Distal posterior inferior cerebellar artery aneurysm with cerebellar arteriovenous malformation treated by open surgery: a case report

The arteriovenous malformation (AVM)-related feeder aneurysm is a relatively rare entity that can be detected in 9% of intracranial AVM cases [1, 2]. Distal posterior inferior cerebellar artery aneurysm (DPICAAn) are reported to coexist or relate with cerebellar AVM [3–5], but DPICAAn comprises < 1% of all intracranial aneurysms. Hence, the coexistence of DPICAAn and cerebellar AVM is particularly rare, and the treatment strategy for cases in which these two entities coexist is debatable. Regarding the treatment of the DPICAAn and the cerebellar AVM, endovascular embolization with or without open surgery is often selected [6]. Here, we report a unique case of a DPICAAn with a cerebellar AVM treated successfully with open surgery, and describe the indications for open surgery for DPICAAn with cerebellar AVM, focusing on the anatomical features of the posterior inferior cerebellar artery (PICA).

This report presents a case of a DPICAAn and a cerebellar AVM, both treated with single-session open surgery.

A study reported the annual bleeding risk of an aneurysm with an AVM to be 7% [6]. SAH with poor morbidity and mortality can occur in 84% of patients with coexistence of the DPICAAn and cerebellar AVM [5, 7]. The management of unruptured AVM remains debatable after a randomized trial of unruptured brain arteriovenous malformations [8]. In our case, the cerebellar AVM showed possible past asymptomatic bleeding on MRI. Moreover, the DPICAAn comprised a bleb; therefore, we believed that the risk of hemorrhagic events could be high. Although we were unable to conclude whether the episode of headache in the present case was related to the AVM or the aneurysm, we considered that the surgical intervention for this patient was appropriate to avoid any hemorrhagic event.

To date, the surgical strategy of the AVM with feeder aneurysm remains disputable. A report supported the preceding treatment of the feeder aneurysm because the change of hemodynamic stress could lead to aneurysmal rupture when the AVM was first removed [9]. Regression of flow-related feeder aneurysms after the complete obliteration of the nidus by surgical intervention for only AVM has also been reported [10, 11]. In the present case, the right PICA was dominant, which was a normal variation of the PICA and was reported to have the aneurysm at a higher rate than normal PICA [12–14] regardless of a coexistent AVM. The DPICAAn in our case could have arisen from the anatomical feature of the dominant PICA, and not from the feeder aneurysm of AVM. Thus, an intervention for only the AVM may not have resolved the DPICAAn. Next, we decided to operate the AVM and DPICAAn in a single-session open surgery in case of operative bleeding. The DPICAAn was located deeply in the operative field, whereas the AVM was located on the surface of the cerebellum. Therefore, to avoid unnecessary hemorrhage and to safely expose the DPICAAn, we first excised the AVM on the cerebellar surface. Gamma knife could also be a treatment option [5]. However, this technique has some disadvantages, including recurrence of the AVM and hemorrhagic events prior to the disappearance of AVM. Moreover, the gamma knife cannot resolve DPICAAn, and the risk of aneurysm rupture remains. Hence, we did not select gamma knife surgery for our patient.

Surgical approaches for the PICA aneurysms can vary based on the aneurysmal location of the PICA segment, ruptured status, and necessity of posterior fossa decompression. The PICA is anatomically divided into five segments: anterior medullary, lateral medullary, tonsillomedullary, telovelotonsillar, and cortical branches [15]. An aneurysm located in the telovelotonsillar segment, like our case, is categorized as the distal aneurysm [16]. The far-lateral approach is a standardized approach, especially for the proximal PICA aneurysm [17], and the midline suboccipital approach could be a good option for the PICA aneurysm distal to the tonsillomedullary segment [18]. Cerebellar AVMs are categorized into the following five subtypes: suboccipital, vermian, tonsillar, tentorial, and petrosal [19]. In our case, the nidus was located on the medial cerebellar surface and belonged to the suboccipital type. Suboccipital AVMs are supplied by distal cortical branches from the SCA superiorly, PICA inferiorly, and AICA laterally [19]. The vascular supply to the nidus in our case was compatible with typical suboccipital AVMs. For suboccipital AVMs, the surgical approach is ascertained with the location of the nidus on the cerebellar surface [19]. Although the lateral suboccipital approach is the most common, the medial lesion, like our case, can be well exposed by the midline suboccipital approach. Hence, the DPICAAn and the cerebellar AVM in our case were successfully approached by the midline suboccipital approach. Furthermore, the dorsal projection and narrow neck of the DPICAAn, which were confirmed preoperatively on reconstructed three-dimensional CTAV, were also considered suitable for neck clipping. In general, harvesting the OA for the bypass procedure should be considered when the neck clipping of the DPICAAn is difficult.

A study reported endovascular embolization of the AVM combined with parent artery occlusion or microsurgical clipping of the DPICAAn associated with the AVM [5]. As ischemic complications with parent artery occlusion are reported to be 17–40% [4], parent artery occlusion for the DPICAAn seems unsuitable for cases in which sacrifice of the single-dominant PICA such as in our case, can result in fatal cerebellar edema. Parent artery occlusion of the PICA can result in spinal cord ischemia as the posterior spinal arteries originate from PICA in 20–50% of cases [20].

Moreover, the primary feeder of the cerebellar AVM in our case was the left SCA. As interventional embolization of the cerebellar AVM could not be attained only with the sacrifice of the PICA, a single-session open surgery for the cerebellar AVM and DPICAAn was suitable in our case.

This report presents a case of a feeder aneurysm in the distal PICA and a cerebellar AVM successfully treated by open surgery. In the event that the dominant PICA feeding to the cerebellar AVM perfuses the bilateral cerebellar hemispheres, as in our case, a single-session open surgery could be suitable option. The anatomical location of the DPICAAn and the nidus of the cerebellar AVM contributed to the decision of opting for single-session open surgery.