The patient was given aspirin 81 mg and clopidogrel 75 mg daily for three days before the procedure. A right common femoral artery puncture was performed and an 8F sheath was inserted after general anesthesia, neuromuscular paralysis, and endotracheal intubation. Using a roadmap and guidewire technique, a Neuron Max 088 support catheter (Penumbra, USA) was advanced to the left vertebral artery. The right AICA-PICA common trunk arose at a sharp angle from the midportion of the basilar artery. The Asahi Intecc micro-guidewire (USA), which provided adequate microcatheter support strength, was used to enter the trunk. The micro-guidewire and microcatheter system (Phenom plus catheter co-axial with Phenom 27 catheter) were advanced past the aneurysm into the AICA-PICA trunk’s inferior bifurcation, which supplied the distal PICA territory. This 1.25 mm-diameter segment was chosen as the flow diverting stent’s distal landing zone. The proximal landing zone, which measured 1.5 mm in diameter, was chosen as the segment between the beginning of the aneurysm and the origin of the AICA-PICA trunk at the basilar artery. The smallest PED-Shield available is 2.5 mm in diameter. As a result, a 2.5 mm×14 mm PED-Shield was successfully deployed without incident (Fig. 4). Final angiograms revealed that the stent was well-positioned, covering the entire neck of the aneurysm, with parent vessel patency (Fig. 5). The patient awoke neurologically intact from endovascular intervention and had an uneventful post-operative period. On post-operative day 2, he was discharged home.

A normal neurological examination was performed at 7 months. The MRI scan revealed no ischemia in the AICA or PICA territories, a patent AICA-PICA vessel distal to the stent, and no aneurysm remnant (Fig. 6). Clopidogrel was stopped, and the patient is now on 81 mg aspirin indefinitely.

A number of anatomical variants of the posterior circulation have been described [14,24,28,36]. Hou et al. [21] recently reviewed the angiography of 500 patients undergoing cerebral angiogram and discovered an AICA-PICA common trunk in 22.1% of cases, further subclassifying these anomalies. Common trunks of the AICA and PICA may arise from the basilar artery (BA type) or the vertebral artery (VA type), and then divide into two branches that supply the territory of the AICA or PICA, respectively [21]. This branching can take place either proximally, at the anterior brainstem segment (BP type), or distally, after the anterior brainstem segment (BD type) [21]. Our case fits the description of a BP-type AICA-PICA common trunk, with the aneurysm originating in the proximal (anterior brainstem) segment.

Injury to this variant at its source, whether during surgical or endovascular treatments, could result in a major stroke involving both the anterior and posterior surface of the cerebellum, as well as a portion of the brainstem, with potentially fatal consequences.

Although AICA-PICA variations are common, AICA-PICA common trunk aneurysms are a rare clinical entity. Only 11 cases have previously been reported, according to our review of the literature [2,7,12,18,32,39]. 5 studies reported surgical treatment, while the remaining 6 cases were managed with coiling, 3 with stent assisted coiling, and 1 with coiling and surgical thrombectomy. A flow diverter was not used to treat this condition in any of the previous studies.

The majority of the studies, as shown in Table 1, were case reports, with the exception of Suh et al. [39] reporting a series of 5 cases and Gopalakrishnan et al. [18] describing 2 cases. Overall, 10 patients were female [2,7,12,18], with ages ranging from 26 to 71 years. The most common type of presentation was subarachnoid hemorrhage, which was described in 7 patients [7,12,18,39], with only 2 incidental findings [39] and 2 cases presenting with mass effect [2,32]. The AICA-PICA variant aneurysm location was proximal in 5 cases [2,39] and distal in 6 [7,12,18,32]. A small aneurysm (10 mm) was found in eight cases [7,12,18,39], a large aneurysm (11-24 mm) was discovered in two cases [2,39], and one giant lesion (25 mm) was described [32].

