Introduction

Superior cerebellar artery (SCA) aneurysms are relatively rare at 1.6 to 4.2% of all aneurysms, and 10 to 13.5% of posterior circulation aneurysms. Reports about specific management of SCA aneurysms are similarly rare and are often incorporated into studies addressing basilar artery or posterior circulation aneurysms. They seem to be more frequently associated with multiple aneurysms (42–43.5%) and often associated with subarachnoid hemorrhage (SAH) (49–81%). A significant association with rupture was found when the aneurysm neck entirely arose from the SCA artery; however, this was not seen in another large study. Furthermore, they seem to present at a relatively small size. Both Nair et al and Peluso et al reported an average size of 7.3 mm with a median of 4.9 and 6 mm, respectively. Jin et al described an overall mean of 6.2 mm with 66% less than 7 mm. Within the 69 SCA aneurysms reported by Iizuka et al, 78% were ≤ 6 mm ( 1 , 2 in algorithm ), and 75% were less than 10 mm in the series of Haw et al; however, Drake et al′s description of 43 SCA aneurysms indicated only 46.5% being less than 12 mm. Interestingly, when examining the raw data from the series of Haw et al, ruptured aneurysms were on average smaller than the unruptured cases (7.2 vs. 8.4 mm).

Major controversies in decision making addressed in this chapter include:

1. 
   

   Whether or not treatment is indicated.

   
   
2. 
   

   Open versus endovascular treatment for ruptured and unruptured SCA aneurysms.

   
   
3. 
   

   Management of SCA aneurysms that present with multiple intracranial aneurysms.

   
   
4. 
   

   Is treatment different for SCA aneurysms when the parent vessel is incorporating into the neck or arises from the SCA–basilar artery junction?

Whether to Treat

Although there is no specific information on SCA aneurysm rupture rates, clinicians tend to have a lower threshold to treat posterior circulation aneurysms, as they have a higher annual rupture rate, with a reported relative risk of rupture of 4:1 compared to their anterior circulation counterparts ( 1 , 2 in algorithm ). The recently developed PHASES score also reflects this higher risk compared to anterior circulation aneurysms. Given their higher association with rupture at presentation and smaller size at rupture relative to most aneurysms, a lower threshold to treat SCA aneurysms is not unreasonable ( 1 , 2 in algorithm ). This must be tempered by the fact that 42 to 65% of these aneurysms incorporate the parent artery into the neck, with its documented inherent treatment difficulties and increased complication rates for either treatment modality. Overall, a treatment threshold of 5 mm for asymptomatic unruptured aneurysms seems reasonable though admittedly somewhat controversial; however, as always, each aneurysm must be managed inherent to its own specific risk factors. Clearly symptomatic and ruptured aneurysms should all be treated.

Anatomical Considerations

The SCA is the most constant of the posterior fossa arteries and arises from the basilar artery at the level of the pontomesencephalic junction. It usually courses below the oculomotor nerve and above the trochlear nerve; however, in some cases, it may arise from the origin of the posterior cerebral artery (PCA) and, in these cases, may course superior to the oculomotor nerve as well. The trigeminal nerve is related to its distal branches. This close relationship underlies the frequent association of the dysfunction of cranial nerves III, IV, and V with SCA aneurysms and potential injury during operative approach.

It is divided into four segments: anterior pontomesencephalic (s1), lateral pontomesencephalic (s2), cerebellomesencephalic (s3), and cortical (s4), which change at the level of the anterolateral brainstem, cerebellomesencephalic fissure, and anterosuperior cerebellar cortical surface, respectively. The SCA usually bifurcates into a rostral and caudal branch, the former supplying vermian and paravermian areas, and the later the hemisphere on the suboccipital surface. In 10% of cases, it may be duplicated; however, the pattern of irrigation is then the same as its usual branches. It supplies perforating branches to the brainstem, cerebellar peduncles and deep cerebellar nuclei, inferior colliculi, and superior medullary velum. The s2 segment initially lies with the tentorial edge; however, as it moves posteriorly, it drops below requiring its division for surgical access.

The proximal SCA is often considered a perforator free zone, thus less dangerous from a surgical viewpoint. Although it generally contains less perforators than the basilar bifurcation region, it is rarely devoid of these important branches, with a mean of 2 (range: 0–7) vessels arising from the s1 segment. Perforators arose on the first millimeter of the s1 in 33% of the cases and the first 2 mm in 50% of the cases. They may be direct, short, or long circumflex in decreasing order of frequency. The distal SCA has good collateral supply via the anteroinferior and posteroinferior cerebellar arteries via the vermian arcade.

Occlusion of the SCA ranges from clinically silent to death; however, it often produces a distinct clinical picture resulting from infarction of the cerebellum, dentate nucleus, brachium conjunctivum (superior cerebellar peduncle), and long sensory pathways in the rostral pontine tegmentum. Vomiting, sudden dizziness, and inability to stand or walk mark onset. Additional symptoms may include ipsilateral intention tremor and Horner′s syndrome, and contralateral loss of pain and temperature, nystagmus, hearing loss, and emotional expression.

There does not appear to be a preference to aneurysms side; however, 90% could be predicted by the inclination of the basilar artery (i.e., aneurysm on the convex side of the basilar artery). When the basilar artery was deemed vertical, there was no such association.

Classification

Iizuka et al subclassified SCA aneurysms into three types, depending on the aneurysm neck location relative to the basilar SCA junction. Type A aneurysms arose from the true basilar SCA junction without involvement of the SCA artery proper. Type B aneurysms incorporated half of the neck into the SCA artery while Type C aneurysm arose solely from the SCA artery. Seventy-six percent of ruptured aneurysms were of type B or C, while 46% of unruptured aneurysms were type A. The angle between the SCA and the PCA vessels on the side of the aneurysms is generally obtuse, compared to an acute angle contralaterally. Typically, these aneurysms have a wide neck and a small dome:neck ratio (defined as < 2) in 67 to 89% of cases.

Aneurysms arising near the basilar SCA junction account for the vast majority of SCA aneurysms and tend to be saccular in nature; however, a small proportion arises distally (8.3%). These distal aneurysms usually present with SAH or trochlear nerve dysfunction and may also be saccular; however, fusiform, traumatic, and dissecting aneurysms are also reported, with the SCA particularly vulnerable to mechanical injury caused by the tentorial edge. SCA aneurysms may also be associated with an arteriovenous malformation feeding artery. Despite being distal in origin, mycotic etiology has seldom been proven in SCA aneurysms.

Workup

Imaging

Typically, the initial imaging workup involves a CT angiography which shows the aneurysm, relevant wall calcifications, and intraluminal thrombus, with most treatment decisions able to be made from this alone. Review of the coronal and sagittal reconstructions is invaluable in assessing cases for surgery and planning the approach. A three-dimensional reconstruction when available is also very helpful in deciding dome:neck ratio and subclassification, which are helpful in deciding which treatment modality is indicated (▶ Fig. 41.1 ). Imaging of the vertebral artery from the aortic arch origin is helpful in planning for neurointervention. Occasionally, a diagnostic cerebral angiography (DSA) is still required to finalize the anatomy and possibly confirm the diagnosis, as there have been case reports of distal SCA aneurysms not showing up on initial imaging (▶ Fig. 41.2 ).

Related posts:

37 Vertebral Artery Aneurysms 38 Midbasilar Artery Aneurysms 39 Basilar Artery Apex Aneurysms 42 Anterior Inferior Cerebellar Artery Aneurysms 46 Dissecting Intracranial Aneurysms of the Posterior Circulation 44 Giant Aneurysms of the Posterior Circulation

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