Abstract 188: An Atypical Cause of Subarachnoid Hemorrhage

Case Presentation Our patient is a 45‐year‐old female that was otherwise healthy who presented acutely with a thunderclap headache and was found to have a subarachnoid hemorrhage. The imaging pattern initially was concerning for a possible perimesencephalic subarachnoid hemorrhage with the blood products centered in the right perimesencephalic cisterns. A computed tomography cerebral angiogram and a conventional digital subtraction angiogram were both negative for a source of bleeding, including aneurysms. It was noted however that there were blood products tracking along the tentorial incisura, following the path of the posterior cerebral artery, raising the concern of a possible dissecting aneurysm. A repeat conventional cerebral angiogram completed 5 days after the initial conventional angiogram demonstrated a right superior cerebellar dissecting aneurysm (Figure 1). This was treated by coil sacrifice of the symptomatic branch of the superior cerebellar artery (Figure 2). The patient did well for the rest of her hospitalization and was discharged with no neurologic deficits. Discussion Perimesencephalic subarachnoid hemorrhage is a relatively rare classification of subarachnoid hemorrhage, representing an estimated 5% of all subarachnoid hemorrhage. The radiographic findings are typically limited to the perimesencephalic cisterns without extension into the sylvian or interhemispheric fissures. The vast majority of cases do not have a causative etiology found. Theories on the etiology of these bleeds include rupture of a small perforating artery or of venous flow dynamics origin. Symptoms are generally more mild and outcomes more favorable. When evaluating for perimesencephalic subarachnoid hemorrhage, it is important to consider the uncommon etiologies. These etiologies include vertebrobasilar system aneurysm rupture, cervicomedullary junction tumors, or vascular malformations. The atypical feature in our case was the distinct asymmetry of blood products along the right tentorial incisura. This allowed us to perform a careful search and eventual identification and treatment of the causative lesion.