In our case, a proximal 9 mm aneurysm was discovered on a right AICA-PICA common trunk variant. Interestingly, when the patient first presented with right-sided balance and hearing problems, indicating cranial nerve VIII dysfunction, an initial CTA revealed no lesion. We believe this was due to a spontaneous dissection that included the origin of the labyrinthine artery, which was not visible on initial imaging due to spatial resolution limitations [37]. A fusiform dissecting aneurysm developed quickly as the dissection progressed.

Dissections of the vertebrobasilar system have previously been reported as an uncommon cause of acute sudden onset sensorineural hearing loss, either directly involving the labyrinthine artery or as an embolic phenomenon [13,31]. Gross et al. [20] described a case of a ruptured dissecting labyrinthine artery aneurysm treated with surgical clipping, where the labyrinthine artery originated directly from the basilar artery.

The specific etiology of AICA-PICA variant aneurysms is unknown. Few hypotheses have been proposed, with the majority of them focusing on increased blood flow as a result of the anatomical variant, a more tortuous course leading to hemodynamic stresses at the vascular turns, and vascular injury such as dissection due to trauma or inflammatory diseases [12]. In our case, no specific cause was found; the patient was only receiving treatment for high blood pressure, with no other known comorbidities or trauma history. Interestingly, the aneurysm grew rapidly, expanding from 0 to 9 mm in less than 4 months, lending support to the theory of a dissection-related fusiform aneurysm.

Several risk factors for aneurysm growth have been reported in the literature (Table 2) [5,6,10]. The only risk factors identified in this case were hypertension, posterior circulation, and non-saccular shape. Furthermore, a meta-analysis of clinical and radiographic risk factors for aneurysm growth and rupture revealed that the majority of risk factors for aneurysm growth were also risk factors for rupture [10]. This consideration influenced our recommendations in favor of prompt treatment.

Surgery for the treatment of posterior fossa aneurysms can be challenging due to the surrounding neurovascular complexity, the narrow surgical field, the rarity and deep locations of these aneurysms [39]. Endovascular techniques, on the other hand, which frequently involve stent placement in the posterior fossa, necessitate dual antiplatelet treatment, which increases morbidity and mortality risks, particularly in ruptured aneurysms [42].

According to the literature, most AICA aneurysms arise from the proximal segment and can be treated with neck clipping or coiling, particularly saccular ones [15,17,23]. Fusiform aneurysms are more difficult to treat, and surgery is considered the first option for large or giant lesions due to the relatively higher morbidity when treated with endovascular techniques [3,22,35]. Distal aneurysms, on the other hand, can be surgically or endovascularly trapped or occluded [17,34,41,44]. The fusiform nature of the aneurysm in our case precluded simple clipping or coiling. Indeed, either surgical or endovascular sacrifice of the parent artery would have increased the risk of ischemic stroke in both the proximal AICA and PICA territories.

In terms of safety and success rate, an endovascular approach with flow diversion was deemed optimal. Flow diversion has recently emerged as a safe and effective treatment option for patients with complex unruptured cerebral aneurysms that would otherwise be difficult to coil or clip microsurgically [8,25]. Flow diversion is also being used to treat ruptured aneurysms [26,33]. Flow diversion has been used successfully in posterior circulation fusiform aneurysms, including those involving the AICA or PICA [19,30]. Wu et al. [43] reported that despite being covered by the flow diverter, all AICA or PICA remained patentable in their cohort. This is in contrast to a previous meta-analysis that found an increased risk of ischemic strokes and perforator infarctions with posterior versus anterior circulation aneurysms [9].

In our case, we used the PED-Shield, a third-generation pipeline flow diverter device. The phosphorylcholine stent surface modification has been shown to reduce the thrombogenicity of the pipeline flow diverter [16], which is important in small caliber vessels that supply eloquent structures, as in this case. Most patients are kept on dual antiplatelet therapy after deployment [4], though single antiplatelet therapy may also be safe [1,11,27,38].