The aneurysm of the anterior inferior cerebellar artery (AICA) are very rare and they account for <1% of all intracranial aneurysms (1, 2). The reported case reports of AICA aneurysms have a single or a few patients (1, 3–62), except for a few larger case series (63), including a large series of AICA aneurysms reported by Drake and colleagues (64). However, in this series, the aneurysms originating from the basilar trunk that is near the origin of AICA were included. AICA aneurysms are more often large in size, which makes their surgical and endovascular treatments challenging. Coiling is the favored treatment of posterior circulation aneurysms (65). However, in selected cases surgery might be a better choice, particularly in larger sized AICA aneurysms (26, 57). The surgical approach depends on location of aneurysm in relation to clivus. Aneurysms laying high can be approached through sub-temporal approach and low laying aneurysms are approached through retro-sigmoidal corridor. The morphological features may be helpful in deciding upon a treatment strategy. Due to the rarity of these lesions, the anatomical and radiological characteristics in unselected patient cohorts with AICA aneurysms have seldom been reported. In this series, we report the radiological features including shape, size, aspect ratio (maximum perpendicular height divided by neck diameter), bottleneck factor (maximum height to width ratio), and frequencies of aneurysm location in different segments of AICA aneurysms in a medium-size series. We report 15 consecutive patients harboring AICA aneurysms admitted from 1968 to 2017 in Helsinki University Hospital, department of Neurosurgery.

AICA aneurysms are rare and have been reported in <1% of all intracranial aneurysms (1, 2). Our data show that the incidence of AICA aneurysms in an unselected cohort was 0.22%. The most common presentation of an AICA aneurysm is subarachnoid hemorrhage (SAH). Seventy three percent of patients in our series presented with SAH, which is similar to previously published data (63). A review of data from already published case reports of 56 patients revealed that 81% of patients presented with SAH, which is slightly greater than our series and the series presented previously. Patients with more insidious onset present with cranial nerve deficits very similar to cerebellopontine angle space occupying lesions. However, patients with SAH may also have cranial nerve deficits often in the seventh and eighth cranial nerves. In our series, 33% (N = 5) of the patients had cranial nerve (VI, VII, VIII, or IX) deficits (Table 1). Rarely, patients with an AICA aneurysm present with convulsive seizure (36). In our series, none of the patients presented with epilepsy. AICA can be anatomically divided into four segments including (67) anterior pontine (a1), lateral pontine (a2), flocculopeduncular (a3), and cortical (a4). The lateral pontine segment has a close relationship with the porus of the internal acoustic meatus and with the facial and cochleovestibular nerves. AICA aneurysms can be categorized into proximal, meatal, and distal (58). Our data shows that 67% (n = 10) of the aneurysms were located proximally, 20% (n = 3) were in the meatal region, and only 13% (n = 2) were located distally. Our data are consistent with a previously published short case series where five out of six aneurysms were located proximally (1). Meatal aneurysms can be further classified into three subtypes (58). Type I (also called remote) are localized in the vascular loop outside the meatus. Type II (plugged) aneurysms are partially buried in the internal-acoustic meatus, and type III (buried) are entirely contained by the internal auditory canal). In our series, we found three aneurysms that were located in the meatal segment. Two of these were type I and one was type II (not shown in the table). The location of the aneurysm facilitates the planning of a suitable surgical approach if surgery is indicated (Figures 1–3). Most of the AICA aneurysms are reached through retrosigmoid approach (Figures 1–3). The mean ± SD size of the AICA aneurysms was 14.8 ± 18.9. The mean ± SD width was 16.9 ± 20.3 mm and the mean ± SD length was 16.9 ± 20.3 (Table 2). It is noteworthy that 73% of the patients were smokers and smoking is a potential risk factor for aneurysm formation and rupture. Further analysis of saccular aneurysms (n = 10) revealed a mean ± SD aspect ratio of 0.92 ± 0.47 and a mean ± SD bottleneck factor of 1.66 ± 1.65 (Table 2). Our data demonstrate that most AICA aneurysms are larger in size, which is consistent with a previous report that analyzed 10 thrombosed AICA aneurysms (20). In most cases in this series, the aneurysm size was >10 mm; two of these aneurysms were giant.

AICA aneurysms are rare posterior circulation lesions predominantly found in females, present predominantly with subarachnoid hemorrhage, are mostly large in size.

Our analysis was retrospective and may not completely reflect the natural history of anatomical and radiological parameters. The aspect ratio was quantified in a few cases after rupture and hence the possibility remains that the aspect ratio might have changed due to morphological changes after rupture and to a compression effect due to subarachnoid blood. Due to the small number of patients, we could not analyze the influence of aspect ratio, bottleneck factor, and segmental location of the aneurysm on aneurysm rupture.