2026 年 13 巻 p. 311-316
Aneurysms associated with arteriovenous malformations occur in up to 20% of cases, but distal anterior inferior cerebellar artery aneurysms associated with arteriovenous malformations are extremely rare. We report a case of a ruptured distal anterior inferior cerebellar artery aneurysm associated with a cerebellar arteriovenous malformation in a 71-year-old man presenting with subarachnoid hemorrhage. Digital subtraction angiography revealed a cerebellar arteriovenous malformation supplied mainly by the right anterior inferior cerebellar artery and a fusiform aneurysm located at the meatal loop within the internal auditory canal. The aneurysm was considered the most likely rupture point based on the hemorrhage distribution and angiographic findings. Endovascular treatment was attempted to prevent re-rupture. Due to challenging distal catheterization and unattainable complete coil embolization of the aneurysm, the aneurysm and a short proximal segment of the parent artery were occluded. Postoperatively, the patient developed a mild hearing loss without brainstem infarction. Subsequent arteriovenous malformation resection via lateral suboccipital craniotomy led to complete obliteration. Parent artery occlusion reduced arteriovenous malformation flow and served as preoperative embolization to facilitate safe surgical removal. This case highlights the importance of individualized treatment strategies for rare distal anterior inferior cerebellar artery aneurysms associated with arteriovenous malformations and suggests that parent artery occlusion with subsequent staged arteriovenous malformation resection can be an effective approach.
Aneurysms associated with arteriovenous malformations (AVMs) appear as a comorbidity in up to 18%-20% of cases.1,2) Compared with other intracranial aneurysms, flow-related aneurysms present a higher risk of hemorrhage and are associated with worse clinical outcomes.3,4) Conversely, anterior inferior cerebellar artery (AICA) aneurysms are rare among all intracranial aneurysms, and distal AICA aneurysms are even rarer.5) To our knowledge, the coexistence of a distal AICA flow-related aneurysm and an AVM is extremely rare. Herein, we present such a case, describe the treatment course, and review the relevant literature.
A 71-year-old male with untreated hypertension and no notable family history of AVMs suffered a sudden severe headache and was transported to the hospital by ambulance. On admission, the patient exhibited full alertness and no neurological symptoms. Computed tomography of the head revealed a subarachnoid hemorrhage, specifically in the infratentorial region (Figure 1a and b). Digital subtraction angiography (DSA) revealed multiple feeders from the right AICA. We identified an AVM in which small vessels originating from the AICA formed small nidus on the cerebellar surface, with venous drainage into the transverse sinus via the right superior petrosal sinus (Figure 1c and d). Furthermore, a small fusiform aneurysm was identified at the meatal loop of the right AICA within the internal auditory canal (Figure 2a and b). It was diagnosed as a flow-related aneurysm of the AVM. The aneurysm exhibited an irregular shape with bleb formation. Furthermore, according to the presence of vasospasm in the proximal vessels and the distribution of the hemorrhage, we considered this aneurysm a probable rupture point, although definitive confirmation was not possible (Figure 2c).
(a, b) Computed tomographic scans depicting significant infratentorial subarachnoid hemorrhage but no intracerebral hematoma.
(c, d) Cerebral digital subtraction angiography (DSA), frontal view, after left vertebral artery injection.
(c) The yellow arrow indicates the high-flow arteriovenous malformation (AVM); the white arrow indicates the anterior inferior cerebellar artery (AICA), which was the main feeding vessel; and the yellow arrowhead indicates a flow-related aneurysm of the AVM observed in the distal AICA.
(d) Anterograde drainage into the transverse sinus via the right superior petrosal sinus was also detected (black arrow).
(a) Three-dimensional digital subtraction angiography (3D-DSA), providing a clearer view of the arteriovenous malformation (AVM) and distal anterior inferior cerebellar artery (AICA) aneurysm. The yellow arrow indicates the nidus of the AVM, the white arrow indicates the AICA, and the yellow arrowhead indicates a flow-related aneurysm of the AVM observed in the distal AICA.
(b) Axial 3D-DSA confirmed the presence of a small fusiform aneurysm (yellow arrowhead) in the canal loop of the AICA within the internal auditory canal.
(c) The aneurysm was irregularly shaped with bleb formation, and the white arrowheads indicate vasospasm of the surrounding vessels.
We selected endovascular treatment for both the AVM and the distal AICA aneurysm. The initial treatment plan was to approach the microcatheter as close as possible to the shunt point to embolize the AVM, followed by definitive treatment of the aneurysm. A Slim Guide 6 Fr catheter (Medikit, Tokyo, Japan) was introduced via the right femoral artery and placed in the left vertebral artery. To access the right AICA, we used a Marathon 1.5 F/2.7 Fr microcatheter (Medtronic, Irvine, USA), coaxially with a Guidepost 3.2 F/3.4 Fr catheter (Tokai Medical Products, Aichi, Japan). We attempted to guide the microcatheter distally to the fusiform aneurysm, but this was difficult. Therefore, we prioritized the prevention of aneurysmal re-rupture, which was considered to be the cause of subarachnoid hemorrhage (SAH).
