Aneurysms of the Craniovertebral Junction
The craniovertebral junction (CVJ) is an intricate anatomical region with transitions not only in bony anatomy but in neural and vascular tissue as well. In this transitional zone, arterial histologic changes occur including the loss of the external elastic lamina and a thinning of the muscularis layer. Additionally, as these arteries traverse the dura, they immediately undergo a serpiginous course, causing a disruption of laminar blood flow. It is perhaps this combination of features that causes aneurysms of the CVJ to be second in frequency only to those of the basilar tip in the posterior intracranial circulation.
Aneurysms of the CVJ consist of lesions that arise from the vertebral artery, the posterior inferior cerebellar artery (PICA), and the vertebrobasilar junction. About 75% of these aneurysms occur near the origin of the PICA.1–5 They occur more often on the left, probably because this artery is frequently the dominant vertebral artery. As with most cerebral aneurysms, lesions of the CVJ are more common in women than men. Due to the complex anatomical configuration of the arteries involved and to the intimate association between vascular and neural tissue at the CVJ, it is imperative that any comprehensive examination of this topic start with the basic vascular anatomy of the region.
Vascular Anatomy of the Vertebral Arteries
Classically, the vertebral artery originates as the first and largest branch off the subclavian artery on each side ( Fig. 16.1A ). In rare cases, it arises directly from the aorta or the external carotid artery. The vertebral artery ascends behind the anterior scalene muscle to enter the foramen transversarium at C6 ~90% of the time. It continues vertically through these transverse foramina passing anteriorly to the exiting nerve roots. After traversing the foramen at C2, the artery curves laterally to reach the foramen of C1 ( Fig. 16.1B ). The vertebral artery then travels posteromedially around the superior articular process of the atlas and into the sulcus arteriosus in the lamina of C1. Next the artery pierces the atlantooccipital membrane and dura while traveling superiorly and ventrally through the foramen magnum. It continues rostrally around the lower medulla, anterior to the most superior dentate ligament and ventral to the lower cranial nerves. From this point, the two vertebral arteries continue superiorly a variable distance along the clivus, before joining to form the basilar artery ( Table 16.1 ).
Throughout its course, the vertebral artery supplies numerous spinal and muscular branches. Its two major arterial branches are the anterior spinal artery and the PICA ( Fig. 16.2 ). The anterior spinal artery usually originates just distal to the PICA and proximal to the vertebrobasilar junction.6 It usually runs inferiorly and medially to form a common trunk with the opposite anterior spinal artery and continues caudally in the ventral median fissure of the spinal cord.
The PICA usually arises from the posterior or lateral side of the vertebral artery at a point 13 to 16 mm proximal to the basilar artery origin ( Fig. 16.3 ).6 Based on data from several large angiographic series, the origin of the artery is caudal to the foramen magnum in 18% of the specimens, at its level in 4%, and above the foramen in 57%. In the remainder of cases, the origin cannot be clearly identified.6,7 The diameter of the PICA measures ~2 mm at its origin (range, 0.5 to 3.4 mm), whereas the diameter of the parent vertebral artery is approximately twice that.8 PICA may be absent or hypoplastic 15 to 20% of the time, and in this case the anterior inferior cerebellar artery (AICA) usually will supply the territory, although the contralateral PICA may also contribute.7,9 In 2% or fewer cases, the PICA arises as a duplicate artery.
Although the course of the PICA is highly variable, Lister and colleagues,8 Rhoton,10 and others6,11,12 have divided the artery into five general segments ( Table 16.2 and Fig. 16.3 ). The short anterior medullary segment traveling around the anterior surface at the medulla is followed by the lateral medullary segment, which includes the first half of the caudal loop of the artery and ends at the approximate origin of cranial nerves IX, X, and XI. The caudal loop of the artery may dip below the level of the foramen magnum in ~35% of cases. This position, therefore, cannot be used as a sign of tonsillar herniation. The tonsillomedullary or posterior medullary segment of the artery continues as the ascending portion of the loop and reaches the upper surface of the cerebellar tonsil. It passes behind or posterior to the rootlets of the lower cranial nerves. The telovelotonsillar segment continues over the superior aspect of the tonsil creating a cranial loop. At this apex, a branch artery is given off to the choroid plexus of the fourth ventricle, creating the “choroidal point.” This point serves as an important diagnostic landmark because of its constant relationship to the fourth ventricle. The artery continues caudally in the retrotonsillar fissure and usually branches into medial and lateral hemispheric or cortical segments.
Perforating arterial branches to the medulla commonly arise from the proximal three segments of PICA. Perforators may be seen from the anterior segment in more than 50% of specimens, from the lateral segment in more than 70%, and from the tonsillomedullary segment in more than 75%.5,8 These arterioles have a diameter of 0.4 mm or less and may be quite numerous.
