SAH of unknown etiology: Natural history and further evaluation

Subarachnoid hemorrhage (SAH) of unknown etiology is a relatively common occurrence, representing approximately 10%-15% of nontraumatic SAHs. The key factors in determining the management strategy for a presumed nonaneurysmal SAH are the distribution, location, and amount of subarachnoid blood. Hemorrhage distribution on computed tomography (CT) can be categorized as follows: perimesencephalic, diffuse, sulcal, primary intraventricular, and CT-negative. The extent of the workup required in determining the etiology of hemorrhage in these patients depends on the distribution of blood. Patients with perimesencephalic, primary intraventricular, and sulcal SAH generally have a lower diagnostic yield of repeat cerebral angiography than those with diffuse SAH. Prognosis also depends on the distribution of hemorrhage as patients with perimesencephalic, sulcal, and CT-negative hemorrhage of unknown etiology generally have a very good prognosis whereas those with intraventricular or diffuse SAH with a poor Fisher grade have worse short- and long-term clinical outcomes. In this chapter, we review the differential diagnosis, acute and long-term follow-up strategies, and outcomes of patients with SAH of unknown etiology.