Perimesencephalic Pattern of Hemorrhage

The perimesencephalic pattern of hemorrhage is a specific pattern of subarachnoid hemorrhage that often (95% of the time) signifies a nonaneurysmal etiology, an uneventful clinical course, and an invariably good prognosis. In only about 5% of cases is a perimesencephalic pattern of hemorrhage due to a vertebrobasilar aneurysm.

A subarachnoid hemorrhage is said to be perimesencephalic if:

• The center of bleeding is located immediately anterior to the midbrain.
• There is no parenchymal hematoma.
• Subarachnoid blood can be seen in the posterior part of the anterior interhemispheric fissure, as long as there is not complete filling of the anterior interhemispheric fissure.
• There may be extension of blood to the basal part of the sylvian fissure, as long as there is no major extension to the lateral sylvian fissure.
• Small amounts of intraventricular blood may layer dependently, but frank intraventricular hemorrhage should not be seen.

References

Velthuis BK, Rinkel GJ, Ramos LM, Witkamp TD, van Leeuwen MS. Perimesencephalic hemorrhage. Exclusion of vertebrobasilar aneurysms with CT angiography. Stroke. 1999 May;30(5):1103-9.