Last Updated: August 23, 2020
Optimizing operative approaches to the posterior fossa requires an understanding of the relationship of the cerebellar arteries to the cranial nerves, brainstem, cerebellar peduncles, fissures between the cerebellum and brainstem, and the cerebellar surfaces (45). When examining these relationships, three neurovascular complexes are defined: an upper complex related to the superior cerebellar artery (SCA); a middle complex related to the anteroinferior cerebellar artery (AICA); and a lower complex related to the posteroinferior cerebellar artery (PICA) (Figs. 2.1 and 2.2) (35).
Other structures, in addition to the three cerebellar arteries, occurring in sets of three in the posterior fossa that bear a consistent relationship to the SCA, AICA, and PICA are the parts of the brainstem (midbrain, pons, and medulla); the cerebellar peduncles (superior, middle, and inferior); the fissures between the brainstem and the cerebellum (cerebellomesencephalic, cerebellopontine, and cerebellomedullary); and the surfaces of the cerebellum (tentorial, petrosal, and suboccipital). Each neurovascular complex includes one of the three parts of the brainstem, one of the three surfaces of the cerebellum, one of the three cerebellar peduncles, and one of the three major fissures between the cerebellum and the brainstem. In addition, each neurovascular complex contains a group of cranial nerves. The upper complex includes the oculomotor, trochlear, and trigeminal nerves that are related to the SCA. The middle complex includes the abducens, facial, and vestibulocochlear nerves that are related to the AICA. The lower complex includes the glossopharyngeal, vagus, accessory, and hypoglossal nerves that are related to the PICA.
In summary, the upper complex includes the SCA, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum, and the oculomotor, trochlear, and trigeminal nerves. The SCA arises in front of the midbrain, passes below the oculomotor and trochlear nerves and above the trigeminal nerve to reach the cerebellomesencephalic fissure, where it runs on the superior cerebellar peduncle and terminates by supplying the tentorial surface of the cerebellum.
The middle complex includes the AICA, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum, and the abducens, facial, and vestibulocochlear nerves. The AICA arises at the pontine level, courses in relationship to the abducens, facial, and vestibulocochlear nerves to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum.
The lower complex includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum, and the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves. The PICA arises at the medullary level, encircles the medulla, passing in relationship to the glossopharyngeal, vagus, accessory, and hypoglossal nerves to reach the surface of the inferior cerebellar peduncle, where it dips into the cerebellomedullary fissure and terminates by supplying the suboccipital surface of the cerebellum.
The Superior Cerebellar Artery
The SCA or its branches are exposed in surgical approaches to the basilar apex, tentorial incisura, trigeminal nerve, cerebellopontine angle, pineal region, clivus, and the upper part of the cerebellum (18, 19).
The SCA is intimately related to the cerebellomesencephalic fissure, the superior half of the fourth ventricular roof, the superior cerebellar peduncle, and the tentorial surface (Figs. 2.3-2.5). The SCA arises in front of the midbrain, usually from the basilar artery near the apex, and passes below the oculomotor nerve, but may infrequently arise from the proximal PCA and pass above the oculomotor nerve. It dips caudally and encircles the brainstem near the pontomesencephalic junction, passing below the trochlear nerve and above the trigeminal nerve. Its proximal portion courses medial to the free edge of the tentorium cerebelli, and its distal part passes below the tentorium, making it the most rostral of the infratentorial arteries. After passing above the trigeminal nerve, it enters the cerebellomesencephalic fissure, where its branches make several sharp turns and give rise to the precerebellar arteries, which pass to the deep cerebellar white matter and the dentate nucleus. On leaving the cerebellomesencephalic fissure where its branches are again medial to the tentorial edge, its branches pass posteriorly under the tentorial edge and are distributed to the tentorial surface. It usually arises as a single trunk, but may also arise as a double (or duplicate) trunk. The SCAs arising as a single trunk bifurcate into a rostral and a caudal trunk. The SCA gives off perforating branches to the brainstem and cerebellar peduncles. Precerebellar branches arise within the cerebellomesencephalic fissure. The rostral trunk supplies the vermian and paravermian area and the caudal trunk supplies the hemisphere on the suboccipital surface. The SCA frequently has points of contact with the oculomotor, trochlear, and trigeminal nerves.
The SCA is divided into four segments: anterior pontomesencephalic, lateral pontomesencephalic, cerebellomesencephalic, and cortical (Fig. 2.1). Each segment may be composed of one or more trunks, depending on the level of bifurcation of the main trunk (Fig. 2.6).
Anterior Pontomesencephalic Segment
This segment is located between the dorsum sellae and the upper brainstem. It begins at the origin of the SCA and extends below the oculomotor nerve to the anterolateral margin of the brainstem. Its lateral part is medial to the anterior half of the free tentorial edge.
Lateral Pontomesencephalic Segment
This segment begins at the anterolateral margin of the brainstem and frequently dips caudally onto the lateral side of the upper pons (Figs. 2.1, 2.7, and 2.8). Its caudal loop projects toward and often reaches the root entry zone of the trigeminal nerve at the midpontine level. The trochlear nerve passes above the midportion of this segment. The anterior part of this segment is often visible above the tentorial edge, but the caudal loop usually carries it below the tentorium. This segment terminates at the anterior margin of the cerebellomesencephalic fissure. The basal vein and the PCA course above and parallel to this SCA.
This segment courses within the cerebellomesencephalic fissure (Figs. 2.7-2.9). The SCA branches enter the shallowest part of the fissure located above the trigeminal root entry zone and again course medial to the tentorial edge with its branches intertwined with the trochlear nerve. The fissure in which the SCA proceeds progressively deepens medially and is deepest in the midline behind the superior medullary velum. Through a series of hairpin-like curves, the SCA loops deeply into the fissure and passes upward to reach the anterior edge of the tentorial surface. The trunks and branches of the SCA are held in the fissure by branches that penetrate the fissure’s opposing walls. Identification of individual branches of the SCA within this fissure is made difficult by the sharp curves of the branches and by the large number of intermingled arterial loops.
This segment includes the branches distal to the cerebellomesencephalic fissure that pass under the tentorial edge and are distributed to the tentorial surface and, if a marginal branch is present, to the upper part of the petrosal surface (Figs. 2.6-2.9).
The SCA is the most consistent of the infratentorial cerebellar arteries in its presence and area of supply (49). Absence
of the SCA, although rare, has been reported (50). In our previous study of 50 SCAs, 43 arose as a single trunk and 7 arose as two (duplicate) trunks (19). Duplicate trunks were present bilaterally in only one of the brains we examined. Triplication of the origin is rare. All but 2 of the 50 SCAs examined arose from the basilar artery. The two exceptions arose solely or in part from the posterior cerebral artery and passed above the oculomotor nerve, after which they followed the typical distal course. The solitary trunk of nonduplicated SCAs and the rostral trunk of duplicate SCAs usually arise from the basilar artery below, but directly adjacent to, the origin of the PCA. The arteries not arising adjacent to the origin of the PCA arise within 2.5 mm of the PCA origin.
The origin of the right and left SCAs and PCAs frequently takes a cruciate configuration in which the limbs cross at the apex of the basilar artery (Fig. 2.2). The height of the bifurcation of the basilar artery is an important determinant of the initial course (47, 59). The level of the bifurcation of the basilar artery is normal if the bifurcation occurs at the pontomesencephalic junction, high if it occurs anterior to the mesencephalon, and low if it is anterior to the pons. The origin of the SCA is above the edge of the tentorium if the bifurcation is high, medial to the free edge if it is normal, and below the tentorium if it is low. In our study, the bifurcation was in a normal position in 18 of the 25 brains that we examined, high in 6, and low in 1. Three of the six arteries with a high bifurcation were associated with a fetal origin of the PCA (47).
The length of the basilar artery ranges from 20 to 40 mm (average, 30) and its diameter is greater at its origin from the vertebral arteries, range from 3 to 8 mm (average, 5–6 mm) than at its apex (range, 3–7 mm; mean, 4–5 mm). The basilar artery is usually straight or deviates a short distance off the midline, but a few will deviate laterally as far as the origin of the abducens nerve or the facial and vestibulocochlear nerves (18, 19).
All of the SCAs that arise as a single vessel bifurcate into two major trunks, one rostral and one caudal (Fig. 2.10). This bifurcation occurs between 0.6 and 34.0 mm (average, 19 mm) from the origin, most commonly near the point of maximal caudal descent of the artery on the lateral side of the brainstem. Rostral and caudal trunks are present in nearly every hemisphere as a result of either a duplicate origin or the bifurcation of a main artery. The rostral and caudal trunks formed by a duplicate origin, referred to as rostral and caudal duplicate SCAs, have a distribution equivalent to that of the rostral and caudal trunks formed by the bifurcation of a solitary SCA.
The rostral trunk terminates by supplying the vermis and a variable portion of the adjacent hemisphere. The caudal trunk supplies the hemispheric surface lateral to the area supplied by the rostral trunk. The diameters of the rostral and caudal trunks are approximately equal, but if one is smaller, it is usually the caudal trunk. If one trunk is small, the other supplies a larger area. The caudal trunk rarely sends branches to the vermis.
