon. The PComA is described above.
The posterior portion of the circle of Willis sends a series of perforating arteries into the diencephalon and midbrain that may become stretched around suprasellar tumors or posterior circle aneurysms (Figs. 2.33 and 2.34). Some of the perforating branches arising from the upper part of the basilar artery overlap with some of those arising from the posterior circle. The risks of occlusion of these vital perforating vessels during tumor or aneurysm surgery include visual loss, somatesthetic disturbances, motor weakness, memory deficits, autonomic imbalance, diplopia, alterations of consciousness, abnormal movements, and endocrine disturbances.
The Posterior Cerebral Artery
The PCA arises at the basilar bifurcation, is joined by the PComA at the lateral margin of the interpeduncular cistern, encircles the brainstem passing through the crural and ambient cisterns to reach the quadrigeminal cistern, and is distributed to the posterior part of the hemisphere (Figs. 2.1, 2.3, 2.7–2.9, 2.12, 2.13, 2.33, and 2.34). The posterior cerebral artery supplies not only the posterior part of the cerebral hemispheres, as its name implies, but also sends critical branches to the thalamus, midbrain, and other deep structures, including the choroid plexus and walls of the lateral and third ventricles. Embryologically, it arises as a branch of the internal carotid artery, but by birth its most frequent origin is from the basilar artery. The basilar bifurcation, and thus the PCA origin, may be located as far caudal as 1.3 mm below the pontomesencephalic junction and as far rostral as the mamillary bodies and adjacent floor of the third ventricle, which may be elevated by a high bifurcation. The artery usually bifurcates opposite the interpeduncular fossa, but some bifurcations may be as low as the upper pons or so high that they indent the mamillary bodies and floor of the third ventricle. The average separation between the basilar bifurcation and mamillary bodies is 8.1 mm (range, 0–14 mm). There is widening of the basilar artery at the bifurcation in 16% of cases, giving the basilar apex and bifurcation a cobra-like appearance (37, 43).
The PCA is divided into four segments, P1 through P4 (Figs. 2.12–2.14 and 2.33) (37, 43).
The P1 segment, also called the precommunicating segment, extends from the basilar bifurcation to the junction with the PComA. A fetal configuration, in which the P1 has a smaller diameter than the PComA and the PCA arises predominantly
from the carotid artery, occurs in approximately one-third of hemispheres. A normal configuration, in which the P1 segment is larger than the PComA, is found in nearly two-thirds of hemispheres. A few hemispheres will have a PComA and P1 of the same diameter. A fetal configuration may be present on both sides.
P1 length varies, being longer if there is a fetal pattern. Average P1 length, which ranges from 3 to 14 mm, is approximately 9.0 mm in the group with a fetal configuration as compared with 7.0 mm in a normal pattern (37). The oculomotor nerve passes below and slightly lateral to the PComA if a normal configuration is present; but if a fetal pattern is present, P1 is longer and the nerve courses beneath or medial to the communicating artery.
The relatively constant branches arising from the P1 are 1) the thalamoperforating artery, which by definition enters the brain through the posterior perforated substance; 2) the medial posterior choroidal artery directed to the choroid plexus in the third ventricle and lateral ventricle; 3) the branch to the quadrigeminal plate; and 4) rami to the cerebral peduncle and mesencephalic tegmentum. The superior cerebellar artery (SCA) arises from the basilar artery at a level between the P1 origin and 7 mm below (average, 2.5 mm) (37). The SCA may infrequently have a common origin with the P1 or arise from P1. The initial segment gives rise to perforating vessels whose termination may overlap with those arising from the basilar apex and P1.
The P2 segment begins at the PComA, lies within the crural and ambient cisterns, and terminates lateral to the posterior edge of the midbrain. The P2 is divided into an anterior and posterior part because the surgical approaches to the anterior and posterior halves of this segment often differ, and because it is helpful in identifying the origin of the many branches that arise from P2. The anterior part is designated the P2A or crural or peduncular segment because it courses around the cerebral peduncle in the crural cistern. The posterior part is designated the P2P or the ambient or lateral mesencephalic segment because it courses lateral to the midbrain in the ambient cistern. Both segments are approximately 25 mm long. The P2A begins at the PComA and courses between the cerebral peduncle and uncus that forms the medial and lateral walls of the crural cistern, and inferior to the optic tract and basal vein that crosses the roof of the cistern, to enter the proximal portion of the ambient cistern. The P2P commences at the posterior edge of the cerebral peduncle at the junction of the crural and ambient cisterns. It courses between the lateral midbrain and the parahippocampal and dentate gyri, which form the medial and lateral walls of the ambient cistern, below the optic tract, basal vein, and geniculate bodies and the inferolateral part of the pulvinar in the roof of the cistern, and superomedial to the trochlear nerve and tentorial edge.