After successful placement of a microcatheter within the aneurysm, coil embolization was attempted. However, complete occlusion proved difficult due to the fusiform morphology of the aneurysm and instability of the microcatheter within the sac. Consequently, both the aneurysm and a minimal segment of the proximal parent artery were occluded (Figure 3a and b). Postoperative DSA confirmed the absence of residual aneurysmal filling. Because the AICA-the main arterial feeder-had been occluded, the flow to the AVM was markedly reduced, with only minor residual flow from the right superior cerebellar artery (SCA) and small cerebellar surface vessels (Figure 3c and d). Postoperatively, the pure-tone average hearing thresholds were 54.0 dB on the right and 28.8 dB on the left. Although preoperative hearing assessment was unavailable, the hearing loss was considered predominantly right-sided. No facial nerve palsy, vertigo, or truncal ataxia was observed, and magnetic resonance imaging (MRI) showed no evidence of infarction.
Intraoperative images.
(a) Digital subtraction angiography. The white arrow indicates the anterior inferior cerebellar artery (AICA), and the yellow arrowhead indicates the distal AICA aneurysm. Because of the vascular tortuosity and stenosis near the aneurysm, the microcatheter was guided with difficulty distal to the aneurysm.
(b) Angiography showing the coils placed in a short segment proximal to the aneurysm to occlude the parent vessel.
(c) Angiography showing that the aneurysm had disappeared, and a shunt distal to the AICA was visualized from small cerebellar surface vessels (white arrowheads).
(d) Early-phase angiography demonstrates that the distal AICA shunt was visualized from a branch of the superior cerebellar artery (SCA) (white arrow).
After resolution of cerebral vasospasm and hematoma resorption, we implemented a direct surgical approach (Figure 4a): Through a lateral suboccipital craniotomy, we coagulated flow in the shunt and transected the nidus. DSA performed 1 week after surgery confirmed complete obliteration, which was similarly demonstrated at 1 month postoperatively (Figure 4b). Postoperative MRI showed a small cerebral infarction, and because of the truncal ataxia, he was transferred to a rehabilitation facility (Figure 4c). Regarding future treatment plans, we will consider the possibility of AVM recurrence and conduct regular follow-up observations with DSA.
Intraoperative and postoperative images.
(a) Intraoperative photograph shows the cerebellum near the shunt being pulled during a right lateral suboccipital craniotomy. Cranial nerves 7 and 8 (white arrowhead) are seen in the center. A large efferent vein (white arrow) and a fine feeder (yellow arrow) are visible on the surface of the cerebellum.
(b) Digital subtraction angiography performed 1 month after craniotomy confirmed complete obliteration of the shunt.
(c) Postoperative fluid-attenuated inversion recovery magnetic resonance image reveals a small cerebral infarction in the right cerebellar hemisphere, but no other significant changes were found.
Although aneurysms associated with AVMs have been widely reported, identifying whether the hemorrhage in acute rupture cases originated from the AVM or the aneurysm can sometimes be difficult. Hematoma patterns can provide important clues: SAH is generally suggestive of an aneurysmal source, whereas focal intraparenchymal or intraventricular hemorrhage more commonly indicates rupture of the AVM.4,6,7) In this case, no intraparenchymal hemorrhage was observed, and the primary hemorrhage consisted of SAH. Additionally, the presence of perianeurysmal vasospasm and aneurysmal blebs supported the diagnosis of aneurysmal rupture.
Aneurysms of the AICA are rare, representing 1.7% of all intracranial aneurysms,5) and distal AICA aneurysms are even rarer, accounting for less than 0.5%.8-11) Distal AICA aneurysms are classified as premeatal, meatal loop, and postmeatal; meatal loop aneurysms account for 44.7% of such aneurysms.8) Meatal loop aneurysms are subcategorized according to their interaction with the internal auditory canal: remote, plugged, and buried.11) Our patient's aneurysm was the buried type, of which 12 cases of rupture have been reported previously.12) Surgical treatment of buried AICA aneurysms requires opening the internal auditory canal, where the facial and vestibulocochlear nerves adhere tightly to an aneurysm; thus, the risk of cranial nerve or internal auditory artery (IAA) injury is high.9,11,13) Furthermore, a formed narrow and deep surgical field may prevent safe clipping of the aneurysm neck, which often necessitates trapping.11,13) Among the 12 reported cases of ruptured buried-type aneurysms, nine were managed surgically with clipping or trapping; however, incomplete clipping or intraoperative rupture occurred in four of these cases. Of the remaining three cases treated endovascularly, postoperative re-rupture occurred in the saccular aneurysm managed with coil embolization, whereas no re-rupture or major complications were observed in the two cases treated with internal trapping.12) In this case, despite unattainable occlusion of the distal parent vessel, occlusion of the aneurysm together with the proximal parent vessel was sufficient to prevent postoperative re-rupture.