These perforating branches are divided into a direct and circumflex type (Fig. 2.7). The direct type pursues a straight course to enter the brainstem. The circumflex type winds around the brainstem before terminating in it. The circumflex perforating arteries are subdivided into short and long types. The short circumflex type travels 90 degrees or less around the circumference of the brainstem. The long circumflex type travels a greater distance to reach the opposite surface. Both types of circumflex arteries send branches into the brainstem along their course.
Perforating branches arise from the great majority of main, rostral, and caudal trunks. Most trunks give rise to two to five perforating branches, although some may give rise to no perforators and others to as many as 10. The most common type of perforating artery arising from the main trunk is the long circumflex type, but it also gives rise to direct and short circumflex branches. In descending order, the main trunk branches terminate in the tegmentum in the region of the junction between the superior and middle cerebellar peduncles, the interpeduncular fossa (usually the direct type), the cerebral peduncle, and the collicular region.
The branches from the rostral and caudal trunk are most frequently circumflex. They course around the brainstem to reach two main areas: the region of the junction of the superior and middle cerebellar peduncles and the quadrigeminal cistern below the sulcus between the superior and inferior colliculi. In descending order, they terminate in the junction between the superior and middle cerebellar peduncles, the inferior colliculus, the cerebral peduncle, and the interpeduncular fossa.
The basilar artery also gives rise to multiple perforating branches to the brainstem. Those arising near the origin of the SCA intermingle with the direct perforating branches arising from the proximal SCA. Those arising above the origin of the SCA enter the interpeduncular fossa.
The precerebellar arteries arise from the trunks and cortical branches within the cerebellomesencephalic fissure (Figs. 2.7- 2.9). As many as eight precerebellar arteries may arise within the fissure and these, along with the trunks and cortical branches and their sharp turns in the fissure, create a complexity that makes arterial dissection and identification difficult. These precerebellar branches tether the distal parts of the trunks and the proximal parts of the cortical arteries in the fissure. The precerebellar arteries consist of a medial group of small branches that pass between the superior medullary velum and the central lobule and a lateral group of larger branches that course between the superior and middle cerebellar peduncles and the wings of the central lobule. The cortical arteries supplying the hemispheric surface lateral to the vermis send precerebellar branches that reach the dentate and deep cerebellar nuclei, and those terminating in the vermis send branches to the inferior colliculi and the superior medullary velum.
The most constant cortical supply of the SCA is to the tentorial surface (Figs. 2.6-2.9). The cortical territory of the SCA is more constant than that of the AICA and PICA, but is reciprocal with them. The SCA usually supplies the majority of the tentorial surface and frequently the adjacent upper part of the petrosal surface. The maximal field of supply includes a full half of the tentorial surface with overlap onto the opposite half of the vermis, the superior part of the suboccipital surface, and the upper two-thirds of the petrosal surface, including both lips of the petrosal fissure. The smallest field of supply includes only the part of the tentorial surface that lies anterior to the tentorial fissure.
The cortical branches are divided into hemispheric and vermian groups (Fig. 2.7). The cortical surface of each half of the vermis is divided into medial and paramedian segments and each hemisphere lateral to the vermis is divided into medial, intermediate, and lateral segments, because the most frequent pattern includes two vermian arteries and three hemispheric arteries corresponding to these segments.
The hemispheric branches arise from the rostral and caudal trunks in the depths of the cerebellomesencephalic fissure. They give rise to the precerebellar arteries, which bind their proximal parts within the cerebellomesencephalic fissure. After leaving the fissure, the hemispheric branches proceed to supply the tentorial surface lateral to the vermis. The rostral and caudal trunks together most commonly give rise to three, but sometimes as many as five, hemispheric branches. There is a reciprocal relationship between the hemispheric arteries. If one is small, the adjacent ones are large and supply the territory normally supplied by the more rudimentary vessel.
The most common pattern is three hemispheric branches: lateral, intermediate, and medial corresponding to the third of the hemispheric surface that they supply. Each branch supplies approximately one-third of the tentorial surface of the hemisphere. However, there are frequent exceptions in which the hemispheric areas are supplied by two branches or by branches from the adjacent hemispheric segments. The medial segment is most frequently supplied from the rostral trunk and the lateral segment is most often supplied from the caudal trunk. The vermian arteries occasionally overlap onto the medial hemispheric segment, and the marginal artery (to be described later) overlaps the lateral hemispheric segment. The whole tentorial hemispheric surface was supplied by a branch of the caudal trunk in one hemisphere and by branches arising from the rostral trunk in one other hemisphere. On reaching the tentorial surface, the hemispheric arteries split into one to seven (average, three) sub-branches, which arborize over the tentorial surface and terminate by disappearing between the cerebellar folia.
The vermian arteries arise from the rostral trunk within the cerebellomesencephalic fissure. The rostral trunk most commonly gives rise to two vermian arteries (maximum four). If the vermian branches on one side are hypoplastic, their area is supplied by branches from the contralateral SCA. The most common pattern is two vermian arteries: one distributed to a medial strip bordering the midline and one distributed to a paramedian strip bordering the hemispheric surface. Anastomoses between vermian branches from the two sides are frequent near the apex of the tentorial surface.
About half of the proximal SCA trunks give rise to a marginal branch to the adjacent petrosal surface (Figs. 2.9 and 2.10). When present, the marginal branch is the first cortical branch. It usually arises from the lateral pontomesencephalic segment and does not enter the cerebellomesencephalic fissure, as do the other cortical branches, but passes from its origin to the cortical surface. It may also arise from the caudal or main trunk or from the basilar artery as a variant of a duplicate origin of the SCA. Its most constant supply is to the part of the petrosal surface adjoining the tentorial surface. Its largest area of supply includes the full extent of the superior part of the petrosal surface and both lips of the petrous fissure. Its area of supply is inversely related to the size of the petrosal surface area supplied by the AICA. The AICA or its branches supply the majority of the petrosal fissure if the marginal artery is small or absent. Anastomoses between the marginal artery and the AICA are frequent and are most prominent if the marginal branch is large. Perforating branches arising from the marginal branch terminate in the region of the middle cerebellar peduncle.
Relationship to the Cranial Nerves
The SCA passes near and frequently has points of contact with the oculomotor, trochlear, or trigeminal nerves (Figs. 2.2, 2.5, and 2.8).
The proximal part of the SCA passes below and is separated from the PCA by the oculomotor nerve (Fig. 2.5). Nearly two-thirds of SCAs have a point of contact with the oculomotor nerve, usually on the inferior surface. The point of contact usually involves the main trunk or, less commonly, the rostral trunk if there is an early bifurcation. This is a contact on the superior surface of the nerve only if the SCA arises from the PCA, as occurs infrequently. Sunderland suggests that the oculomotor nerve may occasionally be constricted between the PCA and SCA (52).
The length of vessel between its origin and its point of contact with the oculomotor nerve averages 4.5 mm (range, 1–9 mm) and the length of the nerve between its origin from the midbrain and the point of contact with the SCA averages 5 mm (range, 1–10 mm) (19). The diameter of the artery at the point of contact averages 2 mm (range, 1–3 mm). There is less likely to be a point of contact with the oculomotor nerve if there is a duplicate origin, a low origin from the basilar artery, or a fetal configuration of the PCA.
The trochlear nerve arises below the inferior colliculus and passes forward in the cerebellomesencephalic fissure (Figs. 2.4, 2.5, and 2.10). It passes from the medial to the lateral side of the branches of the rostral and caudal trunks as it passes forward within the fissure. On reaching the lateral side of the brainstem, it courses between the lower surface of the tentorium and the SCA. The nerve has points of contact with the SCA trunks in almost all cases. This contact may involve the main, rostral, or caudal trunk, or both the rostral and caudal trunks. The point of contact with the nerve averages 17 mm (range, 4–30 mm) from the origin of the nerve and 24 mm (range, 13–38 mm) from the origin of the SCA (18).
The trigeminal nerve arises from the lateral part of the pons and runs obliquely upward (Figs. 2.8 and 2.10). It exits the posterior cranial fossa by passing forward beneath the tentorial attachment to enter Meckel’s cave. The SCA encircles the brainstem above the trigeminal nerve, making a shallow caudal loop on the lateral side of the pons (18). Contact occurs between the SCA and the trigeminal nerve in those cases with the most prominent caudally projecting loops. About half of the SCAs have a point of contact with the SCA, which, depending on the site of bifurcation, may involve the main, rostral, caudal or both the rostral and caudal trunks, or a marginal hemispheric branch. The diameter of the vessel at the point of contact averages 1 to 2 mm, but may range from less than 2 to nearly 3 mm. The distance between the origin of the vessel and the point of contact with the trigeminal nerve varies from 15 to 33 mm (average, 21 mm). The separation between the SCA and the 24 trigeminal nerves, without a neurovascular contact ranges from less than 1 to 8 mm (average, 3 mm).
