Unruptured cerebral aneurysm producing a thunderclap headache.

A sudden and severe headache is the most common presentation of an acutely ruptured cerebral aneurysm. A similar headache in the absence of subarachnoid blood has rarely been ascribed to an unruptured cerebral aneurysm, but may result from acute aneurysm expansion and indicate a high risk of future rupture. We present a patient who developed a sudden, severe, "thunderclap" headache, with no associated neurological deficit. Computed tomogram and lumbar cerebral spinal fluid obtained 5.5 hours after headache onset were negative for subarachnoid hemorrhage. The patient underwent cerebral angiography which revealed a posterior communicating artery aneurysm with an associated daughter aneurysm. Craniotomy and clip obliteration of the aneurysm were performed. The aneurysm dome was very thin and there was no evidence of recent or old hemorrhage. A "thunderclap" headache without subarachnoid hemorrhage may be an important harbinger of a cerebral aneurysm with the potential for future rupture. Early recognition and neurovascular imaging of aneurysms presenting in this rare fashion are warranted.