The P3 or quadrigeminal segment proceeds posteriorly from the posterior edge of the lateral surface of the midbrain and ambient cistern to reach the lateral part of the quadrigeminal cistern and ends at the anterior limit of the calcarine fissure. The PCA often divides into its major terminal branches, the calcarine and parieto-occipital arteries, before reaching the anterior limit of the calcarine fissure. The average length of the P3 segment is 2 cm. The P3s from both sides approach each other posterior to the colliculi. The point where the PCAs from each side are nearest is referred to as the collicular or quadrigeminal point. The separation averages 8.9mm(range, 3.5–17 mm) (43). The artery forming the collicular point is the PCA trunk in approximately half of the hemispheres, and in the other half, in which the PCA bifurcates into its terminal branches before reaching the collicular point, it is formed by the calcarine artery.
The P4 segment includes the branches distributed to the cortical surface. Posteriorly, it begins at the anterior end of the calcarine sulcus.
The PCA gives rise to three types of branches: 1) central perforating branches to the diencephalon and midbrain; 2) ventricular branches to the choroid plexus and walls of the lateral and third ventricles and adjacent structures; and 3) cerebral branches to the cerebral cortex and splenium of the corpus callosum (Fig. 2.33). The central branches include the direct and circumflex perforating arteries, including the thalamoperforating, peduncular perforating, and thalamogeniculate arteries. The ventricular branches are the lateral and medial posterior choroidal arteries. The cerebral branches include the inferior temporal group of branches, which are divided into hippocampal and the anterior, middle, posterior, and common temporal branches, plus the parieto-occipital, calcarine, and splenial branches.
The long and short circumflex and thalamoperforating arteries arise predominantly from P1, and the other PCA branches most frequently arise from P2 or P3. The hippocampal, anterior temporal, peduncular perforating, and medial posterior choroidal arteries most frequently arise from P2A. The middle temporal, posterior temporal, common temporal, and lateral posterior choroidal arteries most frequently arise from P2P. The thalamogeniculate arteries arise only slightly more frequently from P2P than from P2A. The calcarine and parieto-occipital arteries most frequently arise from P3.
The central branches are divided into two groups: direct perforating and circumflex arteries (Figs. 2.34 and 2.35). The direct perforating branches pass directly from the parent trunk to the brainstem. This group includes the thalamoperforating
arteries that arise from P1 and the thalamogeniculate and peduncular perforating arteries that arise from P2. The circumflex branches encircle the brainstem for a variable distance before entering the diencephalon and mesencephalon are divided into long and short groups, depending on how far they course around the brainstem.
An average of four, but as many as a dozen perforating branches, the largest of which may have a diameter of 1.5 mm, arise mainly from the superior and posterior surfaces of the P1, course superiorly and posteriorly, and divide into numerous branches that terminate in the interpeduncular fossa, posterior perforated substance, cerebral peduncle, mamillary bodies, and posterior midbrain. Perforating branches rarely arise from the anterior side of the basilar apex, but they arise from the anterior surface in a third of P1s, and terminate in the posterior perforated substance and mamillary bodies. The largest P1 branch is a thalamoperforating artery (42% of hemispheres), a posterior choroidal artery (40%), or a large trunk from which both arteries arise (18%) (37). The P1s with the larger branches tend to have few perforating branches. P1s having only one or two P1 perforators tend to have larger branches. If the largest P1 branch is relatively small, there will be more P1 branches. More perforating vessels arise on P1 lateral to the largest perforator than medial to it.
The posterior and lateral surfaces of the upper centimeter of the basilar artery is also a rich source of perforating arteries that overlap with those arising from the P1. An average of 8 (range, 3–18) branches arise from the upper centimeter; approximately half arise from the posterior surface and a quarter from each side (37). The more medial branches, called median or paramedian branches, enter the midbrain and pons near the midline, and the lateral ones, called transverse or circumferential branches, terminate in the lateral pons, peduncle, and posterior perforated substance.
