HOW OFTEN IS A PERIMESENCEPHALIC SUBARACHNOID HEMORRHAGE CT PATTERN CAUSED BY RUPTURED ANEURYSMS

Background: Angiography is usually recomended in perimesencephalic subarachnoid haemorrhage (PM SAH) to rule out a basilar artery aneurysm. However it is not known how often aneurysms are found among patients with a CT pattern of PM haemorrhage or the frequency of this CT pattern after rupture of posterior circulation aneurysms. Method: CTs of all SAH caused by posterior circulation aneurysms admited from 1/85 to 12/92 where reviewed by two examiners. Late ( > 72 h) examinations were excluded. The remaining CTs were classified in perimesencephalic (PM) or non PM. Results: 81 posterior circulation aneurysms were collected. Only one PM-like CT pattern was found, due to ruptured posterior communicating artery aneurysm (1.3%; 95% Cl (confidence intervals) = .03.-6.7%). During the same period 37 PM SAH with negative angiographic results were admited. The likehood of finding an aneurysm in a patient with an early CT showing a PM distribution of haematic densities was 2.7% (95% Cl =.07-.14%). Conclusion: Although the probability of finding an aneurysm in a SAH patient with a PM CT pattern is low, a complete 4-vessel angiogram must be obtained.