Perimesencephalic hemorrhage - Exclusion of vertebrobasilar aneurysms withCT angiography

Background and Purpose-It is important to recognize a perimesencephalic pat tern of hemorrhage in patients with subarachnoid hemorrhage (SAH), because in 95% of these patients the cause is nonaneurysmal and the prognosis is ex cellent. The purpose of this study was to investigate whether CT angiograph y can accurately exclude vertebrobasilar aneurysms in patients with perimes encephalic patterns of hemorrhage and therefore replace digital subtraction angiography (DSA) in this setting. Methods-In 40 patients with posterior fossa SAH as shown on unenhanced CT, 2 radiologists independently evaluated unenhanced CT for distinguishing bet ween perimesencephalic and nonperimesencephalic pattern of hemorrhage and a ssessed CT angiography for detection of aneurysms. All patients subsequentl y underwent DSA or autopsy. Results-Observers agreed in 38 of 40 patients (95%) in differentiating peri mesencephalic and nonperimesencephalic patterns of hemorrhage on unenhanced CT. On the CT angiograms, both observers detected a vertebrobasilar aneury sm in 16 patients and no aneurysm in 24 patients. These findings were confi rmed by DSA or autopsy. No patients with a perimesencephalic pattern of hem orrhage were found to have an aneurysm on either CT angiography or DSA. Conclusions-Good recognition of a perimesencephalic pattern of hemorrhage i s possible on unenhanced CT, and CT angiography accurately excludes and det ects vertebrobasilar aneurysms. DSA can be withheld in patients with a peri mesencephalic pattern of hemorrhage and negative CT angiography.