The thalamoperforating arteries arise on the P1 and enter the brain by passing through the posterior perforated substance and the medial part of the cerebral peduncles in the area behind the mamillary bodies in the upper part of the interpeduncular fossa (Fig. 2.35). The branches of the PComA that enter the same area are referred to as premamillary arteries. The majority of thalamoperforating arteries originate on the middle third of P1 as the P1 branch nearest the basilar bifurcation, but they may also arise on the medial or lateral third. If the first branch is not a thalamoperforating artery, it is a circumflex branch that terminates in the peduncle or posterior mesencephalic area. The thalamoperforating artery is the largest P1 branch in most cases (37). They almost always arise from the posterior or superior aspect of P1 and only infrequently from the anterior surface. A P1, even when of normal or large size, may infrequently not give rise to a thalamoperforating artery, in which case, the contralateral side will have well-developed thalamoperforating branches that supply the area normally perfused by the absent thalamoperforating artery. They supply the anterior and part of the posterior thalamus and hypothalamus, the subthalamus and the medial part of the upper midbrain, including the substantia nigra, red nucleus, oculomotor and trochlear nuclei, oculomotor nerve, mesencephalic reticular formation, pretectum, rostromedial floor of the fourth ventricle, and the posterior portion of the internal capsule (39, pp 96–99; 43).
Deficits related to the loss of these arteries include somatesthetic disturbances caused by involvement of the afferent pathways in the medial lemniscus or thalamus; motor weakness caused by involvement of the corticospinal tracts in the internal capsule or peduncle; memory deficits caused by involvement of hypothalamic pathways entering and exiting from the mamillary bodies; autonomic imbalance caused by disturbance of sympathetic and parasympathetic centers in the anterior and posterior diencephalon; diplopia caused by involvement of the extraocular nerves or nuclei in the midbrain; alterations of consciousness caused by ischemia of the midbrain reticular formation; abnormal movements caused by involvement of cerebellothalamic circuits in the midbrain and thalamus; and endocrine disturbances caused by involvement of the hypothalamic-pituitary axis Occlusion of the thalamoperforating arteries, depending on the size of the area of ischemia, may produce a variety of more focal syndromes including contralateral hemiplegia, cerebellar ataxia, or a “rubral” tremor associated with ipsilateral oculomotor nerve paresis (Nothnagel’s syndrome). If the lesion affects the subthalamus, it may produce contralateral hemiballismus, which abates into choreiform movements with time or treatment (43).
Peduncular Perforating Arteries
The peduncular perforating branches, usually two or three, but as many as six, arise from the P2 segment and pass directly from the PCA into the cerebral peduncle. They supply the corticospinal and corticobulbar pathways as well as the substantia nigra, red nucleus, and other structures of the tegmentum, and may send branches to the oculomotor nerve.
The circumflex groups of arteries arise from the P1 and P2 and encircle the midbrain parallel and medial to the PCA. They are divided into a short and long circumflex group. The short circumflex branches reach only as far as the geniculate bodies. The long circumflex branches reach the colliculi. The short circumflex arteries course medial to the P2 and the medial posterior choroidal and the long circumflex arteries, and send branches to the cerebral peduncle as they proceed to their distal termination, which may range from the posterolateral border of the peduncle to the medial geniculate bodies. Those arising from P2 supply only the geniculate bodies and the midbrain tegmentum. The short circumflex arteries may send rami to the area of the interpeduncular fossa and posterior perforated substance, which are supplied predominantly by the thalamoperforating arteries (37).
The long circumflex arteries, referred to as the quadrigeminal arteries, are present in almost all hemispheres, pass around the brainstem to reach the quadrigeminal cistern, and supply the quadrigeminal bodies. They encircle the midbrain medial to the PCA and send small rami to the cerebral peduncle and geniculate bodies and occasionally to the tegmentum, pulvinar, and end at the quadrigeminal plate. They usually arise from the P1 or P2A. The terminal branches of the long circumflex form a rich arterial network over the colliculi, where they anastomose with branches from the superior cerebellar artery. The superior colliculus is supplied by the branches arising from the PCA and the inferior colliculus is supplied by branches of the superior cerebellar artery. Occlusion of the long circumflex (quadrigeminal) artery may result in defects of vertical gauge caused by infarction of the posterior commissure or of the nuclei of Darkschewitsch or Cajal (Parinaud’s syndrome) (40).
