Introduction 

   Aneurysms arising from the posterior inferior cerebellar artery (PICA) are uncommon, with an incidence of 0.49~3% of all intracranial aneurysms.⁴⁾ The majority arise at the vertebral artery (VA) and PICA junction. Distal PICA aneurysms account for only 0.28~1.0% of all intracranial aneurysms.⁹⁾¹¹⁾ To date, there have been more than 200 cases reported in literature.¹¹⁾ However, the infrequency and complexity of the PICA anatomy has limited our understanding of the underlying pathology, natural history, and standard management of distal PICA aneurysms.⁷⁾ 
   In this report, we present two cases of ruptured distal PICA aneurysms treated with surgery successfully and discuss the clinical characteristics and management of patients with distal PICA aneurysms with a review of the literature. 

Case Reports 

Case 1 
   A 48-year-old woman without hypertension or diabetes mellitus was admitted to our hospital because of sudden onset of severe headache and vomiting. Her consciousness was lethargy, but well oriented. On admission, brain computed tomography (CT) revealed intraventricular hemorrhage (IVH) from the fourth ventricle to the lateral ventricle with mild hydrocephalus (Fig. 1). A brain CT angiogram showed no aneurysm. A cerebral angiography showed an aneurysm of the right distal PICA (Fig. 2). A midline suboccipital craniotomy was performed in a prone position. The aneurysm was originated from the medial vermian branch of the PICA. The aneurysm was obliterated with 5mm straight and curved clip (Fig. 3). Postoperative cerebral angiography revealed normal patency of PICA and no aneurysmal remnant (Fig. 4). 
   The patient' postoperative course was uneventful, and she was discharged without neurological deficits. 

Case 2 
   A 74-year-old woman without hypertension or diabetes mellitus was admitted to our hospital because of sudden onset of severe headache and vomiting, which was followed by unconsciousness. On admission, she was in a deep coma (Glasgow Coma Scale 3; E-1, V-1, M-1) with unreactive miotic pupil on both sides. A brain CT showed hematoma in the cerebellar vermis and IVH from the fourth ventricle to the lateral ventricle with severe hydrocephalus (Fig. 5). An emergency extraventricular drainage (EVD) was performed. After EVD, her consciousness was improved to stupor and light reflex was recovered. A cerebral angiography showed an aneurysm of the left distal PICA and anterior inferior cerebellar artery (AICA)-PICA anastomotic channel (Fig. 6). A midline suboccipital craniectomy was performed in a prone position. The aneurysm was originated from the vermian branch of the PICA. The aneurysm was obliterated by proximal parent artery clipping and coagulation. Postoperative cerebral angiography showed no aneurysmal remnant (Fig. 7). 
   At the time of discharge, her consciousness was drowsy and she was able to walk with aid. 

Discussion 

   The first case of a distal PICA aneurysm was reported in 1864 by Fernet.²⁾ Lister et al.⁸⁾ and Lewis et al.⁷⁾ reported that the most common location of a distal PICA aneurysm was in the telovelotonsilar segment. Some reports have described that more than half of aneurysms of the distal PICA arose proximal to or at the choroidal arch.¹⁰⁾¹²⁾ Although aneurysms typically arise at branch points or curves in vessels, distal PICA aneurysms tend to arise at turning points, perhaps because of the complex, tortuous course of an artery with limited branching.³⁾ 
   Vascular anomalies are noted frequently in association with PICA aneurysms, especially hypoplasty of the contralateral PICA or occlusion of the vertebral artery.³⁾ These findings support the concept of an aneurysmal origin from hemodynamic stress in the case of aneurysms in the branch of the PICA, particularly in relation to vascular anomalies. Distal PICA aneurysms arising from its trunk are also associated with vascular anomalies such hypoplasty of the PICA, arteriovenous malformation, primitive trigeminal artery, AICA-PICA anastomotic channels, direct aortic origin of the vertebral artery, and telangiectasia.³⁾ Also in our second case, AICA-PICA anastomotic channel (AICA-PICA complex) was existed. 
   Some authors studied the hemorrhage patterns of PICA aneurysms.^6)7)11)13) They reported that distal PICA aneurysms have high percentage of IVH with an incidence of 85~95%. Lewis et al.⁷⁾ reported that lesions arising distal to the medullary segment of PICA, particularly those originating from the caudal loop are more commonly associated with IVH. Bleeding from proximal PICA segment (medullary segment) aneurysms, tends to produce cisternal hemorrhage without IVH or intracerebellar hemorrhage, however, intracerebellar hemorrhage with IVH or isolated IVH without cisternal SAH was identified in telovelotonsillar and cortical segment aneurysms.⁷⁾ Distal to medullary segment of PICA aneurysms have high percentage of IVH due to the proximity of the 4th ventricle. During the operation, removal of blood clot from the 4th ventricle and cisterna magna could play an important role for a favorable outcome. 
   The incidence of rebleeding of ruptured distal PICA aneurysms is 78%, which is higher than the 34% rebleeding rate reported in all intracranial aneurysms.⁵⁾ So, ruptured distal PICA aneurysms, particularly small size aneurysms should be managed immediately. 
   The best treatment option for a distal PICA aneurysm depends on aneurysm location, morphological structure, and clinical presentation. The ideal treatment of a true saccular lesion is clipping or endovascular obliteration of the aneurysm neck with preservation of parent artery. But, if the lesion cannot be clipped without parent artery occlusion, aneurysm location relative to the origin of brain stem perforators becomes the most important factor for choosing a treatment option. Anterior and lateral medullary segment possess the brain stem perforators. Tonsillomedullary segment may produce some perforating vessels, however telovelotonsillar and cortical segments have no brain stem perforators. So, trapping may be utilized in those lesions arising from or distal to the telovelotonsillar segment.⁷⁾ If the PICA is sacrificed, leptomeningeal communications with the superior cerebellar artery and the AICA are usually sufficient to protect against functionally significant cerebellar infarctions.¹⁾

Conclusion 

   Aneurysms of distal PICA are frequently associated with structural vascular anomalies and high incidence of bleeding when these aneurysms are small. So, distal PICA aneurysms should be obliterated in almost instances whenever they are encountered and these aneurysms should be managed immediately because of the high risk of rebleeding. 

REFERENCES

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