Last Updated: August 23, 2020
In 1979, the author introduced three rules related to the anatomy of saccular aneurysms that should be considered when planning the operative approach to these lesions (18). These three aspects of anatomy are reviewed in this chapter in relation to each of the common aneurysm sites. First, these aneurysms arise at a branching site on the parent artery. This site may be formed either by the origin of a side branch from the parent artery, such as the origin of the posterior communicating artery from the internal carotid artery, or by subdivision of a main arterial trunk into two trunks, as occurs at the bifurcation of the middle cerebral or basilar arteries (Figs. 3.1 and 3.2). Second, saccular aneurysms arise at a turn or curve in the artery. These curves, by producing local alterations in intravascular hemodynamics, exert unusual stresses on apical regions that receive the greatest force of the pulse wave. Saccular aneurysms arise on the convex, not concave, side of the curve. Third, saccular aneurysms point in the direction that the blood would have gone if the curve at the aneurysm site were not present. The aneurysm dome or fundus points in the direction of the maximal hemodynamic thrust in the preaneurysmal segment of the parent artery. Since the original introduction of the three rules, our anatomic studies have revealed a fourth rule. The fourth rule is that there is a constantly occurring set of perforating arteries situated at each aneurysm site that need to be protected and preserved to achieve an optimal result (12, 13, 18).
Aneurysms are infrequently encountered on a straight, nonbranching segment of an intracranial artery. The aneurysms occurring on straight, nonbranching segments are more often found to have sacs that point longitudinally along the wall of the artery in the direction of blood flow and to project only minimally above the adventitial surface. Aneurysms having these characteristics are of a dissecting type, rather than of the congenital saccular type, and their development is heralded more frequently by the onset of ischemic neurological deficits than by the subarachnoid hemorrhage associated with congenital saccular aneurysms. It is rare to find an aneurysm on the concave side of an arterial curve or to find one that points in a direction opposite to that of the flow in the parent artery.
Internal Carotid Artery Aneurysms
These four facets of anatomy, as they apply to aneurysm sites on the supraclinoid portion of the internal carotid artery, are considered first (Figs. 3.1–3.4). If all sites on the supraclinoid portion of the internal carotid artery (C4) are included, it is the most common site of intracranial aneurysms, accounting for approximately 35% of intracranial aneurysms (8). These aneurysms arise at five sites: the upper surface of the internal carotid artery at the origin of the ophthalmic artery, the medial wall at the origin of the superior hypophyseal artery, the posterior wall at the origin of the posterior communicating artery, the posterior wall at the origin of the anterior choroidal artery, and the apex of the carotid artery bifurcation into the anterior and middle cerebral arteries.
The intradural exposure of the supraclinoid carotid is along the sphenoid ridge or orbital roof to the anterior clinoid process and from proximal to distal (Figs. 3.3 and 3.4). Both the internal carotid artery and the optic nerve are medial to the anterior clinoid process. The artery exits the cavernous sinus on the medial side of the anterior clinoid process, beneath and slightly lateral to the optic nerve. It courses posterior, superior, and slightly lateral to reach the lateral side of the optic chiasm, where it turns forward to complete the upper half of the S-shaped curve of the carotid siphon. It bifurcates in the area below the anterior perforated substances to give rise to the anterior and middle cerebral arteries.
The supraclinoid portion of the internal carotid artery is divided into three segments on the basis of the site of origin of the ophthalmic, posterior communicating, and anterior choroidal arteries (Figs. 2.4 and 3.5). The ophthalmic segment extends from the origin of the ophthalmic artery at the roof of the cavernous sinus to the origin of the posterior communicating artery; the
communicating segment extends from the origin of the posterior communicating artery to the origin of the anterior choroidal artery; and the choroidal segment extends from the origin of the anterior choroidal artery to the terminal bifurcation of the internal carotid artery. The ophthalmic segment is the longest and the communicating segment the shortest. Each internal carotid artery gives off from 3 to 16 (average, 8.2) perforating branches with a relatively constant origin and termination (3). The relationships of the perforating branches to each of the common aneurysm sites are reviewed below.
Aneurysms arising at the carotid-ophthalmic artery junction commonly arise from the superior wall of the carotid artery at the distal edge of the origin of the ophthalmic artery at or above the roof of the cavernous sinus, where the superiorly directed intracavernous segment turns posteriorly (Figs. 3.2, 3.3, 3.5, and 3.6). At this turn, the maximal hemodynamic thrust is directed toward the superior wall of the carotid artery just distal to the ophthalmic artery, and the aneurysm projects upward toward the optic nerve.
The origin of the ophthalmic artery is difficult to expose because of its short intradural length and its location under the optic nerve (Fig. 3.6). It arises from the carotid artery below the optic nerve and reaches the orbit by one of three routes. It usually passes through the optic canal to enter the orbit. In a few cases it will arise in the cavernous sinus and enter the orbit through the superior orbital fissure (5). The least common course is for it to penetrate a foramen in the bony strut that separates the optic foramen and the superior orbital fissure, or to arise from the middle meningeal artery (7).
Aneurysms arising in the region of the origin of the ophthalmic artery and the anterior clinoid process are among the most complicated aneurysms because of the variable origin and course of the ophthalmic artery and the involvement of the dural folds in the region of the optic foramen and clinoid process (Fig. 3.6, A–C). Ophthalmic aneurysms are relatively uncomplicated if they arise above the cranial base; however, their complexity increases as they get closer to and involve the segment of the internal carotid artery, referred to as the clinoid segment, exposed by removing the anterior clinoid process (Figs. 3.4 and 3.7) (5). The clinoid segment and its exposure is discussed in Chapter 9 of this issue. The clinoid segment is located at the junction of the intracavernous and subarachnoid segments of the artery, between the dural folds coming off the upper and lower margins of the anterior clinoid process. The dura that extends medially from the top of the anterior clinoid process forms the upper dural ring around the carotid artery. The dura that extends medially from the lower margin of the anterior clinoid surrounds the artery to form the lower dural ring, which marks the lower margin of the clinoid segment. The layer that extends medially to form the lower dural ring separates the lower margin of the clinoid process from the upper surface of the oculomotor nerve. The upper ring forms a tight collar around the artery, but inspection under the operating microscope reveals that there is often a narrow depression in the dura at the site at which the ring hugs the anteromedial aspect of the artery, called the carotid cave. The cave, the short downward pouching, extends a variable distance below the level of the upper dural ring (Fig. 3.6, A and B) and is most prominent on the anteromedial side of the artery, where it may extend down to near the lower ring. The cave seems to become less prominent as the arteries elongate with advancing age. Carotid cave aneurysms are distinct from clinoid segment aneurysms, which arise from the clinoid segment of the internal carotid artery located between the upper and lower dural ring. Aneurysms that arise from the clinoid segment of the internal carotid artery have been referred to as clinoid segment aneurysms, and those located above the upper ring, but extending into the cave adjacent the upper ring, are referred to as carotid cave aneurysms.