The point of contact with the SCA is usually on the superior or superomedial aspect of the nerve. Often a few fascicles of the nerve are indented or distorted by the vessel 3 to 4 mm, but as much as 12 mm peripheral to the point of entry into the pons. In 6 of the 50 specimens we examined, the contact was located at the pontine root entry zone, usually by a loop tucked into the axilla formed between the brainstem and the medial side of the trigeminal nerve. There is no correlation between the configuration of the SCA at its origin and the presence or absence of loops impinging upon the trigeminal nerve; however, the point of bifurcation of the SCA did affect the caliber of the vessel that made contact with the nerve. The contacting vessel is of a smaller caliber if there is an early SCA bifurcation. The significance of these contacts in trigeminal neuralgia is reviewed in the chapter on the cerebellopontine angle (7, 16, 22, 45).
Relationship to the Tentorium Cerebelli
The tentorium incisura (notch), the opening through the tentorium cerebelli, is triangular with the base on the clivus (Figs. 2.6, 2.8, and 2.9) (41). The other two limbs are formed by the right and left free edges that join at an apex located between the colliculi below the occipital lobes above.
The proximal portion of the SCA, usually the main trunk unless there is a duplicate origin or an early bifurcation, courses medial to the anterior third of the free edge. The SCAs with a high origin arise superior to the level of the tentorial edge, but the initial course of all of these slopes caudally. Nearly 20% of SCAs have a point of contact with the free edge of the anterior half of the tentorium. Distally, the SCA loops caudally and passes beneath, sometimes contacting the middle third of the free edge of the tentorium. The interval between the free edge and the SCA as the SCA passes below the free edge averages 3 mm (range, 0–5 mm). The part nearest the lower surface of the free edge is the main trunk in most cases, but may be the rostral or caudal trunk if there is an early bifurcation. Further distally, branches pass medial to the posterior third of the free edge as they enter and exit the cerebellomesencephalic fissure. These branches remain caudal to the level of the free edge in the interval between the colliculi and the occipital lobe, but distally, pass below the tentorium to reach the superior surface of the cerebellum.
The effects of occlusion of a cerebellar artery range from clinical silence to infarction of portions of the brainstem or cerebellum with swelling, hemorrhage, and death (3, 18, 19, 30). Occlusion of the SCA, although uncommon, produces a distinctive clinical picture that results from infarction of the cerebellum, dentate nucleus, brachium conjunctivum, and long sensory pathways in the tegmentum of the rostral pons (32). The onset is marked by vomiting, sudden dizziness, and the inability to stand or walk. Occlusion may result in cerebellar dysfunction caused by involvement of the cerebellum and its deep nuclei and peduncles; ipsilateral intention tremor caused by involvement of the dentate nucleus and the superior cerebellar peduncle; ipsilateral Horner’s syndrome caused by involvement of the descending oculosympathetic fibers; contralateral loss of pain and temperature sensation caused by involvement of the lateral spinothalamic and quintothalamic tracts; nystagmus caused by involvement of the medial longitudinal fasciculus and cerebellar pathways; contralateral disturbance of hearing caused by involvement of the crossed fibers of the lateral lemniscus; and loss of emotional expression on the analgesic side caused by damage to the involuntary mimetic pathways in the upper brainstem. Although a specific clinical syndrome may result from an SCA occlusion, it is worth emphasizing that in the posterior fossa, a given area of parenchyma cannot be as predictably allotted to a specific vessel as in the cerebral circulation, because of the extensive anastomoses over the cerebellum and the variation in arterial distribution.
The recovery and survival of many patients after the intentional occlusion of a major cerebellar artery is attributed to adequacy of the collateral circulation. If the adjacent arteries are unusually small and the artery occluded is large, the collateral circulation is likely to be poor, creating an unfavorable and dangerous situation. Arterial spasm caused by mechanical irritation induced by brain retraction may render the collateral supply less effective. Acute occlusion of any one of the cerebellar arteries is frequently associated with vomiting, dizziness, and the inability to stand or walk.
The SCA is important in both hemorrhagic and ischemic cerebrovascular disease of the posterior fossa. The dentate nucleus, the most common site of spontaneous cerebellar hemorrhage, is supplied by the precerebellar and the penetrating cortical branches of the SCA (8, 49). The area supplied by the SCA is postulated to be the most vulnerable to damage by decreased blood flow in the posterior fossa, because it represents the distal borderline of the vertebral and basilar arteries (49). Infarcts may occur in the area supplied by the SCA in the absence of its occlusion, after occlusion of the vertebral or basilar arteries.
The SCA and its branches may be stretched against the tentorial edge by expanding lesions in the posterior fossa that cause a rostral protrusion of the upper surface of the cerebellum through the tentorial opening. The surface of the vermis and adjacent parts of the lateral lobes are grooved by the free edge of the tentorium, and branches of the SCA may thus be compressed. Symmetrical softening of the cerebellar cortex in the area of supply will result, and similar changes may be found in the dentate nuclei that are supplied by the deep branches (46).
The SCA is exposed in dealing with neoplasms involving the cerebellum, posterior cavernous sinus, tentorial incisura, and cerebellopontine angle; with aneurysms arising at the basilar apex, origin of the SCA and PCA, and, although rare, on the distal SCA; less commonly in dealing with arteriovenous malformations; during vascular decompression of the trigeminal nerve in trigeminal neuralgia; and during a revascularization bypass procedure for posterior fossa ischemia.
Selecting an operative approach to a lesion involving the SCA requires that the arterial segments involved be accurately defined. Lesions located at the front of the brainstem near the origin require a different approach from those located on the back of the brainstem in the quadrigeminal cistern or cerebellomesencephalic fissure. The only supratentorial approach that provides exposures to the SCA origin, anterior and lateral pontomesencephalic and cerebellomesencephalic segments, and the proximal cortical branches is a temporal craniotomy with elevation of the temporal and occipital lobes combined with division and retraction of the tentorium. Extending this approach backward to the quadrigeminal cistern often necessitates obliteration of some of the veins draining the lower surface of the temporal and occipital lobes, with the risk of venous infarction and edema. A similar or even greater exposure of the SCA is achieved with the supra-infratentorial presigmoid approach with tentorial splitting, but this is a much more extensive operation. When the tentorium is divided in either of the above approaches, care must be taken to prevent injury to the trochlear nerve that passes between the lateral pontomesencephalic segment and the tentorial edge. The SCA origin, along with the basilar apex, if located above the dorsum sellae, can be reached through a pterional craniotomy with opening of Liliequist’s membrane. Exposing a low SCA origin by the pterional route may require that the dura roof of the cavernous sinus be opened, a so-called transcavernous approach, and that the posterior clinoid and upper part of the dorsum sellae be removed. Resecting the petrous apex in the subtemporal anterior petrousectomy approach will also aid in exposing a low SCA origin, if it cannot be exposed by dividing the tentorium. A lateral suboccipital craniectomy or, as this writer prefers, a craniotomy, done through a vertical lateral suboccipital incision and extending to the edge of the transverse and sigmoid sinuses, provides excellent exposure of the SCA in the region of the trigeminal nerve and the anterior part of the cerebellomesencephalic fissure. This approach provides satisfactory exposure of the lateral pontomesencephalic segment, but not of the origin or of other segments. An infratentorial- supracerebellar approach directed through a suboccipital craniectomy provides satisfactory exposure of the cortical branches, but not those within the depths of the cerebellomesencephalic fissure or lateral to the brainstem. The occipital transtentorial approach provides a more favorable angle for exposing the branches ipsilateral to the craniotomy near the midline, below the pineal within the cerebellomesencephalic fissure, and in the posterior part of the ambient cistern.
Anteroinferior Cerebellar Artery
The AICA courses through the central part of the cerebellopontine angle near the facial and vestibulocochlear nerve (Figs. 2.5 and 2.11). It or its branches may be exposed in surgical approaches to cerebellopontine angle, basilar or vertebral arteries, clivus, the fourth ventricle and cerebellum, and during approaches directed through the temporal and occipital bones.
The AICA is intimately related to the pons, lateral recess, foramen of Luschka, cerebellopontine fissure, middle cerebellar peduncle, and petrosal cerebellar surface (Figs. 2.1-2.3 and 2.11). The AICA originates from the basilar artery, usually as a single trunk, and encircles the pons near the abducent, facial, and vestibulocochlear nerves. After coursing near and sending branches to the nerves entering the acoustic meatus and to the choroid plexus protruding from the foramen of Luschka, it passes around the flocculus on the middle cerebellar peduncle to supply the lips of the cerebellopontine fissure and the petrosal surface. It commonly bifurcates near the facial-vestibulocochlear nerve complex to form a rostral and a caudal trunk. The rostral trunk sends its branches laterally along the middle cerebellar peduncle to the superior
lip of the cerebellopontine fissure and the adjoining part of the petrosal surface, and the caudal trunk supplies the inferior part of the petrosal surface, including a part of the flocculus and the choroid plexus. The AICA gives rise to perforating arteries to the brainstem, choroidal branches to the tela and choroid plexus, and the nerve-related arteries, including the labyrinthine, recurrent perforating, and subarcuate arteries (34).
The AICA is divided into four segments: anterior pontine, lateral pontine, flocculonodular, and cortical. Each segment may include more than one trunk, depending on the level of bifurcation of the artery (Fig. 2.1).
Anterior Pontine Segment
This segment, located between the clivus and the belly of the pons, begins at the origin and ends at the level of a line drawn through the long axis of the inferior olive and extending upward on the pons. This segment usually lies in contact with the rootlets of the abducent nerve.
