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. Approximately 15 to 20 percent of patients presenting with SAH do not have a vascular lesion on initial four-vessel cerebral angiography [1,2]. The causes of these nonaneurysmal SAH (NASAH) are potentially diverse, and the mechanism of bleeding in these cases is often not identified.
This topic discusses nonaneurysmal subarachnoid hemorrhage. Aneurysmal SAH and perimesencephalic SAH are discussed separately. (See "Clinical manifestations and diagnosis of aneurysmal subarachnoid hemorrhage" and "Treatment of aneurysmal subarachnoid hemorrhage" and "Perimesencephalic nonaneurysmal subarachnoid hemorrhage".)
Perimesencephalic nonaneurysmal subarachnoid hemorrhage — A subtype of NASAH, so-called perimesencephalic nonaneurysmal SAH is characterized by a specific pattern of localized blood on computed tomography (CT), normal cerebral angiography, and a benign course that distinguishes these patients not only from aneurysmal SAH, but also from other patients with nonaneurysmal SAH [2-5]. In some case series, this makes up the majority, up to two-thirds, of patients with NASAH .
The CT findings that define perimesencephalic NASAH (PM-NASAH) include blood isolated to the perimesencephalic cisterns anterior to the brainstem; there may 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) [1,5].
Perimesencephalic nonaneurysmal subarachnoid hemorrhage is discussed separately. (See "Perimesencephalic nonaneurysmal subarachnoid hemorrhage".)