The anatomy of ophthalmic aneurysms varies depending on the site of origin and course of the ophthalmic artery and whether the aneurysm involves the clinoid segment or the carotid cave. If the aneurysm arises on the upper surface of the carotid artery above the upper ring, it will project upward into the optic nerve and involve neither the cave nor the clinoid segment (Fig. 3.6, D and E). If the ophthalmic artery has an even longer subarachnoid segment and arises distal to the upper ring along the superomedial side of the carotid artery, the aneurysm may project medially under the optic nerve in the anterior presellar area and mimic an anteriorly situated superior hypophyseal aneurysm, although it arises at the origin of the ophthalmic artery (Fig. 3.6, F and G). If the aneurysm arises in the carotid cave, the fundus will extend upward out of the carotid cave on the anteromedial aspect of the carotid artery (Fig. 3.6, H and I). The ophthalmic artery also may arise further proximally on the carotid artery and pass through an anomalous foramen in the optic strut, the bridge of bone that separates the lateral margin of the optic canal from the medial edge of the superior orbital fissure, to reach the orbit, rather than passing through the optic canal (Fig. 3.6, J and K). This anomalous foramen in the optic strut is called the ophthalmic foramen (Fig. 7.3L). Aneurysms arising at the origin of an ophthalmic artery that passes through the optic strut have their neck along the anterior or lateral part of the clinoid segment or carotid cave and project upward out of the cave into the subarachnoid space. The fifth variant of the ophthalmic aneurysm is one that is associated with an ophthalmic artery that arises within the cavernous sinus and passes through the superior orbital fissure to reach the orbit (Fig. 3.6, L and M). This aneurysm will point upward, but almost immediately encounters the lower margin of the anterior clinoid process and cannot break into the subarachnoid space.
The ophthalmic artery usually arises from the medial third of the superior surface of the carotid in the area below the optic nerve (Figs. 3.4 and 3.6C). Gentle elevation of the optic nerve away from the internal carotid artery is often required to see the preforaminal segment. The ophthalmic artery, after exiting the carotid, may immediately enter the optic canal, but in most cases, there is a 2- to 5-mm preforaminal segment. Exposure of the neck of this aneurysm may be facilitated by the removal of the anterior clinoid process and adjacent part of the lesser sphenoid wing, by removing the roof of the optic foramen and adjacent part of the orbital roof to allow some mobilization of the optic nerve, and by incision of the falciform process, a thin fold of dura mater that extends medially from the anterior clinoid process to the tuberculum sellae and covers the segment of the optic nerve immediately proximal to the optic foramen. It is helpful to divide the upper and sometimes the lower dural ring to mobilize the carotid artery for clipping aneurysms. Most ophthalmic arteries arise anterior to the tip of the anterior clinoid process, approximately 5 mm medial to the clinoid process (3).
The perforating arteries arising from the ophthalmic segment take origin from posterior or medial aspects of the internal carotid artery and are distributed to the stalk of the pituitary gland, the optic nerve, chiasm, and tracts and floor of the third ventricle around the infundibulum (Fig. 3.5). Ophthalmic aneurysms typically arise on the upper anterior wall of the carotid artery, not on the side from which the perforating arteries arise, and point upward away from the perforating branches arising from the ophthalmic segment. The risk of damaging the adjacent perforating branches is less in clipping an ophthalmic aneurysm than at other sites on the internal carotid artery because ophthalmic aneurysms typically point upward, away from these perforating branches.
Carotid-Superior Hypophyseal Aneurysms
The segment of the carotid artery just distal to the origin of the ophthalmic artery, and from which the superior hypophyseal artery arises, has a medially convex curve in the area lateral to the pituitary stalk (Figs. 3.2, 3.3, 3.5, and 3.6N). It is on this medially convex curve that the superior hypophyseal aneurysm arises. The aneurysm arises at the distal edge of the origin of the superior hypophyseal artery and points medially into the area between the lower surface of the optic chiasm and the diaphragma sellae. The aneurysms are often confused, on lateral angiograms, with intracavernous aneurysms, because they frequently project below the level of the anterior clinoid process, although they are located in the subarachnoid space below the optic chiasm. The superior hypophyseal artery and the ophthalmic segment perforating branches described above are stretched around the neck of this aneurysm.
The superior hypophyseal arteries are small branches, usually two, that arise from the medial or posterior aspect of the ophthalmic segment (Figs. 2.4, 3.2. and 3.5, and 8.1) (3). One branch often predominates. These arteries pass medially to reach the floor of the third ventricle, optic nerves, and the chiasm and pituitary stalk. The perforating arteries and the hypophyseal vascular supply may be compromised if the aneurysm expands medially. Diabetes insipidus and amenorrhea may result from occlusion of these branches. Removing the anterior clinoid process and adjacent part of the roof of the optic canal and orbital roof is often helpful in exposing the neck of the superior hypophyseal aneurysms. In some cases, especially in older individuals, the ophthalmic artery and supraclinoid portion of the internal carotid artery may elongate, thus placing the neck of the ophthalmic aneurysm further posteriorly so that it mimics the position and medial projection under the optic chiasm of the superior hypophyseal aneurysm.
Carotid-Posterior Communicating Aneurysms
The initial segment of the supraclinoid carotid is directed posteriorly, but the segment after the origin of the superior hypophyseal artery turns upward toward the anterior perforated substance to form a curve that is convex posteriorly (Figs. 3.2, 3.3, 3.5, and 3.8). The posterior communicating and anterior choroidal arteries arise from the posterior wall on this convex curve as the carotid artery passes upward toward its bifurcation. The most common carotid aneurysm arises at the carotid-posterior communicating artery junction. These aneurysms arise from the posterior wall of the carotid artery near the apex of this turn, immediately above the distal edge of the origin of the posterior communicating artery. Another important relationship in this area is that of the oculomotor nerve to the internal carotid artery. The oculomotor nerve enters the dura lateral to the posterior clinoid process and medial to the dural band passing from the tentorium cerebelli toward the anterior clinoid process. The oculomotor nerve pierces the dura between 2 and 7 mm (average, 5 mm) posterior to the initial supraclinoid segment. Aneurysms arising at the origin of the posterior communicating artery point downward and backward and may compress the oculomotor nerve at its entrance into the dural roof of the cavernous sinus when they reach 4 to 5 mm in diameter.
The posterior communicating artery is usually found inferomedial and the anterior choroidal artery superior or superolateral to the neck of the aneurysm (Figs. 3.4, 3.7, and 3.8). In exposing the carotid artery beyond the origin of the ophthalmic artery, the surgeon often sees the anterior choroidal artery before the posterior communicating artery, although the anterior choroidal artery arises distal to the posterior communicating artery. This occurs because of three sets of anatomic circumstances. First, the supraclinoidal segment of the internal carotid artery passes upward in a posterolateral direction, placing the origin of the more distally arising branch, the anterior choroidal artery, further lateral to the midline than the origin of the posterior communicating artery, which arises more proximally. Second, the anterior choroidal artery arises further laterally on the posterior wall of the carotid than the posterior communicating artery. Third, the anterior choroidal artery pursues a more lateral course than the posterior communicating artery; the former passes laterally below the optic tract, around the cerebral peduncle, and into the temporal horn, whereas the latter is directed in a posteromedial direction above and medial to the oculomotor nerve toward the interpeduncular fossa. Care should be taken to preserve both the posterior communicating artery and the anterior choroidal artery at the time of obliteration of internal carotid artery aneurysms. Occlusion of either of these arteries may cause a hemiplegia, homonymous hemianopsia, and reduced levels of consciousness.
The posterior communicating artery, which forms the lateral boundary of the circle of Willis, arises from the posteromedial surface of the internal carotid artery and sweeps backward above the sella turcica and above and medial to the oculomotor nerve to join the posterior cerebral artery (Figs. 3.4, 3.7, and 3.8). If the posterior communicating artery remains the major origin of the posterior cerebral artery, the configuration is termed fetal. If the posterior communicating artery is of small or normal size, it courses posteromedially to join the posterior cerebral artery medial to the oculomotor nerve, but if it is of a fetal type, it courses posterolaterally above or above and lateral to the oculomotor nerve.
Fewer perforating branches arise from the communicating segment of the carotid artery than from the ophthalmic or choroidal segments (Fig. 3.5) (3). However, they are of critical importance because some of them may be larger than either the anterior choroidal or the posterior communicating arteries, especially if the latter artery is hypoplastic. These branches arise from the posterior half of the arterial wall at the same site as the neck of the aneurysm and are often stretched around the neck of the aneurysm. These branches terminate in the optic chiasm and tract, floor of the third ventricle, infundibulum, the posterior perforated substance, and medial temporal lobe.