Lateral Pontine Segment
This segment begins at the anterolateral margin of the pons and passes through the cerebellopontine angle above, below, or between the facial and vestibulocochlear nerves and is intimately related to the internal auditory meatus, the lateral recess, and the choroid plexus protruding from the foramen of Luschka (Figs. 2.11 and 2.12). This segment gives rise to the nerve-related branches that course near or within the internal acoustic meatus in close relationship to the facial and vestibulocochlear nerves. This segment is divided into premeatal, meatal, and postmeatal parts, depending on their relationship to the porus of the internal acoustic meatus (Fig. 2.5). These nerve-related branches are the labyrinth artery, which supplies the facial and vestibulocochlear nerves and vestibulocochlear labyrinth; the recurrent perforating arteries, which pass toward the meatus, but turn medially to supply the brainstem; and the subarcuate artery, which enters the subarcuate fossa. This segment not uncommonly dips below the pontomedullary junction, especially if it is tortuous.
This segment begins where the artery passes rostral or caudal to the flocculus to reach the middle cerebellar peduncle and the cerebellopontine fissure (Fig. 2.11). The trunks that course along the peduncle may be hidden beneath the flocculus or the lips of the cerebellopontine fissure.
This segment supplies predominantly the petrosal surface.
The AICA usually originates from the basilar artery as a single vessel, but may also arise as two (duplicate) or three (triplicate) arteries (Figs. 2.2, 2.3, and 2.11). It can arise at any point along the basilar artery, but most commonly arises from the lower half. There is frequent asymmetry in the level of origin from side to side, with one arising significantly above the level of the other. In our previous study, we found that of 50 AICAs 72% arose as a single trunk, 26% as two (duplicate) arteries, and 2% as three (triplicate) arteries (34). From its origin, the AICA courses backward around the pons toward the CPA. Its proximal part lays in contact with either the dorsal or the ventral aspect of the abducens nerve. After passing the abducens nerve, it proceeds to the CPA where one or more of its trunks course in close relationship to the facial and vestibulocochlear nerves and thus are said to be nerve-related.
The AICAs arising as a single trunk usually bifurcate into a rostral and a caudal trunk. The duplicate AICAs referred to as rostral and caudal duplicate AICAs have a distribution similar to the distribution of the rostral and caudal trunks formed by the bifurcation of a single AICA. Approximately two-thirds bifurcated before and one-third bifurcated after crossing the facial and vestibulocochlear nerves. The segment proximal to the bifurcation is the main trunk, and the two trunks formed by the bifurcation are the rostral and the caudal trunks. If the bifurcation is proximal to the facial and vestibulocochlear nerves, either the rostral trunk alone or both of the postbifurcation trunks may be nerve-related. The rostral duplicate AICAs give rise to nerve-related branches more often than the caudal duplicate AICAs. The main trunk of the duplicate AICAs also commonly bifurcate to form rostral and caudal trunks that sent branches to the cerebellum.
After crossing the nerves, the rostral trunk usually courses laterally above the flocculus to reach the surface of the middle cerebellar peduncle and the petrosal fissure to be distributed to the superior lip of the cerebellopontine fissure and the adjoining part of the petrosal surface. The caudal trunks are frequently related to the lateral portion of the fourth ventricle. If the bifurcation is proximal to the facial and vestibulocochlear nerve, the caudal trunk courses caudal to the flocculus to supply the inferior part of the petrosal surface, including a part of the flocculus and the choroid plexus. If the bifurcation is distal to the nerves, the caudal trunk courses posteriorly in the inferior limb of the cerebellopontine fissure near the foramen of Luschka. The caudal trunks often enter the lateral portion of the cerebellomedullary fissure just below the lateral recess before turning laterally to supply the inferior part of the petrosal surface. The distal branches of the caudal trunk often anastomose with the PICA, and those from the rostral trunk anastomose with the SCA. The AICA gives rise to perforating arteries to the brainstem, choroidal branches to the lateral segment of the choroid plexus, and the nerve-related arteries described above.
The nerve-related branches are those that course in or near the porus of the meatus and by the facial and vestibulocochlear nerves (Figs. 2.5 and 2.11-2.14) (34). Each nerve-related segment is composed of one or two arterial trunks. One was most common. The single nerve-related segments were formed from either the main or a rostral trunk, which arise, in decreasing order of frequency, from a solitary AICA, a rostral duplicate AICA, or a caudal duplicate AICA. The double segments result from the presence of one of two anatomic configurations: a) both the rostral and caudal trunks of a solitary AICA or of one duplicate AICA are nerve-related, or b) one trunk from each of duplicate AICAs or one trunk from two of three triplicate AICAs is nerve related.
This segment begins at the basilar artery and courses around the brainstem to reach the facial and vestibulocochlear nerves and the anterior edge of the meatus. The premeatal segment is composed of one or two arterial trunks. In the 50 CPAs we examined, there were 56 nerve-related premeatal segments, 44 CPAs (88%) had solitary, and 6 (12%) had double premeatal segments (34). Most of the premeatal segments, 46 of the 56, were anteroinferior to the nerves. The remainder were anterior, inferior, or anterosuperior to the nerves (Fig. 2.14).
This segment, located in the vicinity of the internal auditory meatus, often forms a laterally convex loop, the medial loop, directed toward or through the meatus. The medial segment was located medial to the porus in about half of CPAs and formed a loop that reached the porus or protruded into the canal in the other half. Sunderland and Mazzoni found the meatal segment at the porus or within the canal in 64 and 67% of CPAs, respectively (36, 51). Mazzoni found that the meatal segment was medial to the porus in 33%, reached the porus in 27%, and entered the canal in 40%, rarely going beyond the medial half of the canal (36).
In the 50 CPAs examined, we found there were 59 nerve-related meatal segments; 41 CPAs (82%) had one, and 9 (18%) had two meatal segments. The majority of the meatal segments coursed below or between the facial and vestibulocochlear nerves (Fig. 2.14). There were three more meatal segments than premeatal segments, because in three CPAs, a premeatal segment bifurcated near the nerves to yield two nerve-related meatal segments. The majority of meatal loops coursed in a horizontal plane above or below the nerves, but some, mostly those passing between the facial and vestibulocochlear nerves, coursed in a vertical or oblique plane.
In some CPAs, the nerve-related loop formed a second laterally convex curve that gave the loop an “M” configuration. This second loop was called the subarcuate loop, because it was directed toward the subarcuate fossa, a small depression in the bone superolateral to the meatus. This loop was located either posterior, posteroinferior, or posterosuperior to the vestibulocochlear nerve. The apex of the loop was occasionally adherent to the dura over the subarcuate fossa at the point where the subarcuate artery arose.
This segment begins distal to the nerves and courses medially to supply the brainstem and the cerebellum. The 59 meatal segments found in our previous study of 50 CPAs gave rise to 60 postmeatal segments; 80% of the CPAs had one, and 10 (20%) had two postmeatal segments. There was one more postmeatal segment than meatal segment, because one meatal segment bifurcated to form two postmeatal segments. The postmeatal segments were most commonly posteroinferior, superior, or posterior to or between the nerves (Fig. 2.14); none were anterior to the nerves. Each of the vessels forming a double segment might pursue similar or separate courses in relation to the nerves.
Branches of Nerve-related AICAs
In their course through the CPA, the nerve-related trunks gives off four branches (Figs. 2.12-2.14): 1) labyrinthine (internal auditory) arteries, which enter the internal auditory canal and reached the inner ear; 2) recurrent perforating arteries, which course medially from their origin to supply the brainstem; 3) subarcuate arteries, which passed through the subarcuate fossa to reach the subarcuate canal; and 4) cerebellosubarcuate arteries, which terminated by sending one branch to the subarcuate canal and one to the cerebellum.
Labyrinthine (Internal Auditory) Arteries
These arteries are the one or more branches of the AICA that enter the internal auditory canal and send branches to the bone and dura lining the internal auditory canal, to the nerves within the canal, and terminate by giving rise to the vestibular, cochlear, and vestibulocochlear arteries that supply the organs of the inner ear (Figs. 2.12-2.14) (34).
The labyrinthine arteries almost always arise from the AICA or one of its branches, although a few have been reported to arise from the basilar artery. In one study, as many as 17% were found to arise from the basilar artery (40, 51, 56). We believe that this discrepancy is explained by differences in the definition of the internal auditory artery and the AICA used in the various studies. In this study and those of Adachi and Fisch, the trunk of origin on the basilar artery of an artery sending a branch to the internal auditory canal was called an AICA if it sent branches, although small, to the cerebellum. The site of origin of the internal auditory artery was defined as the point where the branch to the internal auditory canal arose from the trunk of the AICA sending branches to the cerebellum (1, 13). On the other hand, Nager and Sunderland called a trunk arising from the basilar artery a labyrinthine artery rather than an AICA if the branch entering the meatus was larger than the branch reaching the cerebellum (40, 51). Adachi and Fisch, who did not find a single internal auditory artery that arose from the basilar artery, were always able to find a cerebellar branch, although small, on the vessel entering the meatus (1, 13). Mazzoni reported that the internal auditory artery arose from the PICA in a few cases (36), a finding not confirmed in our study or in the other studies mentioned above. In our study, there was one internal auditory artery in 30% of the CPAs, two in 54%, three in 14%, and four in 2%.
