INTRODUCTION AND DEFINITION
Subarachnoid hemorrhage (SAH) refers to bleeding within the subarachnoid space, which lies between the arachnoid and pia mater and is normally filled with cerebrospinal fluid. Most cases of subarachnoid hemorrhage are caused by rupture of an intracranial aneurysm, but approximately 15 to 20 percent of patients do not have an established vascular lesion on initial four-vessel cerebral angiography [1,2]. The causes of nonaneurysmal SAH (NASAH) are potentially diverse, and often the mechanism of bleeding in these cases is not identified.
In 1985, a subtype of NASAH, so-called perimesencephalic nonaneurysmal SAH was identified in a case series of 13 patients who had a characteristic pattern of localized blood on computed tomography (CT), normal cerebral angiography, and a benign course that distinguished these patients from both aneurysmal SAH and other patients with nonaneurysmal SAH . This observation has subsequently been confirmed by other reports [2,4-7].
The CT finding that defines perimesencephalic NASAH (PM-NASAH) is blood isolated to the perimesencephalic cisterns anterior to the brainstem; there may also be extension into the ambient cisterns or basal parts of the sylvian fissures, but not into the lateral sylvian fissure, anterior interhemispheric fissure, or lateral ventricles (image 1 and figure 1). This CT pattern of hemorrhage is not specific to PM-NASAH; a subset of cases of perimesencephalic SAH are caused by aneurysm rupture. It is important, however, to recognize this pattern of bleeding because invasive diagnostic testing can be limited in these patients and morbidity thereby reduced.
This topic reviews perimesencephalic SAH. Aneurysmal SAH and other causes of NASAH are discussed separately. (See "Clinical manifestations and diagnosis of aneurysmal subarachnoid hemorrhage" and "Treatment of aneurysmal subarachnoid hemorrhage" and "Nonaneurysmal subarachnoid hemorrhage".)
The reported proportion of cases of NASAH that are PM-NASAH varies between 21 to 68 percent [1,2,6,8-11]. In a biracial, population-based, epidemiologic study, the annual rate of PM-NASAH was estimated to be 0.5 per 100,000 persons over 18 years of age .