De novo formation and rupture of an intracranial aneurysm 10 months after normal findings on conventional magnetic resonance angiography in a patient with no history of intracranial lesions: case report.

A 40-year-old male smoker presented with a ruptured saccular aneurysm that formed de novo 10 months after normal magnetic resonance (MR) angiography findings. Computed tomography of the head at the second admission showed subarachnoid hemorrhage in the interhemispheric fissure and anterior basal cistern. MR and conventional cerebral angiography revealed an aneurysm arising from the anterior communicating artery (AcomA). MR angiography performed 10 months earlier, when he experienced transient loss of consciousness, revealed no anomalies at the AcomA. Intraoperatively, the de novo aneurysm was found to be a typical saccular aneurysm with a fragile wall.