Of the 94 internal auditory arteries found in our study, in 50 CPAs, 72 (77%) originated from the premeatal segment, 20 (21%) from the meatal segment, and 2 (2%) from the post-meatal segment (34). They arose proximal to the subarcuate loop in each CPA in which the latter loop was present. Fifty- four percent originated from a solitary AICA, 23% from a duplicate or triplicate AICA, and 23% from a recurrent perforating artery. Mazzoni and Hansen also noted that the internal auditory artery may arise from the recurrent perforating, subarcuate, or cerebellosubarcuate arteries (37).
The internal auditory arteries are divided into two approximately equal-sized groups based on their relationship to the meatus. One group originates medial to the porus and the other arises at the porus or within the auditory canal. Those arising medial to the porus most commonly originate and course anterior, anteroinferior, or inferior to the nerves. Fisch noted that the internal auditory arteries often entered the canal by crossing the anteroinferior rim of the porus (13). Those arising at the porus or within the canal most commonly originate inferior or anteroinferior to the nerves.
Recurrent Perforating Arteries
These perforating arteries arise from the nerve-related vessels and often travel from their origin toward the meatus, occasionally looping into the meatus before taking a recurrent course along the facial and vestibulocochlear nerves to reach the brainstem (Figs. 2.5 and 2.14). They send branches to these nerves and to the brainstem surrounding the entry zone of those nerves. They also send branches, in decreasing order of frequency, to the middle cerebellar peduncle and the adjacent part of the pons, the pons around the entry zone of the trigeminal nerve, the choroid plexus of the CPA, the superolateral medulla, and the glossopharyngeal and vagus nerves. The recurrent perforating arteries give rise to about one- fourth of the internal auditory arteries and 10% of subarcuate arteries.
In our study, recurrent perforating arteries were present in 41 (82%) of the CPAs; one was present in 37 CPAs (74%), two in 3 (6%), and three in 1 (2%) (34). Most arose from the premeatal segment, but they also arose from the meatal loop and the postmeatal segment. There was marked variability in their relationship to the facial and vestibulocochlear nerves. Most originated inferior, anteroinferior or anterior to or between the nerves and coursed medially between or above or below the nerves (Fig. 2.14).
The subarcuate artery usually originates medial to the porus, penetrates the dura covering the subarcuate fossa, and enters the subarcuate canal (Figs. 2.13 and 2.14). In a few cases, it originates in the internal auditory canal. The subarcuate arteries originating in the auditory canal take one of two courses to reach the subarcuate canal; some take a recurrent course through the porus to reach the subarcuate fossa, and others penetrated the meatal wall to reach the subarcuate canal. The artery supplies the petrous bone in the region of the semicircular canals (43). The subarcuate canal is recognized as a potential route of extension of infections from the mastoid region to the meninges and the superior petrosal sinus (40). The AICA is adherent to the dura lining the subarcuate fossa at the site of origin of the subarcuate artery in a few CPAs.
In our study, a subarcuate artery was present in 36 (72%) of the 50 CPAs; 13 (26%) originated from the premeatal segment, 2 (4%) from the meatal segment, and 21 (42%) from the postmeatal segment (34). When present, there was only one subarcuate artery. Most originated posterior and coursed posterosuperior to the nerves to reach the subarcuate fossa. Those originating anterior, inferior, or anteroinferior to the facial nerve crossed inferior to the facial and vestibulocochlear nerves to reach the subarcuate fossa (Fig. 2.14).
Nager noted that the subarcuate artery is mentioned rarely in descriptions of the arteries in this area (40). This is probably because the artery and its connection to the bone were destroyed when the brain was removed from the skull. Nager found that its most frequent site of origin was the labyrinthine artery rather than the AICA, a difference explained by the difference in definitions of the AICA and the internal auditory artery previously mentioned (40). He reported that the subarcuate artery may also have a double origin; one branch may enter the subarcuate canal by penetrating the subarcuate fossa and the other may penetrate the wall of the internal auditory canal to reach the subarcuate canal.
The cerebellosubarcuate artery is a small branch of the AICA that sends one branch to the subarcuate fossa and another to the cerebellum, as reported by Mazzoni (37). It usually originates proximal to the meatal loop, passing inferior to the facial and vestibulocochlear nerves before coursing superolateral to reach the subarcuate fossa. At the fossa, it gives rise to a subarcuate artery and turns medially to supply the cerebellum. A cerebellosubarcuate artery was present in four of the CPAs we investigated (34). The artery originates anteroinferior or inferior to the nerves entering the meatus. The cerebellar branch terminates on the flocculus and on the adjacent cerebellar cortex below the flocculus.
The most common pattern is for the AICA to supply the majority of the petrosal surface, but the cortical area of the supply is quite variable (Fig. 2.11). It can vary from a small area on the flocculus and adjacent part of the petrosal surface to include the whole petrosal surface and adjacent part of the tentorial and suboccipital surfaces. After crossing the nerves, the rostral trunk usually courses above the flocculus to be distributed to the superior lip of the cerebellopontine fissure, and the caudal trunks course caudal to the flocculus to supply the inferior part of the petrosal surface. If the PICA is absent, the caudal trunk may supply almost all of the ipsilateral suboccipital hemisphere and vermis. Overlap of the SCA onto the upper part of the petrosal surface and the PICA onto the lateral part of the suboccipital surface in not uncommon.
Occlusion of the AICA results in syndromes related predominantly to softening of the lateral portions of the brainstem and cerebellar peduncles, rather than to involvement of the cerebellar hemisphere, including palsies of the facial and vestibulocochlear nerves caused by involvement of the nerves and their nuclei; vertigo, nausea, vomiting, and nystagmus caused by lesions of the vestibular nuclei and their connections with the nuclei of the vagus nerves; ipsilateral loss of pain and temperature sensation on the face and corneal hypesthesia caused by interruption of the spinal tract and nucleus of the trigeminal nerve; Horner’s syndrome caused by interruption of the descending pupillodilator fibers in the lateral portion of the pons and medulla; cerebellar ataxia and asynergia ascribed to a lesion in the cerebellar peduncles; and an incomplete loss of pain and temperature sensation on the contralateral half of the body (the absence of a complete contralateral hypalgesia is caused by the extreme lateral and posterior position of the lesion, which spares a portion of the lateral spinothalamic tract) (2, 3). All of the syndromes caused by its occlusion are not identical, because of the variability of the AICA. The symptoms usually are sudden in onset and unaccompanied by a loss of consciousness (2). The most prominent symptom is vertigo, often associated with nausea and vomiting, followed by a facial paralysis, deafness, sensory loss, and cerebellar disorders. Notable by their absence are signs of involvement of the corticospinal tract and medial lemniscus, which are nourished from midline tributaries of the vertebral and basilar arteries.
The recovery and survival of many patients after the intentional occlusion of the AICA at operation is attributed to adequacy of the collateral circulation from the other cerebellar arteries (34). The size of the area of infarction after AICA occlusion is inversely related to the size of the PICA and SCA and to the size of the anastomoses with those arteries. If the PICA is unusually small and the AICA is large, the collateral circulation is likely to be poor, creating an unfavorable and dangerous situation in the event of AICA occlusion. Arterial spasm caused by mechanical irritation induced by the brain retractor used during tumor removal may render the collateral supply less effective.
The AICA is most commonly exposed in operations for tumors of the cerebellopontine angle. Aneurysms involving the AICA are rare and if not located at the origin, are most likely located at or near the internal acoustic meatus (25, 31). The displacement and management of the nerve-related arteries with acoustic neuromas are reviewed in greater detail in the chapter on the cerebellopontine angle. Arteriovenous malformations located infratentorially are uncommon compared with those in supratentorial locations, and not infrequently involve the other cerebellar arteries, in addition to the AICA and the brainstem, thus increasing the management risk (9, 39, 44). Compression of the facial and vestibulocochlear nerves by tortuous arteries is postulated to cause dysfunction of these nerves, a concept that is reviewed in Chapter Four on the cerebellopontine angle (18, 19, 34).
The AICA may be approached by a lateral suboccipital (retrosigmoid), middle fossa, translabyrinthine or combined supra-infratentorial presigmoid approach. The suboccipital exposure is excellent for lesions involving the meatal and postmeatal segments of the AICA, the lateral part of the mid- and lower brainstem below the trigeminal nerve, and the area near the internal acoustic meatus. A subtemporal middle fossa approach, with division of the tentorium and possibly combined with a medial petrosectomy, may be selected for lesions in which the AICA has a high origin, or also involves the SCA and basilar arteries and is medial to the trigeminal nerve. In the middle fossa approach to the internal meatus, only a short segment of the artery located near the meatus is exposed and sometimes only if the artery loops into the meatal perus. The translabyrinthine approach exposes the AICA, at and for a short distance proximal and distal to the internal acoustic meatus and along the anterior part of the petrosal surface. The supra-infratentorial presigmoid approaches with various degrees of resection of the semicircular canals, vestibule, and cochlea may be selected for lesions located deep in front of the brainstem, especially those located near the AICA origin. The AICA origin may be exposed in the anterior approaches directly through the clivus only if the origin is near the midline, but not if the origin is from a tortuous basilar artery that loops laterally into the cerebellopontine angle lateral to the medial aspect of the cavernous sinus and petrous carotid, which limit the lateral extent of the anterior exposures of the prepontine cistern.