The thalamogeniculate arteries arise directly from the P2 beneath the lateral thalamus and penetrate the part of the roof of the ambient cistern formed by the geniculate bodies and surrounding area. The PCA most commonly gives origin to two or three thalamogeniculate arteries, but there may be as many as seven. They arise near the junction of the crural (P2A) and ambient (P2P) segments, with a nearly equal number arising from each segment.
The thalamogeniculate arteries supply the posterior half of the lateral thalamus, posterior limb of the internal capsule, and the optic tract (39, pp 96–99). They meet the thalamoperforating branches of P1 near the middle of the thalamus and the thalamic branches of the PComA anteriorly in the lateral nucleus. The long and short circumflex and medial posterior choroidal arteries also send branches to this area as they encircle the brainstem, but the term thalamogeniculate arteries is reserved for those branches arising from the P2 and passing through the geniculate bodies and adjacent part of the roof of the ambient cistern.
Infarction of the area supplied by the thalamogeniculate arteries results in the thalamic syndrome of Dejerine and Roussy, consisting of a contralateral loss of superficial and particularly of deep sensation with an intense, intractable, hyperpathic pain on the affected side, with extreme hypersensitivity to mild touch, pain, and temperature stimuli, a contralateral hemiplegia, often transient and sometimes associated with choreoathetoid or dystonic movements of the paralyzed side, with possibly a homonymous hemianopsia (7, 22). There is usually a permanent disturbance of deep sensibility (position sense, heavy contact, and deep pressure) and, although the threshold to cutaneous stimuli is elevated, a threshold stimulus evokes a disagreeable burning, agonizing type of pain response, and there may be spontaneous pain. The limbs are affected more than the face.
In one such case reported in 1906, Dejerine and Roussy (7) found infarction in the posterior third of the lateral thalamic nucleus, part of the medial and centromedian nuclei and the pulvinar, the posterior limb of the internal capsule, and posterior part of the lentiform nucleus, but they did not find an occlusion of any PCA branch. The fact that the area is supplied not only by multiple thalamogeniculate arteries, but also by the circumflex and choroidal branches of the PCA, makes it unlikely that occlusion of a single thalamogeniculate artery would produce the complete syndrome. It would more likely be caused by a PCA occlusion proximal to the origin of all of these branches. Arterial occlusion is the most common cause of a typical thalamic syndrome, although vascular malformations or tumors of the thalamus may be a cause (43).
Ventricular and Choroid Plexus Branches
The posterior choroidal arteries, the branches of the PCA that enter the lateral and third ventricles to supply the choroid plexus and ventricular walls, are divided into medial and lateral groups referred to as the medial posterior (MPChA) and lateral posterior choroidal arteries (LPChA), depending on the origin and area of supply (Figs. 2.12, 2.13, and 2.33) (13). The MPChAs most frequently arise from the posteromedial aspect of the proximal half of the PCA or one of its branches, encircle the midbrain medial to the main trunk of the PCA, turn forward at the lateral side of the pineal gland to enter the roof of the third ventricle between the thalami, and finally course through the choroidal fissure and foramen of Monro to enter the choroid plexus in the lateral ventricle. The MPChAs send branches along their course to the peduncle, tegmentum, geniculate bodies (medial and lateral, but primarily the former), the colliculi, pulvinar, pineal gland, and medial thalamus.
Most hemispheres have a single MPChA, but there may be as many as three (43). Most arise in the P2, but they may arise from the P3 or from the parieto-occipital and calcarine branches. Those MPChAs arising from the parieto-occipital and calcarine arteries and the distal PCA course in a retrograde fashion from their origin to enter the roof of the third ventricle.
The LPChAs arise from the PCA or its branches and pass laterally through the choroidal fissure to supply the choroid plexus of the lateral ventricle. The number of LPChAs in one hemisphere ranges from one to nine (average, four) (13). They most commonly arise directly from the P2P, but may also arise from the P2A or P3, or from some of the PCA branches. The largest LPChAs arise directly from the P2P in the ambient cistern, pass laterally through the choroidal fissure to the choroid plexus of the temporal horn and the glomus of the plexus in the atrium, and anastomose on the choroid plexus within the branches of the AChA and MPChA. The LPChAs may send branches to the cerebral peduncle, posterior commissure, part of the crura and body of the fornix, the lateral geniculate body, pulvinar, dorsomedial thalamic nucleus, and the body of the caudate nucleus (13, 43).
The cortical branches of the PCA are the inferior temporal, parieto-occipital, calcarine, and splenial branches (Figs. 2.36 and 2.37).