Carotid-Anterior Choroidal Aneurysms
The apex of the posteriorly convex curve of the supraclinoid carotid may also be located at the level of the origin of the anterior choroidal artery, which shifts the hemodynamic force distally from the level of origin of the posterior communicating artery to that of the anterior choroidal artery (Figs. 3.2, 3.3, and 3.5). An aneurysm arising at the level of the anterior choroidal artery is usually located just distal, superior, or superolateral to the origin of the anterior choroidal artery. They point posteriorly or posterolaterally, usually well above the oculomotor nerve. In opening the sylvian fissure, the origin and proximal portion of the anterior choroidal artery is often exposed before the posterior communicating artery, because of its more lateral origin and course.
The anterior choroidal artery arises from the posterolateral aspect of the carotid artery (Figs. 3.4, 3.7, and 3.8) (19). It may arise as two or duplicate arteries. Perforating branches arising in this area may be as large as the anterior choroidal artery. From its origin, it courses posteriorly below the optic tract and terminates by joining the choroid plexus in the temporal horn. Occlusion causes a variable deficit that includes contralateral hemiplegia, hemianesthesia, and hemianopsia.
Aneurysms arising from the choroidal segment commonly have more perforating branches stretched around their neck than those arising from the communicating or ophthalmic segment because the choroidal segment has a greater number of perforating branches arising from it and the majority arise from the posterior wall, where the neck of the aneurysm is situated (Figs. 3.5 and 3.9). On average, four, but as many as nine, perforating branches arise from the posterior wall of this segment. These branches pass superiorly behind the choroidal segment and the bifurcation of the internal carotid artery to enter the anterior perforated substance with the perforating branches of the anterior cerebral, recurrent, middle cerebral, and anterior choroidal arteries and ascend to the internal capsule (3, 19). An oculomotor nerve deficit, as frequently occurs with a carotid-posterior communicating artery aneurysm, is uncommon and rarely occurs before rupture.
Carotid Bifurcation Aneurysms
The fifth aneurysm site on the internal carotid artery is at its bifurcation. These aneurysms most easily fit the four principles described above (Figs. 3.2, 3.3, 3.5, and 3.9). These aneurysms arise at the apex of the T-shaped bifurcation. They point upward in the direction of the long axis of the prebifurcation segment of the artery toward the anterior perforated substance. The perforating branches arising from the choroidal segment of the internal carotid and the proximal part of the anterior and middle cerebral arteries are stretched around the back side of the neck and wall of the aneurysm and should be dissected free of the aneurysm (Figs. 3.4, 3.5, 3.7, and 3.9).
Middle Cerebral Artery Aneurysms
The middle cerebral artery is one of the most common sites of saccular aneurysms. These aneurysms also conform to the four anatomic precepts (Figs. 3.9 and 3.10) (2). They most commonly arise at the level of the first major bifurcation or trifurcation of the artery. The angulation with which the bifurcating trunks arise from the main trunk forms the turn or curve. These aneurysms usually point laterally in the direction of the long axis of the prebifurcation segment of the main trunk.
The middle cerebral artery is divided into four segments, M1 to M4. The M1 segment begins at the origin of the middle cerebral artery and extends laterally below the anterior perforated substance to where the M2 segment begins at the point the artery turns sharply posterior, at a turn called the genu, to reach the insula. It is on the M1 or junction of the M1 and M2 segments that saccular aneurysms arise. The M1 segment is subdivided into a prebifurcation and a postbifurcation part. The prebifurcation part is composed of a single main trunk that extends from the origin to its first major division, which is a bifurcation in most hemispheres. The bifurcation occurs proximal to the genu in most hemispheres. The small cortical branches arising from the M1 segment proximal to the bifurcation, called early branches, may be the site of origin of aneurysms arising proximal to the bifurcation. The early branches are directed to the frontal and temporal lobes.
The middle cerebral artery branches to the anterior perforated substance are called the lenticulostriate arteries (Figs. 2.30, 2.31, 3.9, and 3.10). On average, there are 10 (range, 1–20) lenticulostriate arteries per hemisphere (19). Eighty percent of lenticulostriate arteries arise from the prebifurcation part of the M1 segment, 17% arise from the postbifurcation part of the M1 segment, and 3% arise from the proximal part of the M2 segment near the genu. The earlier the bifurcation, the greater the number of branches arising distal to the bifurcation. An aneurysm may infrequently arise at the origin of a large lenticulostriate branch. The lenticulostriate arteries are divided into medial, intermediate, and lateral groups (Figs. 2.30 and 3.9) (19). Each group has a unique origin, composition, and characteristic distribution in the anterior perforated substance. The distinct morphology of each group has led to the medial group being referred to as straight because they pursue a straight course, the intermediate group as candelabra because of their complex branching as they approach the anterior perforated substance, and the lateral group as S shaped, describing their curved course. All three groups are encountered in splitting the sylvian fissure and following the artery medially. The number and type of perforating branches stretched around the neck of the aneurysm is dependent on the level of the bifurcation (Figs. 3.9 and 3.10). If the prebifurcation segment is very short, the neck of the aneurysm may have the straight or candelabra branches stretched around the neck, whereas an aneurysm arising at the apex of a long prebifurcation segment may involve the area of the S-shaped lenticulostriate branches.
Instruments helpful in dissecting the neck and in separating the perforating arteries from the wall of the aneurysm include the 40-degree-angled teardrop dissectors and the 1-, 2-, or 3-mm wide spatula dissectors (Fig. 3.11) (14, 15). A small angled curette with a 1.5-mm cup is useful in removing the dura over the clinoid process. A 5-French suction, 10-cm long provides a useful suction dissector. Bayonet scissors with 9.5-cm shafts are the appropriate length to divide arachnoidal bands. For grasping and separating arachnoidal adhesions, bayonet tissue forceps with fine serrations on the inside of the tips of the forceps are needed. Brain spatulas tapered from 10 or 15 mm at the base to 5 or 10 mm at the tip are suitable for elevating the brain at most aneurysm sites.
Anterior Communicating Aneurysms
The most common aneurysm site on the anterior cerebral artery is at the level of the anterior communicating artery (Fig. 3.12). These aneurysms are made complex by the frequently associated variants of anatomy and the difficulties in fully visualizing the major arterial trunks and perforating arteries in the area (12). The segment of the anterior cerebral artery between the internal carotid and anterior communicating arteries is referred to as the A1 segment, and the segment between the anterior communicating artery and the rostrum of the corpus callosum is referred to as A2 segment. Aneurysms usually occur in the setting where one A1 segment is hypoplastic and the dominant A1 gives rise to both A2s (Fig. 3.12). The aneurysm arises at the point where the dominant A1 segment bifurcates at the level of the anterior communicating artery to give rise to both the left and right A2 segments. These aneurysms usually point away from the dominant segment toward the opposite side. They may also project in other directions. The direction in which the fundus points is determined by the course of the anterior cerebral arteries proximal to their junction with the anterior communicating artery. Tortuosity of the arteries may create a situation in which the hemodynamic thrust varies, so that these aneurysms may project not only to the opposite side, but also in the anterior, posterior, or inferior direction (Fig. 3.12).