Posteroinferior Cerebellar Artery
The PICA has the most complex, tortuous, and variable course and area of supply of the cerebellar arteries. It may be exposed in surgical approaches to the foramen magnum, fourth ventricle, cerebellar hemisphere, brainstem, jugular foramen, cerebellopontine angle, petrous apex, and clivus (30).
The PICA is intimately related to the cerebellomedullary fissure, the inferior half of the ventricular roof, the inferior cerebellar peduncle, and the suboccipital surface (Figs. 2.1- 2.5). The PICA, by definition, arises from the vertebral artery near the inferior olive and passes posteriorly around the medulla. At the anterolateral margin of the medulla, it passes rostral or caudal to or between the rootlets of the hypoglossal nerve, and at the posterolateral margin of the medulla it courses rostral to or between the fila of the glossopharyngeal, vagus, and accessory nerves. After passing the latter nerves, it courses around the cerebellar tonsil and enters the cerebellomedullary fissure and passes posterior to the lower half of the roof of the fourth ventricle. On exiting the cerebellomedullary fissure, its branches are distributed to the vermis and hemisphere of the suboccipital surface. Its area of supply is the most variable of the cerebellar arteries (26). Most PICAs bifurcate into a medial and a lateral trunk. The medial trunk supplies the vermis and adjacent part of the hemisphere, and the lateral trunk supplies the cortical surface of the tonsil and the hemisphere. The PICA gives off perforating, choroidal, and cortical arteries. The cortical arteries are divided into vermian, tonsillar, and hemispheric groups.
The PICA is divided into five segments: 1) anterior medullary, 2) lateral medullary, 3) tonsillomedullary 4) telovelotonsillar, and 5) cortical (Figs. 2.1 and 2.15). These segments are often longer than the distance around the medulla or the tonsil because the PICA frequently has a tortuous course and forms complex loops on the side of the brainstem among the lower cranial nerves, near the tonsil, and caudal to the roof of the fourth ventricle. Each segment may include more than one trunk, depending on the level of bifurcation of the artery.
Anterior Medullary Segment
This segment lies anterior to the medulla. It begins at the origin of the PICA anterior to the medulla and extends backward past the hypoglossal rootlets to the level of a rostro-caudal line through the most prominent part of the inferior olive that marks the boundary between the anterior and lateral surfaces of the medulla. Those PICAs arising lateral rather than anterior to the medulla do not have an anterior medullary segment. An anterior medullary segment is more likely to be present if the PICA arises from the superior part of the vertebral artery, because the vertebral artery courses from the lateral side of the medulla below to the anterior surface of the medulla above. An anterior medullary segment is present if the vertebral artery at the level of origin of the PICA has passed to the anterior surface of the brainstem. From its origin, the PICA usually passed posteriorly around or between the hypoglossal rootlets, but occasionally loops upward, downward, laterally, or medially before passing posteriorly around or between the hypoglossal rootlets.
Lateral Medullary Segment
This segment begins where the artery passes the most prominent point of the olive and ends at the level of the origin of the glossopharyngeal, vagus, and accessory rootlets. This segment is present in most PICAs. Its course varies from passing directly posterior to reach the glossopharyngeal, vagal, and accessory rootlets to ascending, descending, or passing laterally or medially to form one or more complex loops in the cistern on the side of the brainstem before passing between these nerves.
This segment begins where the PICA passes posterior to the glossopharyngeal, vagus, and accessory nerves and extends medially across the posterior aspect of the medulla near the caudal half of the tonsil (Figs. 2.3, 2.4, 2.15, and 2.16). It ends where the artery ascends to the midlevel of the medial surface of the tonsil. The proximal portion of this segment usually courses near the lateral recess and then posteriorly to reach the inferior pole of the tonsil. This segment commonly passes medially between the lower margin of the tonsil and the medulla before turning rostrally along the medial surface of the tonsil. The loop passing near the lower part of the tonsil, referred to as the caudal or infratonsillar loop, has been reported to form a caudally convex loop that coincides with the caudal pole of the tonsil, but it may also course superior or inferior to the caudal pole of the tonsil without forming a loop. In some cases it dips below the caudal margin of the tonsil and even below the level of the foramen magnum. A caudally convex loop is not present if the PICA passes directly medial between the tonsil and medulla, if the PICA ascends along the lateral surface of the tonsil to reach the hemispheric surface, or if the artery has a low origin from the vertebral artery and ascends posterior to the medulla to reach the tonsil (Fig. 2.17).
The relationships between the tonsillomedullary segment and the cerebellar tonsil and foramen magnum varies (Fig. 2.17). In our previous study of 42 PICAs, the caudal limit of this segment was located superior to the caudal pole of the tonsil in 23, inferior in 8, and at the same level in 11 (30). This segment passed medially in a location 10.0 mm inferior to 13.0 mm superior (average, 1.6 mm superior) to the caudal tip of the tonsil. The caudal limit of this segment was superior to the foramen magnum in 37 PICAs, inferior in 4, and at the same level in 1. It was located 7.0 mm inferior to 18.0 mm superior (average, 6.9 mm superior) to the foramen magnum.
This is the most complex of the segments. It begins at the midportion of the PICA’s ascent along the medial surface of the tonsil toward the roof of the fourth ventricle and ends where it exits the fissures between the vermis, tonsil, and hemisphere to reach the suboccipital surface (Figs. 2.15-2.18). In most, but not all, hemispheres, this segment often forms a loop with a convex rostral curve, called the cranial loop (20, 38, 57). This loop is located caudal to the fastigium between the cerebellar tonsil below and the tela choroidea and posterior medullary velum above. The apex of the cranial loop usually overlies the central part of the inferior medullary velum, but its location varies from the superior to the inferior margin and from the medial to the lateral extent of the inferior medullary velum. The apex of the cranial loop is inferior to the level of the fastigium of the fourth ventricle in most cases, but may also extend to the level of the fastigium. This segment gives rise to branches that supply the tela choroidea and choroid plexus of the fourth ventricle.
This segment begins where the trunks and branches leave the groove between the vermis medially and the tonsil and the hemisphere laterally, and includes the terminal cortical branches. The bifurcation of the PICA often occurs near the origin of this segment. The cortical branches radiate outward from the superior and lateral borders of the tonsil to the remainder of the vermis and hemisphere.
The PICA Origin and the Vertebral Artery
The PICA is defined here, in agreement with others, as the cerebellar artery that arises from the vertebral artery (Figs. 2.19 and 2.20) (49, 55). The PICA is less commonly defined as the cerebellar artery that supplies the posteroinferior part of the cerebellum and generally arises from the vertebral artery, but may also arise from the basilar artery (4, 56).
Of 50 cerebellar hemispheres examined in our previous study, all but 1 had vertebral arteries, and 42 of the 49 vertebral arteries gave rise to PICAs (30). Both a vertebral artery and the associated PICA were absent in a few hemispheres. If a PICA is present, it is the largest branch of the vertebral artery. It is rarely absent bilaterally, but may arise as a double or duplicate PICA. Forty-one of the 42 PICAs arose as a single trunk and 1 arose as a duplicate trunk. The vertebral artery sometimes terminates in a PICA.
The vertebral artery enters the dura lateral to the cervicomedullary junction, courses superior, anterior, and medial to reach the front of the medulla and joins its mate from the opposite side at approximately the level of the pontomedullary junction to form the basilar artery. The site of the origin of the PICA from the vertebral artery varies from below the foramen magnum to the vertebrobasilar junction. A few of the PICAs arising below the foramen magnum may arise from the vertebral artery in an extradural location (Fig. 2.21) (12). Thirty-five of the 42 PICAs examined in our previous study arose above the level of the foramen magnum, and 7 vessels originated below. The origin was located 14.0 mm below to 26.0 mm above the level of the foramen magnum (average, 8.6 mm above) (30). The origin was located 0 to 35.0 mm (average, 16.9 mm) below the junction of the vertebral and basilar arteries.
The PICA arises from the posterior or lateral surfaces of the vertebral artery more often than from the medial or anterior surfaces (Fig. 2.19). On leaving the parent vessel, the initial course of the PICA is posterior, lateral, or superior more often than anterior, medial, or inferior (Fig. 2.20). The vertebral artery’s diameter is greater at its entrance through the dura (range, 1.8–6.2 mm; average, 4.4 mm) than at the PICA origin (range, 1.6–5.7 mm; average, 3.9 mm) or at its termination (range, 1.7–5.5 mm; average, 3.7 mm). The diameter of the PICA at its origin ranges from 0.5 to 3.4 mm (average, 2.0 mm). The origin was 1.0 mm or less in diameter in 4 cerebellae. The PICA has been reported to be hypoplastic in 5 to 16% of cerebellar hemispheres (33, 48).
Most PICAs bifurcate into a smaller medial and a larger lateral trunk; the trunk before the bifurcation is referred to as the main trunk. The medial trunk supplies the vermis and adjacent part of the hemisphere and the lateral trunk supplies most of the hemispheric and tonsillar parts of the suboccipital surface. The PICAs that do not bifurcate are usually small and supply only a small area on the tonsil and adjacent part of the vermis and hemisphere.
