Nonaneurysmal Perimesencephalic Subarachnoid Hemorrhage

In approximately 15% to 30% of patients with spontaneous subarachnoid hemorrhage, no cause of bleeding can be identified on the initial cerebral angiogram (1,2,3,4,5). While patients with negative angiography after a subarachnoid hemorrhage have a benign overall prognosis with a low yield from repeated angiography, it is important to discern between the subgroups of patients within this population to avoid missing potentially serious or treatable lesions. Within this population of patients with negative cerebral angiography, a particular subgroup was first described in 1985 in the Netherlands (6) with an almost universally benign and favorable outcome. Extent and volume of hemorrhage is mild and confined to the prepontine and perimesencephalic area, and the clinical condition is characterized by headache and signs of meningeal irritation only (Figs. 24-1–24-3). The clinical entity of “nonaneurysmal perimesencephalic subarachnoid hemorrhage” (alternatively named “pretruncal nonaneurysmal hemorrhage”) has since been validated in other countries (7,8). This group probably accounts for 5% of all patients presenting with nontraumatic SAH (9) or a third of the angiogram-negative nonaneurysmal population, with the following features:

Onset of subarachnoid hemorrhage is spontaneous and presents with severe headache and meningeal irritation. Focal neurologic deficits, focal signs, and diminished levels of consciousness are not seen.
The CT pattern of hemorrhage is symmetric or virtually so, confined to the perimesencephalic area, the prepontine cistern and interpeduncular fossa, the posterior part of the suprasellar cistern, the posterior part of the interhemispheric fissure, and to a minimal degree the medial aspects of the Sylvian fissures. Dense clots of blood are uncharacteristic. Extension into the anterior extent of the interhemispheric fissure, into the lateral aspects of the Sylvian fissures, and into the ventricles, or intraparenchymal hematoma are not seen with this entity, and such findings would qualify the condition as diffuse nonperimesencephalic nonaneurysmal hemorrhage (see below) (10). The CT images used to define this pattern must be obtained within 48 hours of the bleed.
Hydrocephalus may develop in a minority of patients with nonaneurysmal perimesencephalic subarachnoid hemorrhage but is usually mild and resolves spontaneously (4,10). Hydrocephalus can occur in patients in whom encirclement of the mesencephalon by blood obstructs flow of CSF through the tentorial incisura (11).
A familial or genetic predisposition seems likely as several instances of siblings or first-degree relatives with this condition have been recognized (12,13).
The prognosis for patients with this pattern of hemorrhage and with negative cerebral angiography is excellent with a low risk of vasospasm, rehemorrhage, or of neurologic sequelae (5,6,14,15).

It is thought that the source of hemorrhage in this group of patients may be venous, possibly from a ruptured prepontine vein or an angiographically occult vascular malformation at the pial surface, arterial microaneurysms of the basilar perforators (16,17), or venous stenotic disease due to congenital causes or acquired narrowings (18). The combination of a typical CT pattern of perimesencephalic subarachnoid hemorrhage and a negative angiogram in a patient with a good clinical grade is very reassuring that the prognosis for the patient is favorable. The likelihood of vasospasm in this group of patients is also very low and rarely of clinical concern. Several publications suggest rates of significant hydrocephalus or clinically symptomatic vasospasm can be seen in up to 25% of patients, but it is likely that many papers have included in their analysis patients with more diffuse patterns of nonaneurysmal subarachnoid hemorrhage (19,20).

The chance of finding an aneurysm in the setting of this pattern of hemorrhage is between 3% and 9% (20,21).

Fewer than 2% of aneurysms present with this pattern of hemorrhage according to most publications (11,22,23), but up to 16.6% of ruptured aneurysms of the posterior circulation can do so (21). Therefore, most authors still agree on the need for at least one cerebral angiogram even when the initial CT pattern is typical for nonaneurysmal perimesencephalic subarachnoid hemorrhage (24), and preferably two angiograms when the pattern of hemorrhage is extensive or atypical (21). Aneurysms typically found hiding under a perimesencephalic pattern of hemorrhage include the tip of the basilar artery; the vertebrobasilar junction possibly associated with a fenestration; the origin of the posterior inferior cerebellar artery contralateral to the side of vertebral artery injection; the distal posterior cerebral artery; or the posterior communicating artery (Figs. 24-4–24-9) (25).

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