The anterior cerebral artery gives rise to numerous perforating branches (Figs. 2.16, 2.24, 3.9, and 3.13). The branches arise from two sources. First, the A1 segment gives rise to branches that pass directly to the anterior perforated substance; and second, the A1 and the proximal part of the A2 segments give rise to the recurrent artery. The recurrent branch of the anterior cerebral artery is the largest and longest of the branches directed to the anterior perforated substance. It may be the first artery seen on elevating the frontal lobe to approach the anterior communicating aneurysm (Fig. 3.13). It is unique among arteries in that it doubles back on its parent vessel, passing above the carotid bifurcation, and accompanying the middle cerebral artery into the sylvian fissure before entering the anterior perforated substance. If the A1 segment is hypoplastic, the recurrent artery on that side may be as large as the hypoplastic A1 segment and might even be confused with it, since both will be passing along the area between the carotid bifurcation and interhemispheric fissure (Figs. 2.24 and 3.13). The recurrent artery may lie in any direction from the A1 segment. Its origin may adhere to the wall of the anterior communicating aneurysms. The inverting adventitia of A1 may so obscure the recurrent artery that inadvertent occlusion by a clip may easily occur, even under the operating microscope. The recurrent artery pursues a long, redundant path, looping forward on the gyrus rectus or the posterior part of the orbital surface of the frontal lobe where it could be damaged and occluded in removing the posterior 1 or 2 cm of the gyrus rectus, as is common practice in exposing anterior communicating aneurysms (Fig. 3.9). It may arise from a common stem with the frontopolar artery (Fig. 3.13). Ischemia in the area supplied by Heubner’s artery may cause hemiparesis with facial and brachial predominance, because of compromise of the branch supplying the anterior limb of the internal capsule, and may cause aphasia if the artery is on the dominant side (19).
The anterior communicating artery is the site of origin of as many as four perforating branches to the dorsal surface of the optic chiasm and suprachiasmatic area (Figs. 2.16 and 2.24) (11). These perforating branches perfuse the fornix, corpus callosum, and septal region. Their occlusion results in personality and memory disturbances.
The next most common aneurysm site on the distal anterior cerebral artery is at the level of origin of the callosomarginal artery from the pericallosal artery, usually in close proximity to the anterior part of the corpus callosum, near the point of greatest angulation of the artery at the genu (Figs. 2.22 and 3.14). The curve is formed by the angulation of the branching and the artery’s passage around the rostrum of the corpus callosum. The aneurysm points distally into the interval between the junction of the pericallosal and callosomarginal arteries. Unusual variants, such as a connection between the two pericallosal arteries at their major bifurcation, may cause aneurysms by producing alterations in hemodynamics.
Vertebral and Basilar Artery Aneurysms
Approximately 15% of saccular aneurysms occur in the vertebrobasilar system, the majority of which (63%) occur at the basilar bifurcation. The incidence of anomalies consisting of either a hypoplastic communicating or a fetal posterior cerebral origin is more common with aneurysms than in normal groups. Aneurysms arising on the branches of the vertebral and basilar arteries also share the same four facets of anatomy described above. They arise at an apical branching site on a curve, point in the direction the blood would have followed if the curve were not present, and are surrounded by a constantly occurring set of perforating branches (Fig. 3.15). The basilar apex aneurysm arises at the branching of the posterior cerebral arteries from the basilar artery and points upward in the direction of the long axis of the basilar artery (Figs. 3.15 and 3.16, A and B). Because of these variations, posterior cerebral artery aneurysms may be visualized on carotid as well as on vertebral angiography, especially when the P1 segment is hypoplastic (fetal type).
Aneurysms arising from the basilar artery at the level of origin of the superior cerebellar or anteroinferior cerebellar artery, or from the vertebral artery at the level of origin of the posteroinferior cerebellar artery, initially seem to conform poorly to the first three facets of anatomy applicable to the other aneurysms because the basilar and vertebral arteries are often pictured as straight arteries, with the cerebellar arteries arising at right angles from them (Fig. 3.15) (18). However, most of the arteries harboring aneurysms are tortuous, and the change in direction of flow associated with the curves creates hemodynamic stress on the wall of the basilar or vertebral arteries near the origins of the cerebellar arteries. These aneurysms point in the direction the blood would have gone had there not been a curve at the level of origin of the involved branch.
Basilar Apex Aneurysms
The majority of the 15% of aneurysms occurring in the vertebral-basilar system are located on the posterior part of the circle of Willis at the bifurcation of the basilar artery (Figs. 3.4, 3.15, and 3.16, A and B). The basilar apex aneurysm arises at the branching of the posterior cerebral arteries from the basilar artery. The curve at the aneurysm site is related to the change from the vertical direction of the basilar artery to a lateral direction of the posterior cerebral arteries. These aneurysms project upward in the direction of the long axis of the basilar artery. The basilar bifurcation is most commonly situated opposite the interpeduncular fossa, but it may be located as far as 1.3 mm below the pontomesencephalic junction in front of the pons, or as far rostral as the mamillary bodies (20). High bifurcations may indent and push the mamillary bodies and floor of the third ventricle upward. High or low bifurcations are best approached by the subtemporal rather than the pterional route.
In the subtemporal approach for basilar aneurysm, the neck of the aneurysm at the bifurcation is best found by following the inferior side of the posterior cerebral artery medially as it curves around the peduncle, because the inferior surface is the most infrequent site of origin for perforating branches, thus making it the safest approach to the P1 and basilar bifurcation (Figs. 3.17 and 3.18).
The region of the basilar bifurcation may be the site of multiple anomalies (20, 22). The segment of the posterior cerebral artery between the basilar bifurcation and the posterior communicating artery is referred to as P1 and the segment just distal to the communicating as P2. A normal posterior circle, defined as one in which both P1 segments have a diameter larger than their posterior communicating arteries—and the latter are not hypoplastic—is found in approximately half of cases. In the remainder, anomalies are found consisting of either a hypoplastic posterior communicating artery or a fetal arrangement in which the P1 segment is hypoplastic and the posterior communicating artery provides the major supply to the posterior cerebral artery.
A hypoplastic posterior communicating artery, or a fetal configuration in which the posterior cerebral artery arises predominantly from the carotid artery, may be found on one or both sides (Figs. 2.8 and 2.34). Transection of a hypoplastic posterior communicating artery or P1 segment has been recommended to gain access to basilar bifurcation aneurysms on the assumption that they have fewer branches. However, the number and diameter of perforating branches is relatively constant, regardless of trunk size; therefore, a hypoplastic segment supplies the same perforating area as a larger vessel, despite its smaller size (20).
The posterior portion of the circle of Willis sends a series of perforating arteries into the diencephalon and midbrain that may become stretched around basilar apex aneurysms. The most important and largest of these are the thalamoperforating arteries, which arise from the P1 in the region of the basilar apex aneurysm (Figs. 3.18 and 3.19) (20, 22). They originate from P1 and enter the brain behind the maxillary bodies through the posterior perforated substance in the interpeduncular fossa and medial cerebral peduncles. They are both the largest branches of the P1 and the branch nearest the bifurcation in most cases. One P1 may not give rise to a thalamoperforating artery, in which case a well-developed or dominant thalamoperforating branch on the contralateral side will supply the area normally perfused by the branches of both P1s. The risks from occlusion of these vital perforating vessels include visual loss, paralysis, somesthetic disturbances, weakness, memory deficits, autonomic and endocrine imbalance, abnormal movements, diplopia, and depression of consciousness.
The posterior and lateral surfaces of the upper centimeter of the basilar artery is also a rich source of perforating arteries. An average of 8 (range, 3–18) branches arise from the upper centimeter (Figs. 2.34 and 2.35) (20, 22). Approximately half arise from the posterior surface and a quarter arise from each side. Perforating branches rarely arise from the anterior surface of the basilar artery. The patient with basilar bifurcation aneurysms has been viewed more gravely than the patient with aneurysms in other areas because of the greater tendency of vital perforators to be involved in aneurysm dissection and clipping. In basilar bifurcation aneurysms, the more posterior the aneurysm, the poorer the prognosis, because the tendency for vital perforators to be involved becomes greater as the aneurysm projects more posteriorly (1). The anterior surface of the basilar bifurcation is infrequently the site of perforators, thus surgical results are better with anteriorly projecting aneurysms. The rich plexus on the posterior basilar surface, 2 to 3 mm below the bifurcation, entering the interpeduncular fossa and terminating in the medial midbrain makes this the most dangerous site. The basilar apex is intermediate in risk because the thalamoperforating artery is easier to identify at surgery, and there are fewer perforators than on the posterior aspect of the bifurcation.