Inferior Temporal Arteries
The inferior temporal group of arteries arises from the PCA and the superior temporal arteries arise from the MCA. The inferior temporal arteries include the hippocampal and the anterior, middle, posterior, and common temporal arteries. These arteries supply the inferior parts of the temporal lobe. Branches of the inferior temporal arteries pass around the lower margin of the hemisphere to gain access to the lateral cerebral surface, reaching the middle temporal gyrus in 42% of hemispheres (43). They also give rise to some LPChAs. The inferior temporal arteries are divided into five groups based on the branches present and the area they supply:
- Group 1. All of the inferior temporal branches (hippocampal and anterior, middle, and posterior temporal arteries) are present (10% of hemispheres).
- Group 2. A single large trunk, the common temporal artery, arises from the PCA and branches to supply the entire inferior temporal lobe (16%).
- Group 3. Anterior, middle, and posterior temporal branches are present, but no hippocampal artery is present (20%).
- Group 4. Anterior and posterior temporal branches are present, but no hippocampal or middle temporal arteries are present (10%).
- Group 5. Hippocampal and anterior and posterior temporal branches are present, but no middle temporal artery is present. This is the most frequent pattern, present in 44% of hemispheres (43).
The hippocampal artery, if present, arises in the crural or ambient cistern and is the first cortical branch of the PCA. It
supplies the uncus, anterior parahippocampal gyrus, hippocampal formation, and the dentate gyrus. A small branch may extend to the lateral surface of the temporal lobe and forward to the temporal tip. If the first cortical branch supplies a significant portion of the inferior temporal lobe in addition to the hippocampal gyrus, the branch is classified as an anterior temporal artery. Bilateral occlusion of the vessels to the medial temporal area supplied by the hippocampal artery may cause a severe memory loss and a deficit resembling Korsakoff’s syndrome (43).
Anterior Temporal Artery
The anterior temporal artery is usually the second cortical PCA branch. It is the first branch if there is no hippocampal artery. It usually arises in the proximal part of the ambient cistern and supplies the anteroinferior surface of the temporal lobe, occasionally reaching a portion of the temporal pole and the lateral cerebral surface in the region of the middle temporal sulcus and gyrus.
Middle Temporal Artery
This artery arises in the crural and ambient cisterns and supplies the inferior surface of the temporal lobe. It is the smallest, is frequently absent, and has the fewest branches of the inferior temporal arteries.
Posterior Temporal Artery
This artery, present in almost all hemispheres, arises from the inferior or lateral aspect of the PCA, most commonly in the ambient, but occasionally in the crural or quadrigeminal cisterns, and runs obliquely posterolateral toward the occipital pole to supply the inferior temporal and occipital surfaces, including the occipital pole and lingual gyrus. It has the largest trunk diameter and number of branches of any temporal artery except a common temporal artery from which all the temporal branches arise. Deficits after occlusions of the posterior temporal artery include dysphasia, which has usually been mild and transient, an amnestic syndrome, usually transient with homonymous hemianopsia, but without hemiparesis or sensory loss and inability to match colors to their names (21).
Common Temporal Artery
The common temporal artery, seen in slightly fewer than 20% of hemispheres, arises in the crural or ambient cisterns as a single PCA branch that supplies the majority of the inferior surface of the temporal and occipital lobes.
The parieto-occipital artery, one of the two terminal branches of the PCA, is present in almost all hemispheres. It consistently arises as a single branch and runs in the parieto-occipital fissure to supply the posterior parasagittal region, cuneus, precuneus, lateral occipital gyrus, and, rarely, the precentral and superior parietal lobules. It arises in the ambient or quadrigeminal cisterns. The arteries with a more proximal origin tend to be larger and donate branches to the midbrain, thalamus, pulvinar, and lateral geniculate bodies as they pass posteriorly within the hippocampal fissure. Those arteries with a proximal origin also send branches through the choroidal fissure to the choroid plexus in the lateral ventricle. This artery occasionally sends branches to the third ventricle in the area supplied by the MPChA or to the splenium of the corpus callosum.
The calcarine artery, a terminal PCA branch, is present in all hemispheres. It courses within the calcarine fissure to reach the occipital pole, and has branches that fan out to the lingual gyrus and the inferior cuneus. It usually arises directly from the PCA in the ambient or quadrigeminal cisterns, but occasionally is a branch of the parieto-occipital artery. The calcarine artery supplies the visual cortex, and the hallmark of an occlusion of this vessel is a homonymous visual field defect, usually with macular sparing. Occlusion may be associated with pain in the ipsilateral eye. Bilateral occipital lobe infarction may result in blindness with preserved pupillary reflexes or in Anton’s syndrome, in which there is cortical blindness, confabulation, denial of blindness, and preservation of the pupillary reaction to light. The visual field may recover after ligation or occlusion of the calcarine artery (19).
