42 Anterior Inferior Cerebellar Artery Aneurysms
Abstract
Anterior inferior cerebellar artery (AICA) aneurysms are very rare, accounting for less than 2% of intracranial aneurysms. A majority of these aneurysms are diagnosed following rupture and thus require treatment. However, they constitute a diverse group and are named according to their location along the artery, in relation to the internal acoustic meatus: premeatal, meatal, and postmeatal. The choice of treatment depends on the precise location of the aneurysm along the AICA and on the height at which the AICA originates along the basilar artery (BA). In addition, the main consideration for surgery is a comprehensive appreciation of the aneurysm in relation to the surrounding neurovascular structures, as it determines ease or difficulty of access. Most AICA aneurysms are proximal, at the level of the mid-third of the BA. Nowadays, they are most often treated by endovascular means, which is usually associated with less morbidity and mortality than surgical clipping, although with a higher risk of recurrence. For more distally located aneurysms, a surgical approach is often preferred as it allows a better chance for the parent artery to be preserved. This chapter gives a description of various surgical techniques as well as an overview of basic endovascular principles, as knowledge of both is necessary to avoid complications and to improve outcome.
Introduction
Anterior inferior cerebellar artery (AICA) aneurysms are very rare, accounting for less than 2% of intracranial aneurysms. In the Co-operative Study of Intracranial Aneurysms and Subarachnoid Haemorrhage, published in 1966, which gathered the largest series examining the management and the outcome of ruptured intracranial aneurysms, AICA aneurysms represented only 0.06%. Since then, improvements in neurological imagery have increased the ability to diagnose these aneurysms. Because of its rarity, experience in the management of AICA aneurysms is limited. This chapter is based on a summary review of 150 cases found in the literature.
Major controversies in decision making addressed in this chapter include:
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Whether or not to treat.
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Open versus endovascular treatment, based on the location of the aneurysm.
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Which surgical approach to choose when clipping.
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When to consider indirect surgical techniques (e.g., extracranial–intracranial vascular anastomosis).
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What are the indications for the various modalities of endovascular treatment.
Whether to Treat
The majority of AICA aneurysms reported in this summary review are ruptured aneurysms (75.3%). Because of their very grim natural history, treatment of ruptured aneurysms is clearly indicated, unless prognosis is poor due to severe medical conditions and/or poor neurological status ( 1 in algorithm ).
As for unruptured aneurysms, the indication for treatment depends on multiple factors, in which the risk of bleeding is weighed against the risks associated with the particular treatment. Factors to consider are those related to the patient (age, medical condition, family history, smoking habits, and prior subarachnoid hemorrhage [SAH] from another aneurysm) and those related to the aneurysm itself (size, morphology, location along the AICA, and increase in size).
First to consider is whether the aneurysm is symptomatic or not. It has been reported that in general, symptomatic aneurysms bear a fourfold increased risk of rupture. The majority of unruptured AICA aneurysms in the literature presented with a sign of compression of the brainstem or cranial nerve (CN). Generally for all types of aneurysms, early treatment is indicated for patients presenting with CN palsy ( 2, 4 in algorithm ).
Another important factor is the size of the aneurysm. According to the International Study of Un-ruptured Intracranial Aneurysms (ISUIA), the risk of rupture increases with size. In general, treatment of aneurysms is indicated when size is 7 mm or more. In addition, ISUIA and other studies have suggested posterior circulation aneurysms are at a higher risk of rupture than those of the anterior circulation. Because of this, even though there is no randomized study supporting it, one may assume that AICA aneurysms smaller than 7 mm may also be considered for treatment ( 2, 4, 5, 6 in algorithm ). Another consideration for treatment of an aneurysm smaller than 7 mm is whether the patient had a prior bleeding from another aneurysm, as ISUIA reported a significantly greater risk of hemorrhage in such patients compared to patients with similarly sized aneurysms and no prior history of rupture. Treatment of smaller aneurysms may also be indicated in patients with a family history of intracranial aneurysms. Also to be considered for treatment are aneurysms that have increased in size since previous imaging.
Anatomical Considerations
The AICA is intimately related to the pons, the middle cerebellar peduncle, and the petrosal surface of the cerebellum (
Fig. 42.1 ).
Origin: The AICA usually originates at the lower third of the basilar artery (BA; 75%), but may at times arise at the mid-third (16%) or close to the vertebrobasilar junction (9%). It originates as a unique vessel in 72% of cases, but may also be a duplicate artery (26%) or even a triplicate one (2%).
Segments: The AICA is commonly divided into four segments. (1) Anterior pontine segment is located between the clivus and the belly of the pons. It begins at its origin and ends at the level of a line drawn by the long axis of the inferior olive, passing anteriorly to the sixth CN. (2) Lateral pontine segment begins at the anterolateral margin of the pons and passes through the cerebellopontine angle (CPA) to end at the level of the medial border of the flocculus. This second segment is usually adjacent to the eighth CN in the CPA. (3) Flocculo-peduncular segment begins when the artery passes rostral or caudal to the flocculus and ends in the cerebellopontine fissure. (4) Segment branches out to feed the petrosal surface of the cerebellum. Branches: The AICA gives rise to perforating branches to the pons, choroidal branches to the lateral part of the choroid plexus, and branches to the seventh and eighth CNs.
In its premeatal segment, the AICA essentially gives rise to perforating branches to the brainstem. These perforating branches to the brainstem may also arise from the flocculonodular segment of the AICA (82% of cases), in which case they are named the recurrent perforators, in reference to their anterior course in the CPA. The labyrinthine or internal meatal arteries are branches of the AICA that enter in the internal meatal canal to end by branching out into the vestibular, cochlear, and vestibulo-cochlear arteries that supply the inner ear. The labyrinthine arteries arise in the premeatal segment of the AICA in 77% of cases, in the meatal segment in 21%, and in the postmeatal segment in only 2%.
Pathophysiology
The review of the literature revealed various causes for AICA aneurysms ( Table 42.1 ). As in most saccular aneurysms, degeneration of the wall, possibly due to factors contributing to atherosclerosis, remains the main cause. However, AICA can be associated with cerebellar or dural arteriovenous malformation, and cerebellar hemangioblastoma. In addition, 9.4% of AICA aneurysms have been found when the AICA provides a dominant vascular supply (AICA/PICA [posterior inferior cerebellar artery] variant), also suggesting to be the result of hemodynamic factors. Another factor may be direct injury of the artery wall as AICA aneurysms have also been reported following stereotactic radiosurgical treatment of a vestibular schwannoma (4.8% of AICA aneurysms).
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