An aneurysm of the posterior cerebral artery distal to the origin is uncommon. The most common site is at the origin of the first major branch, as the posterior cerebral artery winds around the midbrain either on the P1 or P2 in the crural or ambient cisterns. Distal posterior cerebral artery aneurysms tend to become larger than other aneurysms before their identification, often mimicking neoplasms in the region. The most frequent neurological deficit with posterior cerebral aneurysms is a partial or complete oculomotor nerve deficit.
Basilar Trunk Aneurysms
The basilar aneurysm at the level of the superior cerebellar artery often arises where there is a curvature and tilt of the upper basilar artery, so that the hemodynamic thrust created by flow along the basilar artery is just above the origin of the superior cerebellar artery rather than at the basilar apex (Figs. 3.15 and 3.16C) (4). The aneurysm located at the origin of the anteroinferior cerebellar artery commonly arises from the convex side of the curve in the basilar artery and points in the direction of the long axis of the basilar segment immediately proximal to the aneurysm (Fig. 3.16D) (10).
The most common aneurysm site on the vertebral artery is at the level of origin of the posteroinferior cerebellar artery. The vertebral artery is often depicted as being straight; however, if an aneurysm is present, the vertebral artery is usually found to have a convex upward curve with an apex where the posteroinferior cerebellar artery arises (Figs. 3.15 and 3.16F) (6). The aneurysm arises from the apex of this curve at the origin of the posteroinferior cerebellar artery and points upward.
Aneurysms arising infrequently at the junction of the two vertebral arteries with the basilar artery may initially seem difficult to fit into these precepts. When examined in multiple angiographic projections, however, they are often found to conform to these same anatomic principles applied in predicting the site and direction of projection of the more common saccular aneurysms. These aneurysms often arise on the convex side of a tortuous curve formed at the vertebrobasilar junction (Figs. 3.15 and 3.16E). One vertebral artery is often dominant and the smaller vertebral artery acts as the branch site. If this tortuous configuration is not present, it is likely that the aneurysm is associated with a fenestration in the lower part of the basilar artery.
Anatomic Principles Directing Surgery
The following basic surgical principles are helpful in directing the attack on intracranial aneurysms.
- The parent artery should be exposed proximal to the aneurysm. This allows control of flow to the aneurysm if it ruptures during dissection. Exposure of the internal carotid artery above the cavernous sinus will give proximal control for aneurysms arising at the level of the posterior communicating or anterior choroidal artery. Exposure of the internal carotid artery at the level of the ophthalmic and superior hypophyseal arteries is commonly achieved by removing the anterior clinoid process, the adjacent part of the roof of the optic canal, and the posterior part of the orbital roof to gain access to the clinoid segment of the internal carotid artery. An operative plan that permits cervical internal carotid occlusion in the neck, either by balloon catheter or by direct exposure, should be considered if anterior clinoid removal and proximal supraclinoid exposure is unlikely to yield adequate proximal control. The supraclinoid carotid or the preaneurysmal trunks of the middle cerebral or anterior cerebral arteries should also be exposed initially to obtain proximal control of middle cerebral and anterior cerebral artery aneurysms. The exposure can be directed laterally from the internal carotid artery for middle cerebral aneurysms and medially over the optic nerves and chiasm for anterior communicating aneurysms. For basilar apex aneurysms, control of the basilar artery proximal to the aneurysm can be obtained by following the inferior surface of the posterior cerebral artery or the superior surface of the superior cerebellar artery to the basilar artery and then working up the side of the basilar artery to the neck of the aneurysm. An operative plan that includes proximal balloon may also be considered. There are several operative routes, discussed below, under Operative Approaches, that increase the length of basilar artery below the apex that can be exposed.
- If possible, the side of the parent vessel away from or opposite to the site on which the aneurysm arises should be exposed before dissecting the neck of the aneurysm. The dissection can then be carried around the wall of the parent vessel to the origin of the aneurysm.
- The aneurysmal neck should be dissected before the fundus. The neck is the area that can tolerate the greatest manipulation, has the least tendency to rupture, and is to be clipped. Unfortunately, it is the portion of the aneurysm that is most likely to incorporate the origin of a parent arterial trunk or perforating vessel. Therefore, dissection of the neck and proximal part of the fundus should be performed carefully, with full visualization, to prevent passage of a clip around the parental arterial trunk or significant perforating branches that arise near the neck of the aneurysm. The dissection should not be started at the dome, because this is the area most likely to rupture before or during surgery.
- All perforating arterial branches should be separated from the aneurysmal neck before passing the clip around the aneurysm. Before the use of magnification, there was a tendency to keep dissection of aneurysms to a minimum because of the hazard of rupture. The use of magnification has permitted increased accuracy of dissection of the aneurysmal neck and more frequent preservation of the perforating arteries. Thus the risk of occlusion of perianeurysmal perforating arterioles that results from placement of a clip on an inadequately exposed aneurysm is greater than the hazard of rupture with microsurgical dissection. Separating perforating arteries from the neck of an aneurysm requires appropriately sized microdissectors. Small spatula dissectors 1- or 2-mm wide (Rhoton No. 6 or 7) or 40-degree-angle teardrop dissectors are suitable. Separating the perforators, if tightly packed against or adherent to the aneurysm, may be facilitated by lowering the blood pressure or by temporary clipping of the parent artery. In other cases, where the middle portion of the body, but not the neck of the aneurysm can be separated from the perforating arteries, placing a clip around the middle portion will sometimes reduce the width of the aneurysm neck so that the perforators can be separated from the neck before moving the clip to the aneurysm neck. Perforators may also be placed in the open area of a fenestrated clip in some cases where one cannot separate the perforator from the neck. An endoscopic view of the neck with angled endoscopes may aid by revealing the position of perforating branches not seen in the view provided by the surgical microscope.
- If rupture occurs during microdissection, bleeding should be controlled by applying a small cotton pledget to the bleeding point and concomitantly reducing mean arterial pressure. If this technique does not stop the hemorrhage, temporary occlusion with a clip or occluding balloon can be applied to the proximal blood supply, but only for a brief time.
- The bone flap should be placed as low as possible to minimize the need for retraction of the brain in reaching the area (Figs. 3.4, 3.7, 3.17, 3.20, and 3.21). Most aneurysms are located on or near the circle of Willis under the central portion of the brain. Cranial-base resection, such as is performed in the orbitozygomatic, anterior petrosectomy, presigmoid, or far lateral approaches, should be used if it will minimize brain retraction, improve vascular exposure, and broaden the operative angle available for attacking the aneurysm.
- A clip with a spring mechanism that allows it to be removed, repositioned, and reapplied should be used.
- After the clip is applied, the area should always be inspected, sometimes with intraoperative angiography, to make certain the clip does not kink or obstruct a major vessel and that no perforating branches are included in it.
- If an aneurysm has a broad-based neck that will not easily accept the clip, the neck may be reduced by bipolar coagulation. Nearby perforating arteries are protected with a cottonoid sponge during coagulation. The tips of the bipolar coagulation forceps are inserted between adjacent vessels and the neck of the aneurysm, and are gently squeezed during coagulation. Short bursts of low current are used, and the tips of the forceps are relaxed and opened between applications of current to prevent them from adhering to the aneurysm, and to evaluate the degree of shrinkage.
