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This article reviews the current knowledge of the pathogenesis, clinical features, diagnostic evaluation, and treatment of intracranial aneurysms. The vast majority are acquired lesions, likely as a result of hemodynamic stress on the relatively unsupported bifurcations of the intracranial arteries. Cigarette smoking is an emerging important risk factor for their development. The natural history of unruptured aneurysms is controversial and may be subject to revision. A benign perimesencephalic pattern of subarachnoid hemorrhage has been recognized that usually is not due to a ruptured aneurysm. Current treatment includes surgical clipping and endovascular treatment with Guglielmi detachable coils, which produces comparable results in some instances. Epidemiologic studies suggest that the outcome of patients with aneurysmal subarachnoid hemorrhage remains poor, with a 30-day death rate of 45%, although the outcome of patients who reach the hospital alive may have improved in recent years.