The bifurcation usually occurs posterior to the brainstem as the PICA courses around the tonsil (Figs. 2.16, 2.17, and 2.22). The most common site of the bifurcation is in the telovelotonsillar fissure as the artery courses around the rostral pole of the tonsil. The medial trunk usually ascends in the vermohemispheric fissure to reach the vermis, and the lateral trunk passes laterally out of the telovelotonsillar fissure to reach the hemispheric surface. If the bifurcation occurs at a more proximal site in relation to the tonsil, the medial trunk usually ascends along the medial tonsillar surface and through the vermohemispheric fissure, and the lateral trunk passes posteriorly over the tonsillar surface near the point of bifurcation to reach the hemispheric surface. If the bifurcation occurs proximal to the lateral margin of the tonsil, the medial trunk commonly pursues a course around the medial surface of the tonsil to reach the vermohemispheric fissure, and the lateral trunk passes directly to the hemispheric surface.
The medial trunk terminates by sending branches over the inferior part of the vermis and adjacent part of the tonsil and hemisphere. The lateral trunk divides into a larger hemispheric trunk that gives off multiple branches to the hemisphere and smaller tonsillar branches that supply the posterior and inferior surfaces of the tonsil. This division of the lateral trunk into tonsillar and hemispheric branches may occur at various sites in relation to the tonsil, but is most commonly located near the posterior margin of the medial surface of the tonsil. The trunks passing through the tonsillomedullary fissure send branches to the medulla, and the trunks passing through the telovelotonsillar fissure send ascending branches to the dentate nucleus (55).
The PICA gives rise to perforating branches to the medulla, choroidal arteries that supply the tela choroidea and choroid plexus, and cortical arteries. The cortical arteries are divided into median and paramedian vermian; tonsillar; and medial, intermediate, and lateral hemispheric arteries. The cortical branches arising near the superior pole of the tonsil send branches upward to supply the dentate nucleus.
The perforating arteries are small arteries that arise from the three medullary segments and terminate in the brainstem. They are divided into direct and circumflex types. The direct type pursues a straight course to enter the brainstem. The circumflex type passes around the brainstem before terminating in it. The circumflex perforating arteries are divided into short and long types. The short circumflex type does not travel more than 90 degrees around the circumference of the brainstem. The long circumflex type travels a greater distance to reach the opposite surface. Both types of circumflex arteries send branches into the brainstem along their course. The perforating arteries have numerous branches and anastomoses that create a plexiform pattern on the medullary surface. In our previous study, the anterior medullary segments gave rise to 0 to 2 (average, 1.0) perforating branches per hemisphere, which were most commonly of the short circumflex posterior type and supplied the anterior, lateral, or posterior surfaces of the medulla (30). The lateral medullary segments gave rise to 0 to 5 (average, 1.8) branches per hemisphere that supplied the lateral or posterior medulla predominately as short circumflex arteries. The tonsillomedullary segment gave rise to more perforating branches than the other segments (range, 0–11 per hemisphere; average, 3.3). They were either of the direct or short circumflex type, but the former predominated. They terminated in the lateral and posterior surfaces of the medulla.
The perforating branches of the PICA intermingle and overlap with those arising from the vertebral artery (Fig. 2.5). The segment of the vertebral artery distal to the origin of the PICA more frequently gives rise to perforating arteries than the segment proximal to the PICA origin. The perforating branches arising between the entrance of the vertebral artery into the dura mater and origin of the PICA are most commonly of the short circumflex or direct type and terminate predominately on the lateral side of the medulla. Those arising between the PICA origin and the vertebrobasilar junction are predominately of the short circumflex type and terminate on the anterior and lateral surfaces of the medulla. The segment of the vertebral artery distal to the PICA origin also gives rise to a few branches that enter the choroid plexus protruding from the foramen of Luschka.
The PICA gives rise to branches that supply the tela choroidea and choroid plexus of the fourth ventricle, usually supplying the choroid plexus near the midline of the roof of the fourth ventricle and in the medial part of the lateral recess (Figs. 2.16 and 2.23) (15). This includes all of the medial segment and the adjacent part of the lateral segment of the choroid plexus. More choroidal branches arise from the tonsillomedullary and telovelotonsillar segments than from the lateral or anterior medullary segment. The AICA usually supplies the portion of the choroid plexus not supplied by the PICA, commonly that part in the cerebellopontine angle and the adjacent part of the lateral recess.
The most constant area supplied by the PICA includes the majority of the ipsilateral half of the suboccipital surface of the cerebellum (Figs. 2.15, 2.16, and 2.22). This includes the majority of the suboccipital surface of the ipsilateral hemisphere and tonsil, the ipsilateral half of the vermis, and the anterior aspect of the tonsil. The largest area supplied by a PICA includes all of the ipsilateral half of the suboccipital surface with overlap onto the contralateral half of the suboccipital surface and the adjacent parts of the tentorial and petrosal surfaces. The smallest area supplied by a PICA is confined to the inferior part of the ipsilateral cerebellar tonsil. The cortical area supplied by the PICA is more variable than that supplied by the AICA and the SCA. If the PICA is absent on one side, the contralateral PICA or the ipsilateral AICA supplies most of the area normally supplied by the absent PICA.
The cortical branches are divided into hemispheric, vermian, and tonsillar groups. The vermian branches usually arise from the medial trunk, and the hemispheric and tonsillar branches from the lateral trunk. Each half of the vermis is divided into median and paramedian segments, and the hemisphere lateral to the vermis is divided into medial, intermediate, and lateral segments. There is a reciprocal relationship with frequent overlap in the areas supplied by the tonsillar, hemispheric, and vermian branches.
The hemispheric branches most commonly arise from the lateral trunk within or distal to the vermohemispheric fissure. They appear to radiate outward to the hemispheric surface from the superior and lateral margin of the tonsil. In our previous study, the number of hemispheric branches given off from a PICA ranged from 0 to 9 (average, 2.8). Four PICAs had no hemispheric branches (30). A common pattern was for there to be three branches with an individual branch being directed to the medial, intermediate, and lateral segments of the suboccipital surface. The medial hemispheric segment is occasionally supplied by the medial trunk. The ipsilateral AICA often gives rise to branches that overlap onto the lateral hemispheric segment, and the SCA often overlaps onto the superior part of the three hemispheric segments.
The vermian arteries usually arise from the medial trunk in the vermohemispheric fissure. A common pattern is for there to be one or two vermian branches. If two are present, they are often directed to the median and paramedian segments. If no vermian branches are present, the vermian area is usually supplied by the contralateral PICA.
The tonsillar branches usually arise from the lateral trunk and most commonly supply the medial, posterior, inferior, and part of the anterior surfaces of the tonsil. If there are no branches directed predominately to the tonsil, the tonsil is supplied by the adjacent hemispheric and vermian branches.
Relationship to the Cranial Nerves
The PICA has the most complex relationship to the cranial nerves of any artery (27, 30, 52). The vertebral artery courses anterior to glossopharyngeal, vagus, accessory, and hypoglossal nerves, and the proximal part of the PICA passes around or between and often stretches or distorts the rootlets of these and adjacent nerves.
The inferior olive protrudes from the anterolateral surface of the medulla near the vertebral artery and the origin of the PICA (Fig. 2.24). The hypoglossal nerve joins the brainstem on its anterior border and the glossopharyngeal, vagus, and accessory nerves on its posterior border. Most PICAs arise at the level of the olive, but some will arise rostral or caudal to that level. The PICA origins at the level of the olive are either lateral or anterior to the olive. The PICA origin is anterior to the olive if the vertebral artery pursues its usual course anterior to the olive, but if the vertebral artery is tortuous and kinked posteriorly, the PICA origin is lateral to the olive.
The hypoglossal nerve arises as a line of rootlets that exits the brainstem along the anterior margin of the caudal two-thirds of the olive in the preolivary sulcus, a groove between the olive and the medullary pyramid (Fig. 2.24). The hypoglossal rootlets, in their course from the preolivary sulcus to the hypoglossal canal, pass posterior to the vertebral artery, except in the rare instance in which they pass anterior to the artery. If the vertebral artery is elongated or tortuous and courses lateral to the olive, it stretches the hypoglossal rootlets dorsally over its posterior surface. Some tortuous vertebral arteries stretch the hypoglossal rootlets so far posteriorly that they intermingle with the glossopharyngeal, vagus, and accessory nerves.