Ninety-five percent of aneurysms are found at one of five sites, all of which are located in close proximity to the circle of Willis (Fig. 3.1). These sites are 1) the internal carotid artery between the posterior communicating and the anterior choroidal arteries; 2) the anterior communicating artery area; 3) the initial bifurcation or trifurcation of the middle cerebral artery; 4) the internal carotid bifurcation; and 5) the basilar bifurcation. The frontotemporal craniotomy with slight modifications is commonly selected for approaching all of these aneurysms arising from the anterior circle of Willis, and for some originating from the upper basilar artery (21). A frontotemporal flap centered at the pterion (pterional craniotomy) may be used for internal carotid artery aneurysms (Figs. 3.4, 3.20, and 3.21). The flap may be enlarged posterosuperiorly for reaching aneurysms of the middle cerebral artery and of the internal carotid artery bifurcation, forward for approaches to the anterior communicating area, and posteriorly to provide a pterional-pretemporal or anterior subtemporal approach for an aneurysm of the basilar apex.
The scalp incision for this flap begins above the zygoma and extends across the temporal region and forward to the frontal region behind the hairline. The method of opening the scalp for the frontotemporal exposure varies, depending on the site of the aneurysm (Figs. 3.20 and 3.21). If the aneurysm is located at the level of or above the posterior communicating artery, the skin, galea, pericranium, and temporalis muscle and fascia are reflected as a single layer. If the aneurysm is located at the level of the ophthalmic or superior hypophyseal artery, the skin and galea are elevated in one layer and the temporalis muscle and fascia are elevated in a second layer. The two-layer scalp opening provides a lower exposure and better access for removing the anterior clinoid process and adjacent part of the orbital roof than the single-layer flap.
A small, free bone-flap, having the center of its base below the pterion, is elevated. The opening in the cranium is extended inferiorly and medially by removing the sphenoid ridge and reducing the thickness of the orbital roof and lateral wall to a thin shell of bone. The time required to prepare this flap, in which all of the soft tissue layers are reflected together, is less than that required to separate and reflect each layer individually. The incidence of weakness of the frontalis muscle is reduced with the single-layer exposure because the layers superficial to the temporalis fascia, in which the facial nerve branches to the frontalis muscle, are not disturbed. Decreased dissection around the temporalis muscle diminishes the incidence of contractures that limit opening of the mouth and reduces cosmetic deformities caused by scarring and atrophy of the temporalis muscle. Any burr holes or craniectomy site that would heal with a cosmetic deformity are closed with cranioplasty material or nonmagnetic plates. The cranioplasty material is molded into position and allowed to harden under direct vision to ensure that the hardened material fits the natural contour of the area.
The frontotemporal scalp flap is modified so that the scalp and galea are elevated as one layer and the temporalis muscle and fascia are elevated as a second layer if the aneurysm is located at the origin of the superior hypophyseal or ophthalmic artery or if a basilar apex aneurysm is to be reached by this approach (Fig. 3.21). This allows the temporalis muscle to be reflected into the posteroinferior part of the exposure and provides a lower exposure for removal of the anterior clinoid process, roof of the optic canal, and adjacent part of the roof of the orbit, which are commonly needed to manage aneurysms that arise proximal to the posterior communicating artery.
Cranial-base approaches, such as orbitozygomatic osteotomy, anterior petrosectomy, and various modifications of the presigmoid and far lateral approaches, have been used with increasing frequency in dealing with aneurysms because they reduce the need for brain retraction, increase the width of the operative route, and broaden the angle for dissection and clip application. The orbitozygomatic craniotomy, with elevation of the superior and lateral orbital rim and the zygomatic arch, may facilitate the exposure of all aneurysms on the supraclinoid carotid and circle of Willis, but the benefits are greatest with ophthalmic and superior hypophyseal aneurysms (Figs. 3.7 and 3.22). The orbitozygomatic craniotomy may be combined with any of the following: anterior clinoidectomy and removal of the roof of the optic canal and orbital apex for ophthalmic and superior hypophyseal aneurysms; anterior clinoidectomy opening of the roof of the cavernous sinus; and posterior clinoidectomy (transcavernous approach) or anterior petrosectomy for reaching a low-lying basilar apex or basilar trunk aneurysm (Figs. 3.7, 3.17, 3.22, and 3.23). The far lateral approaches that expose the vertebral artery as it courses behind the atlanto-occipital joint are used with increasing frequency for vertebral, vertebrobasilar, and lower basilar trunk aneurysms (Figs. 3.24 and 3.25). The presigmoid approaches with varying degrees of temporal bone resection may be considered for aneurysms located in the central part of the posterior fossa, although many of these aneurysms may be reached with the various modifications of the orbitozygomatic, anterior petrosectomy, or far lateral approaches (Figs. 3.26 and 3.27). The various modifications of the orbitozygomatic approach are reviewed in Chapter 9 of this issue and the far lateral and presigmoid approaches were reviewed in the Millennium issue of Neurosurgery (16, 17).
After the pterional or orbitozygomatic bone flap is elevated and the dura opened, the arachnoid is opened, usually beginning below the pars triangularis of the inferior frontal gyrus. The frontal lobe adjoining the anterior part of the sylvian fissure may be elevated to expose the sphenoid ridge to the depth of the anterior clinoid process. The sylvian veins emptying into the anterior part of the cavernous sinus are usually preserved (Fig. 4.12). The arachnoid walls of the cistern around the optic nerve and carotid artery are opened. The surgeon is at the desired location if the aneurysm arises from the internal carotid artery (Figs. 3.3, 3.4, and 3.7). Exposure of the neck of ophthalmic and superior hypophyseal aneurysms is facilitated by the removal of the anterior clinoid process, unroofing the optic canal and adjacent part of the orbital roof, and incision of the falciform process of the dura extending above the optic nerve to allow mobilization of the optic nerve. The anterior clinoid removal for exposure of an aneurysm is usually performed intra- rather than extradurally.
In approaching posterior communicating aneurysms, the anterior or anterolateral surface of the supraclinoid carotid is exposed initially before exposing the wall on the posterior or posteromedial side from which the aneurysm arises (Fig. 3.8). It has been suggested that the posterior communicating artery can be clipped with the neck of the aneurysm, especially if the artery is hypoplastic (9). However, hypoplastic segments of the circle of Willis give rise to the same number and size of perforating branches as do normal or large segments.
In approaching internal carotid aneurysms along the sylvian fissure, the origin and proximal portion of the anterior choroidal artery is often exposed before the posterior communicating artery because of its more lateral origin and course. The anterior choroidal aneurysm usually projects posterolaterally above and medial to the anterior choroidal artery, thus providing an angle of separation for safe application of a clip. The neck is inferior, medial, or inferior and medial. The aneurysm may also arise within a multivessel origin of the anterior choroidal artery and displace its branches both laterally and medially. It may be helpful to work over the carotid bifurcation to expose a portion of the neck.
The anterior communicating area is most commonly approached by the pterional route and less frequently by a subfrontal, bifrontal, or anterior interhemispheric approach. For anterior communicating artery aneurysms, the dissection in the pterional approach is directed superiorly to the bifurcation of the internal carotid artery and over the optic nerve and chiasm along the anterior cerebral artery to the neck of the aneurysm (Figs. 3.4 and 3.12). The majority of the aneurysms point anteriorly, inferiorly, and toward the side opposite the dominant A1. An approach along the pterion facilitates exposure of the base before the fundus. Some surgeons approach all anterior communicating aneurysms from the right side. The author has selected the left side if a left frontal hematoma is present, if the fundus of the aneurysm projects toward the right, or if the left anterior cerebral artery is dominant and the right is hypoplastic. It is important to have control of the dominant anterior cerebral artery, because the majority of these aneurysms occur in association with dominance of one A1 and hypoplasia of the other. Gyrus rectus removal is not necessary if the aneurysm is exposed in the subarachnoid cistern above the chiasm. If resection is required to visualize both A1s and proximal A2s and the recurrent and anterior communicating arteries, it should be kept to a minimum.
