RUPTURED VERTEBROBASILAR ANEURYSMS - FREQUENCY OF THE NONANEURYSMAL PERIMESENCEPHALIC PATTERN OF HEMORRHAGE ON CT SCANS

PURPOSE: To determine the frequency of the computed tomographic (CT) p attern of nonaneurysmal perimesencephalic subarachnoid hemorrhage in t he setting of ruptured posterior fossa aneurysms. MATERIALS AND METHOD S: Four neuroradiologists independently and retrospectively reviewed c ranial CT scans of 169 patients with ruptured vertebrobasilar aneurysm s (44 cases of posteroinferior cerebellar artery aneurysm, 20 cases of superior cerebellar artery aneurysm, and 105 cases of basilar-tip ane urysm). RESULTS: The mean frequency of nonaneurysmal perimesencephalic pattern of subarachnoid hemorrhage in ruptured vertebrobasilar aneury sms was 7.1% (48 of 676 readings) among the four readers. The mean fre quency of aneurysms with the pattern of hemorrhage for each location w as as follows: basilar tip, 9.8% (41 of 420 readings); superior cerebe llar artery, 5.0% (four of 80 readings); and posteroinferior cerebella r artery, 0%. In 75% (six of eight) of the cases in which the CT patte rn of hemorrhage was deemed compatible by all readers with nonaneurysm al hemorrhage, the clinical presentation was mild. CONCLUSION: Because ruptured posterior fossa aneurysms manifest with the nonaneurysmal pa ttern of hemorrhage in approximately 10% of cases, a high degree of su spicion should be maintained even if the pattern of subarachnoid hemor rhage suggests a nonaneurysmal origin and clinical symptoms are mild.