The relation of the origin and proximal part of the PICA to the hypoglossal rootlets varies markedly. The PICA arises either rostral or caudal or at the level of the hypoglossal rootlets. The majority of the PICAs arise at the level of the hypoglossal rootlets near the junction of the hypoglossal rootlets with the medulla (Fig. 2.24). The PICAs that arise superior or inferior to the hypoglossal rootlets usually course superior or inferior to, rather than between, the hypoglossal rootlets. The hypoglossal rootlets are frequently stretched around the origin and initial segment of the PICAs that arise at the level of the caudal two-thirds of the olive, in addition to being stretched posteriorly by the vertebral artery. About half of the PICA origins are located anterior to and half posterior to or at the level of the rostrocaudal line drawn through the exits of the hypoglossal rootlets from the medulla. The vertebral artery courses from the lateral side of the inferior part of the medulla to the anterior surface of the superior part of the medulla. Those PICAs arising inferior to the olive, arise posterior to the level of the hypoglossal rootlets if the vertebral artery at the site of origin of the PICA has not coursed far enough anterior to reach the level of the hypoglossal rootlets. The PICA origin is anterior to the hypoglossal rootlets if the vertebral artery, on reaching the hypoglossal rootlets, was anterior to the olive. The PICA origin is located at the level of or posterior to the hypoglossal rootlets if the vertebral artery at the site of origin of the PICA courses lateral to the olive and stretches the hypoglossal rootlets posteriorly.
The initial segment of the PICA has a variable course in relation to the hypoglossal rootlets. The most common course is for the PICA to arise from the vertebral artery and pass directly posteriorly around or between the hypoglossal rootlets. However, some PICAs will loop upward, downward, or laterally in front of the hypoglossal rootlets before passing posteriorly between or around them.
Glossopharyngeal, Vagus, and Accessory Nerves
After coursing posterior to the hypoglossal rootlets, the PICA encounters the rootlets of the glossopharyngeal, vagus, and accessory nerves (Fig. 2.25). The glossopharyngeal, vagus, and accessory nerves arise as a line of rootlets, then exit the brainstem along the posterior edge of the olive in the retroolivary sulcus, a shallow groove between the olive and the posterolateral surface of the medulla. The glossopharyngeal nerve arises as one or rarely two rootlets posterior to the superior third of the olive, just inferior to the pontomedullary junction and anterior to the foramen of Luschka and the rhomboid lip of the lateral recess of the fourth ventricle. The vagus nerve arises inferior to the glossopharyngeal nerve as a line of tightly packed rootlets posterior to the superior third of the olive. The accessory nerve arises as a widely separated series of rootlets that originates from the medulla and upper cervical cord, inferior to the vagus nerve below the level of the junction of the upper and middle third of the olive. The glossopharyngeal and vagus nerves arise rostral to the level of origin of the hypoglossal rootlets. The accessory rootlets arise at both the level of and inferior to the origin of the hypoglossal rootlets.
The PICA commonly passes from the lateral to the posterior aspect of the medulla by passing between the rootlets of the glossopharyngeal, vagus, and accessory nerves. The PICA may be ascending, descending, or passing laterally, or medially or be involved in a complex loop that stretches and distorts these nerves as it passes between them. Of the 42 PICAs found in 50 cerebellae in a previous study, 16 passed between the rootlets of the accessory nerve, 10 passed between the rootlets of the vagus nerve, 13 passed between the vagus and accessory nerves, 2 passed above the glossopharyngeal nerve between the latter nerve and the vestibulocochlear nerve, and 1 passed between the glossopharyngeal and vagus nerves (30).
Facial and Vestibulocochlear Nerves
The facial and vestibulocochlear nerves arise superior to the glossopharyngeal nerve at the level of the pontomedullary junction. The proximal part of the PICA usually passes around the brainstem inferior to the facial and vestibulocochlear nerves. However, in some cerebellopontine angles, the proximal part of the PICA, after coursing posterior to the level of the hypoglossal rootlets, loops superiorly toward, even compressing, the facial and vestibulocochlear nerves before descending to pass between the glossopharyngeal, vagus, and accessory rootlets (Figs. 2.11 and 2.12).
The consequences of a PICA occlusion vary and may be overshadowed by the effects of occlusion of the parent vertebral artery. The effects range from a clinically silent occlusion to infarction of portions of the brainstem or cerebellum with swelling, hemorrhage, and death (53). Nearly all occlusions of the PICA, but only slightly more than half of occlusions of the vertebral artery, result in medullary or cerebellar infarction (5, 11). The incidence of medullary and cerebellar infarction in vertebral artery occlusion increases greatly if the origin of the PICA is included in the occlusion. Occlusion of the PICA is usually the result of thrombosis of a preexisting atherosclerotic stenosis and is less commonly caused by embolization (5).
Occlusion of the PICA causes an infarct in the lateral medulla, dorsal to the inferior olivary nucleus. The syndrome of occlusion of the PICA, referred to as the lateral medullary syndrome, includes ipsilateral numbness of the face caused by injury to the spinal tract of the trigeminal nerve; loss of pain and temperature on the contralateral half of the body caused by damage to the spinothalamic tract; dysphagia, dysarthria, and hoarseness as a result of homolateral weakness of the palate, pharynx, vocal cord, and occasionally the sternoclinoid muscle caused by a lesion in the nucleus ambiguis; ataxia, dizziness, vertigo, nystagmus, and homolateral cerebellar signs caused by damage to the vestibular nuclei, cerebellar tracts in the brainstem, and the cerebellum; an ipsilateral Horner’s syndrome caused by disruption of the oculosympathetic fibers in the lateral medullary reticular substance; and vomiting caused by involvement of the nucleus and tractus solitarius. Other less common accompaniments include nystagmus and diplopia caused by a lesion in the dorsal medulla and the medial longitudinal fasciculus; and facial weakness caused by damage to the facial motor nucleus (10, 14, 17).
The syndrome associated with lateral medullary infarction may be caused by occlusion of either the PICA or the vertebral artery, but it is most commonly attributable to vertebral artery occlusion (14, 17). Fisher et al. noted that 75% of cases of lateral medullary syndrome were associated with a vertebral artery occlusion and that only 12% had a PICA occlusion (14). The site of the infarct with a PICA occlusion does not differ significantly from that with a vertebral artery occlusion. Symptoms, if present with the other manifestation of the lateral medullary syndrome, suggest vertebral artery rather than PICA occlusion include paresis of the trunk, limb, and tongue muscles, crossed sensory loss with dysphagia, visual loss suggesting calcarine cortex involvement, diplopia with an abducens nerve palsy, loss of hearing, or a facial palsy.
Occlusion of the branches of the PICA distal to the medullary branches produces a syndrome resembling labyrinthitis and includes rotatory dizziness, nausea, vomiting, inability to stand or walk unaided, and nystagmus without appendicular dysmetria. The dizziness, unsteadiness, and nystagmus are postulated to caused by involvement of the flocculonodular complex. The lack of brainstem signs in this syndrome indicates that the occlusion is distal to the medullary branches of the PICA. Branch occlusions are usually caused by emboli and result in infarction of the suboccipital portion of the cerebellar hemisphere and vermis. Massive acute cerebellar infarction is most frequently caused by PICA or vertebral artery occlusion, with the most common site of cerebellar infarction being in the PICA territory (53).
The PICA is exposed in dealing with neoplasms involving the cerebellopontine angle, foramen magnum, cervicocranial junction, clivus, jugular foramen, fourth ventricle, and cerebellum; aneurysms arising at the PICA origin, the most common site in the posterior fossa below the basilar apex, and less frequently from the distal segments (30); arterial dissections at the PICA-vertebral junction (54, 58); arteriovenous malformations, which also commonly involve the other cerebellar arteries and the brainstem as well as the cerebellum (6); posterior fossa ischemia requiring bypass because of the PICAs easy accessibility through a suboccipital craniotomy and the proximity to the occipital artery (28); anomalies at the cranio-cervical junction, like the Chiari malformation and osseous deformities; and dysfunction of the lower cranial nerves like glossopharyngeal neuralgia (21, 23, 24, 29, 42).
The PICA can arise outside the dura, and at any point from along the intradural course of the vertebral artery. The origin can be located along the lateral side of the medulla, if the artery arises near the passage of the vertebral artery through the dura, or in front of the brainstem, if the origin is high near the vertebrobasilar junction. Exposing a low-lying PICA origin, either extra- or immediately intradurally, at the level of the foramen magnum can be achieved by a midline suboccipital or a far-lateral approach. If an artery with a low-lying origin has to be followed upward into the cerebellopontine angle or there is a need to mobilize the site of the vertebral artery’s passage through the dura, a far-lateral or transcondylar modification approach are to be considered. A retrosigmoid craniotomy may be sufficient to expose a PICA arising from the midportion of the vertebral artery on the lateral side of the brainstem in the lower part of the cerebellopontine angle. If there is a need to expose the origin deep in the midline near the vertebrobasilar junction, a supra-infratentorial presigmoid approach with some added degree of labyrinth resection may be required, depending on the depth of the PICA origin and the pathology. A midline suboccipital craniectomy, possibly combined with removal of the posterior atlantal arch, is usually sufficient to expose pathology involving the tonsillomedullary and telovelotonsillar segments of the artery. Lesions involving the PICA in the walls in the fourth ventricle, vermis, and paravermian areas are usually exposed by a midline suboccipital approach. Lesions involving the hemispheric branch can be exposed through a vertical suboccipital incision and craniotomy centered over the pathology. The anatomy of PICA compression of the lower cranial nerves and medulla is reviewed in the section on the cerebellopontine angle.
Contributor: Albert L. Rhoton, Jr., MD
Content from Rhoton AL. The Posterior Cranial Fossa: Microsurgical Anatomy and Surgical Approaches. Neurosurgery 47(3), 2000, 10.1097/00006123-200105000-00065. 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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