Significance of a small bulge on the basilar artery in patients with perimesencephalic nonaneurysmal subarachnoid hemorrhage. Report of two cases.

Perimesencephalic nonaneurysmal subarachnoid hemorrhage (SAH) is a distinct type of hemorrhage with a characteristic bleeding pattern and an excellent clinical outcome. The cause of this benign form of SAH remains unknown. The authors report on two cases of perimesencephalic nonaneurysmal SAH in which a small bulge on the basilar artery (BA) was demonstrated on three-dimensional rotational angiography studies. Based on data from these cases, one may infer that the lesion on the BA is responsible for the SAH. The possible pathogenesis is discussed.