Vascular Anatomy of the Basilar Artery
At the level of the pontomedullary sulcus, the vertebral arteries join to form the basilar artery trunk. Traveling within the prepontine cistern, the artery courses superiorly between the sixth cranial nerves before arching slightly posteriorly after entering the interpeduncular cistern close to its termination. The trunk has an average length of 32 mm and diameter of 3 to 4 mm.13 The artery often dilates rostrally by 0.3 to 0.8 mm. Only 25 to 50% of cases demonstrate a straight arterial course for the basilar artery.14 The remainder assume an S-shaped configuration, which is especially prominent in older patients and those with extensive atherosclerotic disease. In this latter group, the diameter of the artery is also frequently enlarged. Fenestrations are relatively common in the basilar trunk, most often in its lower half. The incidence of this anomaly appears to be between 1 and 5% of cases.15 In one report of 59 vertebrobasilar junction aneurysms, 21 (35.5%) arose in a fenestration of the proximal basilar artery.15
Considerable variation occurs in the level of the basilar artery at both its origin and termination. Normally situated at the lower end of the pons, the basilar origin may be several millimeters above or 1.5 cm below this level. Alternatively, the upper termination varies depending on the patient′s age, clival configuration, and vessel integrity. In 51% of the specimens, the basilar artery terminates at the level of the posterior clinoid process (or dorsum sellae), whereas in 30% it is above and in 19% it is below this level.6 In infants, the basilar bifurcation typically is located well above the posterior clinoid process. The basilar artery gives rise to various branches including numerous paramedian, short, and long circumflex pontine rami, as well as the anterior/inferior and superior cerebellar arteries. The basilar artery terminates into the bilateral P1 segments of the posterior cerebral artery. Lang found an average of 6.2 perforators per dissection (mean diameter, 0.3 mm).14 On average, there were two arteries at each paramedian, short, and long circumflex location. The territory irrigated by these rami includes the pontine tegmentum, the middle cerebral peduncle, and the adjacent cranial nerves.
AICA most often arises from the proximal third of the basilar artery (52 to 75%). In 16 to 41% of the specimens, it branches off the middle third of the parent vessel. This artery is most often single but may be multiple in as many as 26% of specimens.14 It is rarely absent (2 to 7%). The artery has been divided into premeatal, meatal, and postmeatal segments. The initial arterial course is downward and lateral with angles of ~45 degrees. AICA gives rise to several recurrent perforating arteries. The first two perforators most often arise from the premeatal segment. The internal auditory artery is rarely duplicated and passes immediately into the internal auditory canal to supply the nerve roots and ear sensory organs. Most commonly, it is ventral and medial to the nerves. AICA divides into a medial and lateral branch within the cerebellopontine angle in close relation to cranial nerves VII and VIII. The lateral branch curls around the flocculus and into the horizontal fissure. Here hemispheric branches anastomose with those of PICA and the superior cerebellar artery. The medial branch travels inferomedially, supplying primarily the biventral lobule, as does PICA. The size of AICA is inversely related to the size of PICA. In ~8% of cases, PICA itself may arise from the basilar artery.14
Like aneurysms in other locations, those at the CVJ follow the three basic anatomical principles of formation defined by Rhoton.10 First, intracranial saccular aneurysms arise at branch points off a parent artery, either at the site of a side branch or at a bifurcation of the parent artery. For CVJ lesions, those arising at the PICA–vertebral artery branch and those arising at the vertebrobasilar junction ( Fig. 16.4 ) illustrate both aspects of this principle, respectively. The second tenet suggests that saccular aneurysms tend to occur at curves or loops in arteries due to local hemodynamic stresses in these regions from directional changes in flow. Aneurysms that arise in the peripheral segments of PICA demonstrate this principle by occurring most commonly at either the cranial or caudal loops of PICA ( Fig. 16.5 ).11,16,17 The association of saccular aneurysms occurring in relation to a fenestrated basilar artery also may typify this point.15,18,19 Third, an aneurysm, and more specifically its dome, points in the direction of blood flow irrespective of the accompanying side branch or curve associated with it. Prime examples of this tenet are aneurysms arising at the origin of PICA ( Fig. 16.6 ). This lesion often occurs such that the rostral half of the PICA origin is involved as part of the aneurysm neck, with the dome most often pointing superiorly, a position that corresponds with the direction of blood flow in the parent artery.
Additionally, the CVJ harbors a disproportionately high number of dissecting aneurysms.20 Dissecting aneurysms most often involve the intracranial V4 portion of the vertebral artery ( Fig. 16.7 ).21,22 As with saccular aneurysms, these pathological lesions tend to congregate around the PICA–vertebral artery junction.23 It is hypothesized that the dissection itself is the main cause of fusiform aneurysmal formation. Other factors such as atherosclerotic plaques, fibromuscular dysplasia, or collagen vascular disease can also contribute to dissecting/fusiform aneurysm formation. The V4 segment of the vertebral artery is also exposed to sheer forces from head movement as it passes through the dura. This stress point is associated with tapering of the tunica media and adventitia, making this location more vulnerable to spontaneous dissection. When the dissection occurs between the internal elastic lamina and the tunica media, the lumen narrows, producing ischemic symptoms. When the dissection occurs between the tunica media and the adventitia, subarachnoid hemorrhage (SAH) results. SAHs from dissecting aneurysms are of concern as they are associated with considerably higher rates of early recurrent hemorrhage. Immediate attention and treatment are therefore indicated.2,25,32,33 Traumatic aneurysms of the CVJ are rare26 and should be treated like traumatic aneurysms elsewhere.
Related posts:
The Rheumatoid Neck: Changing Pathology Requires Altering Surgical Strategies
Primary Extramedullary Tumors of the Craniovertebral Junction
Radiosurgical Management of Lesions of the Craniovertebral Junction
Management of Vertebral Artery Dissections and Vascular Insufficiency
Traumatic Injuries of the Craniovertebral Junction
Cavernous Malformations of the Cervicomedullary Junction
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