The recurrent artery of Heubner is frequently exposed before the A1 segment in defining the neck on anterior cerebral aneurysms because it commonly courses anterior to A1 (Figs. 3.9 and 3.13). The first artery seen on frontal lobe elevation may be the recurrent artery. If A1 is hypoplastic, the recurrent artery on that side may be nearly as large as the A1 segment and might even be confused with it because it may have the same course as the A1. The recurrent artery may lie in any direction from the A1 segment, but if followed, usually joins the A2 segment just distal to the anterior communicating artery. The recurrent artery may be adherent to the wall of aneurysms. It may loop forward or cross the gyrus rectus where it could be occluded in removing the posterior part of the gyrus rectus, as performed in the gyrus rectus approach. The investing adventitia of A1 may so obscure Heubner’s artery that inadvertent occlusion by a clip may easily occur, even under the microscope. Hypoplastic A1s should be preserved because they may give rise to perforating branches even when very small. Temporary clips should be placed on the A1 at a site that avoids the perforating branches, the majority of which arise from the lateral half of the A1 segment. Placement of a clip on an inadequately exposed aneurysm risks occlusion of perianeurysmal perforating arterioles, and is to be avoided.
Aneurysms of the distal anterior cerebral artery are located in or near the midline. They should be approached from the nondominant right side through a unilateral frontal craniotomy anterior to the coronal suture and extending up to the midline as needed to obtain exposure along the falx without undue retraction (Fig. 3.14). The craniotomy is preferably placed far enough forward that the proximal part of the pericallosal artery can be exposed and temporarily occluded if bleeding should occur during exposure. The craniotomy may be modified so that a second aneurysm, which occurs more frequently than with aneurysms in other sites, can also be approached at the same operation. The distal portion of the anterior cerebral artery is difficult to expose because of its location deep in the interhemispheric fissure. At no other location do the main trunks of two major cerebral arteries run side by side as do the distal anterior cerebral arteries and because of cross-over of branches from one side to the other, injuries to one anterior cerebral artery may cause infarction in the contralateral cerebral hemisphere. A less satisfactory, more difficult approach, suitable only for lesions of the proximal A2, is through a pterional or subfrontal craniotomy with elevation of the frontal lobe and following the anterior cerebral artery distally from near the carotid origin. Before retracting the medial surface of the frontal lobe, it may be necessary to sacrifice a bridging vein passing from the superior margin of the hemisphere to the sagittal sinus. Most frequently, only one vein must be sacrificed. From this point, the surgery is often tedious because of the limited exposure provided by the interhemispheric fissure, the frequent attachment of the aneurysm to the falx, and because aneurysms at this site are more prone to rupture during exposure than other supratentorial aneurysms.
Intracerebral hemorrhage occurs after rupture slightly more frequently with aneurysms of the distal anterior cerebral artery than with aneurysms in other locations, because of the absence of a subarachnoid cistern into which to bleed and the closely applied cerebral surfaces. The hemorrhage may be into the hemisphere opposite the anterior cerebral artery harboring the aneurysm. A significant hematoma may dictate that the approach be on the side of the hematoma. The pericallosal and callosomarginal arteries and variants of normal anatomy should be identified before dissecting the aneurysm (Fig. 2.22). Connections between the two anterior cerebral arteries may occur proximal or distal to the area of the aneurysm, or the aneurysm may occur at the apex of a single pericallosal artery created by a fusion of the pericallosal arteries from both sides to form a single artery. The necks of distal anterior cerebral artery aneurysms are often wide and atherosclerotic.
Middle cerebral artery aneurysms are exposed by splitting the sylvian fissure (Figs. 3.4, 3.9, and 3.10). Usually, opening the sylvian fissure and working in the superior part of the exposure below the frontal lobe will allow the proximal M1 segment and its postbifurcation trunks to be exposed before encountering the neck and fundus of the aneurysm. These aneurysms usually arise distal to the lenticulostriate arteries near the genu at the M1 bifurcation or trifurcation, but they may also arise at the origin of an early branch of the M1 segment to the frontal or temporal lobes. Aneurysms arising at an early branch site arise from the same part of the M1 segment from which the lenticulostriate arteries arise. An aneurysm may also arise at the origin of a large lenticulostriate artery. These aneurysms arising at the genu, the most common site, point downward, forward, and laterally and may be attached to the sphenoid ridge, in which case the operative approach may need to be modified to avoid avulsing the fundus of the aneurysm at the sphenoid ridge.
There are several approaches to basilar apex aneurysms. They may be exposed through a pterional, pretemporal, anterior subtemporal, or subtemporal approach. The four routes to the apex of the basilar apex that can be accessed through a frontotemporal (pterional) craniotomy are: 1) through the opticocarotid triangle, located between the internal carotid artery, optic nerve, and anterior cerebral artery; 2) between the bifurcation of the internal carotid artery below and the optic tract above; 3) through the interval between the carotid artery and the oculomotor nerve and above the posterior communicating artery; and 4) between the internal carotid artery and oculomotor nerve and below the posterior communicating artery (Figs. 3.4 and 3.28).
Some basilar apex aneurysms may be exposed through the opticocarotid triangle if the interval between the optic nerve, carotid artery, and A1 is sufficiently wide and the aneurysm projects superiorly or anteriorly (Figs. 3.4 and 3.28). The triangle is widened if the supraclinoid carotid and A1 are elongated, and is small if these arteries are short. If this approach is used, care should be taken to preserve the vital perforating branches that arise on the internal carotid artery and cross this space to supply the optic nerve and tract and diencephalon. Aneurysms arising on a high basilar bifurcation may also be exposed through the interval between the bifurcation of the internal carotid artery below and the optic tract above, usually by depressing the bifurcation, but again, the perforating arteries crossing this interval must be protected (Figs. 3.4 and 3.28). The approach may be applicable if the supraclinoid carotid is short so that there is a wide space between the carotid bifurcation, lower surface of the optic tract, and anterior perforated substances. In the pterional route, the aneurysm is more commonly approached through the space between the internal carotid artery and the oculomotor nerve (Figs. 3.4 and 3.28). This exposure is facilitated by elevating the carotid artery and proximal M1 segment. After exposing the area between the carotid artery and the oculomotor nerve, a decision must be made regarding whether to expose the aneurysm by operating above or below the posterior communicating artery. If a basilar aneurysm arises from the posterior aspect of the upper basilar artery, it is best to elevate the temporal lobe and approach the area along the floor of the middle fossa (Figs. 3.4, 3.17, and 3.18).
Most basilar artery aneurysms are approached through an anterior subtemporal approach (Figs. 3.17 and 3.18). The anterior subtemporal and subtemporal approaches are facilitated if the pterional scalp incision and bone flap are extended backward in a question-mark incision above the anterior part of the ear and downward onto the zygomatic arch near the tragus to facilitate exposure along the floor of the middle fossa. Turning the temporalis muscle and fascia as a separate layer from the scalp and folding the temporalis muscle downward and forward facilitates the exposure along the middle fossa floor. Elevating the anterior part of the temporal lobe provides an anterior subtemporal exposure with visualization of the oculomotor nerve as it arises from the medial surface of the cerebral peduncle and passes between the posterior cerebral and superior cerebellar arteries to enter the roof of the cavernous sinus. Elevating the posterior communicating artery and temporal lobe exposes the basilar apex, both oculomotor nerves, and the junction of the right posterior communicating artery with the right posterior cerebral artery. The subtemporal approach, when combined with sectioning of the tentorium cerebelli posterior to the junction of the trochlear nerve with the tentorial edge, accesses aneurysms arising on a low basilar bifurcation or at the origin of the superior cerebellar artery. Aneurysms arising at the origin of the anteroinferior cerebellar arteries may also be approached by this route if the origin is high on the upper basilar artery (Fig. 3.17).