Proximal AICA occlusion makes complete compensation by adjacent arteries difficult, leading to brainstem perforator infarction.14) Although temporary balloon test occlusion is the most reliable method for assessing the safety of parent vessel occlusion, it is not feasible for the AICA due to its narrow artery.15) Therefore, predicting the risk of parent vessel occlusion in this region is difficult. However, according to Zager et al.,16) if the aneurysm is located in the AICA beyond branches supplying the brainstem, distal occlusion can be performed without neurological sequelae. Furthermore, previous reports have suggested that collateral circulation may develop via the SCA or posterior inferior cerebellar artery depending on hemodynamic requirements.17,18) Therefore, in this case, the occlusion segment should be as short as possible, as proximal AICA occlusion may lead to brainstem infarction. Coils provide more predictable embolization than liquid agents, such as N-butyl-2-cyanoacrylate, particularly when occluding a short segment of blood vessel.19-21) Furthermore, the high-flow nature of AVMs elevates the risks of distal migration of liquid embolic agents or partial outflow vein occlusion, which can precipitate a rupture.22,23) Coils minimize these risks and were therefore selected for our patient. In this case, the only symptom of AICA occlusion during endovascular treatment was mild hearing loss, with no brainstem infarction. Inoue et al.12) revealed that hearing loss occurred in eight of 12 patients after treatment of buried-type aneurysms, indicating that it may represent a frequent complication. This hearing loss may be related to involvement of the IAA. The IAA is small and often challenging to visualize on conventional angiography, and detailed MRI identifies the artery in only 2.7% of cases.24) The IAA was identified intraoperatively in only two cases even in direct surgery for buried-type aneurysms.11,25) In this case, neither DSA nor three-dimensional-DSA demonstrated the presence of the IAA. The IAA is generally considered to originate from the meatal loop; however, anatomical studies have revealed that it more commonly originates from perforating branches supplying the brainstem and cerebellar peduncle, predominantly from the premeatal segment (92.3%).26) Therefore, given that the IAA was not clearly identified and preoperative hearing assessment could not be performed in this case, it is possible that the IAA was occluded concomitantly with proximal vessel occlusion, although this remains speculative. However, in light of these considerations, when occluding the parent vessel in this region, the risk of hearing loss should be carefully considered, and the surgery needs to be performed with extreme caution, with the occlusion length kept as short as possible.
Single-session treatment of the AVM and aneurysm was planned; however, advancing the catheter distal to the aneurysm was technically difficult. Therefore, coil embolization was performed to occlude both the aneurysm and the parent AICA. The presence of an aneurysm associated with an AVM has increased the risk of hemorrhage, particularly in infratentorial AVMs; therefore, such aneurysms warrant prompt treatment.1,2,6) Thompson et al.27) revealed that abrupt hemodynamic changes during AVM treatment may lead to rupture of flow-related aneurysms and recommended that treatment of the aneurysm should be prioritized. Early prevention of aneurysmal rupture is therefore an appropriate strategy before AVM craniotomy, and occlusion of the main feeding vessel recedes blood flow to the AVM, effectively functioning as preoperative embolization. Previous reports have indicated that this strategy reduces intraoperative blood loss and shortens operative time during AVM resection, thereby contributing to safer removal.28,29) Furthermore, performing craniotomy in the subacute phase after hematoma evacuation enables accurate assessment of vascular anatomy and architecture.30) This approach may reduce surgical risk and technical complexity in AVM resection, thereby contributing to favorable clinical outcomes. However, recurrence cannot be ruled out, as treatment in this case was limited to coagulation and excision of the nidus. Accordingly, periodic imaging follow-up is warranted.
This case of ruptured distal AICA aneurysm associated with an AVM is rare. In the absence of an established standard treatment strategy, treatment selection should be flexible and individualized, based on the shunt site as well as aneurysm location and morphology. Further accumulation of evidence from detailed case reports is essential for optimizing future treatment strategies.
ConclusionsWe treated a distal AICA aneurysm associated with an AVM, which represents an extremely rare clinical entity with few reported cases to our knowledge. Reports describing the treatment of aneurysms in such rare locations are lacking; therefore, selection of the optimal management strategy can be challenging. In this case, endovascular treatment was performed to prevent aneurysm re-rupture and to reduce blood flow to the AVM. The outcome was successful; thus, flexible and individualized treatment planning is essential in such cases.
All authors have no conflict of interest.