The PCA, or its branches, gives rise to branches supplying the splenium of the corpus callosum in all hemispheres. They may arise from the following arteries: parieto-occipital, calcarine, medial posterior choroidal, posterior temporal, and lateral posterior choroidal. The splenial arteries anastomose with branches of the pericallosal artery a few centimeters anterior to the posterior tip of the splenium as previously noted. Retrograde filling of this artery through the pericallosal artery suggests occlusion of the PCA proximal to the origin of the splenial artery. Infarction of the dominant occipital pole (producing a hemianopsia) plus the splenium of the corpus callosum in the distribution of the splenial artery interrupts the fibers between the intact occipital pole and contralateral angular gyrus, resulting in the syndrome of dyslexia without dysgraphia (43).
Lateral Convexity Branches
All the cortical branches of the PCA may send branches to the lateral surface of the hemisphere, but of the seven cortical arteries, the posterior temporal artery is the most common site of origin of lateral cortical branches. The next most common source is the parieto-occipital artery. If a revascularization procedure using microvascular anastomoses between the superficial temporal or occipital arteries and a cortical branch of the PCA were undertaken, the area supplied by the posterior temporal artery would show the most promise of revealing a vessel of sufficient caliber to be used as a recipient, there being a higher than 75% chance of finding a vessel of sufficient size within this area (43). This corresponds with the region immediately anterior to the preoccipital notch. The majority of the cortical branches of the PCA are 0.4 to 0.6 mm in diameter when they pass around the margin to the lateral cerebral surface.
IIIrd and IVth Cranial Nerves
The relationship between the oculomotor and trochlear nerves and the PCA and SCA is constant (Figs. 2.1 and 2.3) (32). The oculomotor nerve consistently passes between the PCA and SCA near their origin, and the trochlear nerve passes between the two on the lateral margin of the brainstem. The relationship is unaltered even when the superior cerebellar origin is duplicated. When the SCA arises as duplicate trunks, the nerves pass between the superior trunk of the SCA and the PCA. The PCA consistently courses above the trochlear. A tortuous SCA may occasionally loop above a trochlear nerve.
The PCA, more than any other intracranial vessel, subserves the function of vision. It supports a long list of ocular functions that include papillary reflexes, eye movement, visual memory, intrahemispheric transfer of visual information, binocular and visual spatial integration through its supply to the optic tracts, geniculate bodies, colliculi, extraocular nerves and their nuclei, the geniculocalcarine tracts, and the striate and peristriate cortex. The dysfunction caused by occlusion of the
individual PCA branches has been reviewed in the subsection related to those branches. Occlusion of various branches may also lead to somesthetic disturbances caused by involvement of afferent pathways in the medial lemniscus or thalamus, motor weakness caused by involvement of the corticospinal tracts in the internal capsule or peduncle, memory deficits caused by involvement of the hypothalamic pathways entering and exiting the mamillary bodies, autonomic imbalance caused by disturbances of the sympathetic and parasympathetic pathways in the anterior and posterior diencephalon, alterations of consciousness caused by ischemia of the midbrain reticular formation, abnormal movements caused by involvement of cerebellothalamic circuits in the midbrain and thalamus, and endocrine disturbances caused by involvement of the hypothalamic pituitary axis.
Vascular complications in pituitary surgery result mainly from carotid artery injury and circulatory embarrassment after occlusion of the carotid artery. Occlusion of the perforating branches of the posterior circle is commonly neglected in discussions regarding complications in pituitary surgery. The arterial branches reviewed in this study, which would be stretched around the margin of suprasellar tumors, have the potential, when occluded, to cause personality disorders, memory disturbances, extraocular palsies, visual loss, and altered states of consciousness (12, 34). The branches stretched around pituitary tumors are discussed further in Chapter 8.
Contributor: Albert L. Rhoton, Jr., MD
Content from Rhoton AL. The Supratentorial Cranial Space: Microsurgical Anatomy and Surgical Approaches. Neurosurgery 51(1), 2002, 10.1097/00006123-200210001-00001. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton Jr., MD
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