In the subtemporal approaches, the neck of the aneurysm at the basilar bifurcation is best found by following the inferior side of the posterior cerebral artery medial as it curves around the peduncle. The inferior surface of the P1 is the most infrequent site of origin for perforating branches, thus making it the safest approach to the proximal part of the posterior cerebral artery and the basilar bifurcation (Figs. 3.17 and 3.18). The approach under the anterior temporal lobe in front of the vein of Labbé gives better exposure of the perforating arteries that commonly arise from the posterior aspect of the basilar artery than does the pterional approach along the sphenoid ridge. These perforating branches are especially important because they supply diencephalic areas controlling consciousness. Transection of a hypoplastic posterior communicating artery or P1 may be considered to gain access to basilar bifurcation aneurysms and some tumors on the assumption that they have fewer branches and the brain is less dependent on them. However, the number and diameter of perforating branches are relatively constant, regardless of trunk size. If a hypoplastic segment is divided, care should be taken not to sacrifice any small perforating branches (20). In ligating or placing clips on the posterior cerebral artery, the small circumferential arteries on its medial surface that may not be visible from the lateral subtemporal route must be avoided. These small circumferential arteries are often incorporated into the same arachnoid bundle with the posterior cerebral artery trunk and can be preserved only by dissecting them away from the main trunk.
Cranial-base approaches have been used with increasing frequency in dealing with basilar apex aneurysms. An orbitozygomatic craniotomy, in which the orbital roof and lateral wall and the zygomatic arch are removed, increases the angle of exposure, whether the approach be transsylvian, pretemporal, anterior subtemporal, or midsubtemporal (Figs. 3.7 and 3.22). Two other modifications that have been used to reach the low basilar bifurcation are the orbitozygomatic craniotomy combined with a transcavernous approach, in which the anterior and posterior clinoid processes and the roof of the cavernous sinus are removed (Figs. 3.7 and 3.22). An alternative to the transcavernous approach is the anterior petrosectomy approach, in which the part of the petrous apex behind the petrous carotid artery and under the trigeminal nerve is removed extradurally before opening the dura, either through a frontotemporal or orbitozygomatic craniotomy (Figs. 3.17 and 3.23). After the drilling is complete, the dura is opened and the tentorium divided. The exposure allows the trigeminal nerve to be depressed, thus significantly increasing the length of basilar artery that can be exposed as compared with that seen with tentorial section without petrosectomy.
Aneurysms arising at the vertebrobasilar junction are approached through a subtemporal transtentorial exposure if the aneurysm and junction are high in the posterior fossa, through a combined supra- and infratentorial presigmoid exposure if the junction is deep in the middle part of the posterior fossa, or through a lateral suboccipital or far lateral approach if the vertebrobasilar junction is low (Figs. 3.16E and 3.24–3.27). Vertebral aneurysms arising at the origin of the posteroinferior cerebellar artery are approached through lateral suboccipital craniectomy or far lateral approach if they are located low in the posterior fossa, and through a combined supra and infratentorial presigmoid exposure if they are deep in the middle portion of the posterior fossa (Figs. 3.16F and 3.24– 3.27). If the far lateral suboccipital approach is selected, the ipsilateral half of the posterior C1 arch may be removed to provide adequate exposure of the segment of the vertebral artery proximal to the aneurysm. The side for the suboccipital approach should be selected only after carefully reviewing the angiogram, because aneurysms of one vertebral artery may lie on the side of the brainstem opposite the side of the vertebral artery from which it fills because of extreme tortuosity of these arteries.
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
- Drake CG: Bleeding aneurysms of the basilar artery: Direct surgical man- agement in four cases. J Neurosurg 18:230–238, 1961.
- Gibo H, Carver CC, Rhoton AL Jr, Lenkey C, Mitchell RJ: Microsurgical anatomy of the middle cerebral artery. J Neurosurg 54:151–169, 1981.
- Gibo H, Lenkey C, Rhoton AL Jr: Microsurgical anatomy of the supraclinoid portion of the internal carotid artery. J Neurosurg 55:560–574, 1981.
- Hardy DG, Peace DA, Rhoton AL Jr: Microsurgical anatomy of the superior cerebellar artery. Neurosurgery 6:10–28, 1980.
- Inoue T, Rhoton AL Jr, Theele D, Barry ME: Surgical approaches to the cavernous sinus: A microsurgical study. Neurosurgery 26:903–932, 1990.
- Lister JR, Rhoton AL Jr, Matsushima T, Peace DA: Microsurgical anatomy of the posterior inferior cerebellar artery. Neurosurgery 10:170–199, 1982.
- Liu QL, Rhoton AL Jr: Middle meningeal origin of the ophthalmic artery. Neurosurgery 49:401–407, 2001.
- Locksley HB: Natural history of subarachnoid hemorrhage, intracranial aneurysms and arteriovenous malformations: Based on 6368 cases in the cooperative study. J Neurosurg 25:219–239, 1966.
- Lougheed WM, Marshall BM: Management of aneurysms of the anterior circulation by intracranial procedures, in Youmans JR (ed): Neurological Surgery. Philadelphia, W.B. Saunders Co., 1973, vol 2, pp 731–767.
- Martin RG, Grant JL, Peace D, Theiss C, Rhoton AL Jr: Microsurgical relationships of the anterior inferior cerebellar artery and the facial-vestibulocochlear nerve complex. Neurosurgery 6:483–507, 1980.
- Perlmutter D, Rhoton AL Jr: Microsurgical anatomy of the anterior cerebral- anterior communicating-recurrent artery complex. J Neurosurg 45:259–272, 1976.
- Rhoton AL Jr: Anatomy of saccular aneurysms. Surg Neurol 14:59–66, 1980.
- Rhoton AL Jr: Microsurgical anatomy of saccular aneurysms, in Wilkins RH, Rengachary SS (eds): Neurosurgery. New York, McGraw-Hill, 1985, vol 2, pp 1330–1340.
- Rhoton AL Jr: Micro-operative techniques, in Youmans JR (ed): Neurological Surgery. Philadelphia, W.B. Saunders Co., 1990, vol 2, ed 3, pp 941–991.
- Rhoton AL Jr: Instrumentation, in Apuzzo MLJ (ed): Brain Surgery: Complication Avoidance and Management. New York, Churchill-Livingstone, 1993, vol 2, pp 1647–1670.
- Rhoton AL Jr: Far lateral approach and its transcondylar, supracondylar, and paracondylar extensions. Neurosurgery 47[Suppl 1]:S195–S209, 2000.
- Rhoton AL Jr: Temporal bone and transtemporal approaches. Neurosurgery 47[Suppl 1]:S211–S265, 2000.
- Rhoton AL Jr, Saeki N, Perlmutter D, Zeal A: Microsurgical anatomy of common aneurysm sites. Clin Neurosurg 26:248–306, 1979.
- Rosner SS, Rhoton AL Jr, Ono M, Barry M: Microsurgical anatomy of the anterior perforating arteries. J Neurosurg 61:468–485, 1984.
- Saeki N, Rhoton AL Jr: Microsurgical anatomy of the upper basilar artery and the posterior circle of Willis. J Neurosurg 46:563–578, 1977.
- Yas ̧argil MG, Fox JL: The microsurgical approach to intracranial aneurysms. Surg Neurol 3:7–14, 1975.
- Zeal AA, Rhoton AL Jr: Microsurgical anatomy of the posterior cerebral artery. J Neurosurg 48:534–559, 1978.
Please login to post a comment.