Pre-mesencephalic subarachnoid hemorrhage: rupture of tiny aneurysms of the basilar artery perforator

Subarachnoid hemorrhage (SAH) around the midbrain without evidence of aneurysm, a so-called perimesencephalic SAH, has been considered a typical nonaneurysmal SAH. Recently, we have encountered several patients with SAHs that could have been classified as having perimesencephalic SAH, but a common cause of the bleeding was demonstrated. In this article, we describe clinical and radiologic characteristics of these patients. Clinical and radiologic data from patients with spontaneous SAH (total number 339) who were treated at Seoul National University Bundang Hospital between May 2003 and December 2007 were reviewed. Of the 13 patients that could be classified as having perimesencephalic SAH, three had common radiologic features that were distinct from others. On computed tomography, the main hemorrhage (hematoma) was found localized in front of the midbrain (interpeduncular and/or peduncular cistern). Angiographically, the cause of the bleeding was not seen on conventional views and rotational angiograms. In three-dimensional reconstructed angiographic images, very small-sized (tiny) aneurysmal lesions were visible at the origin of mesencephalic perforators from the basilar artery. All the aneurysms were positioned at the exact site corresponding to the pre-mesencephalic clots. Follow-up angiography was performed on the three patients and all showed complete disappearance of the lesions at 1 month, 15 months, and 16 months follow-up, respectively. Based on our experience, we suggest a subtype of spontaneous SAH that has unique hemorrhage localization (pre-mesencephalic cistern), specific cause (tiny aneurysms at the origin of the mesencephalic perforator), and a common benign clinical course.