A 51-year-old female was admitted complaining of headache and vomiting. On admission her consciousness was clear, and neurological examination showed no abnormality except for nuchal stiffness. Computed tomographic scans demonstrated subarachnoid hemorrhage (SAH) and a hematoma in the fronto-temporal lobe. However, cerebral angiography revealed neither aneurysmal shadow nor other causes of bleeding. On the day of admission, exploratory surgery was undertaken. Aneurysms or other vascular lesions were not found on the C
1 and C
2 of the internal carotid artery, M
1 and M
2 of the middle cerebral artery (MCA), and A
1 of the anterior cerebral artery, while a small mass 1 cm in diameter was found on a branch of the anterior temporal artery derived from the M
1 segment. Trapping of the feeding and draining vessels was performed and the mass was resected. Histopathologically the wall of the mass was lacking in the media and internal elastic lamina and was composed of lamina interna and adventitia. Inflammatory, tumorous and dissecting lesions were excluded. Postoperative course was uneventful, and the patient was discharged with no neurological deficits.
SAH of unknown etiology is thought to be benign lesions, while some of the patients diagnosed as SAH of unknown etiology occasionally present with rebleeding, and the prognosis is not always good. Exploratory surgery frequently delineates the cause of bleeding in those patients diagnosed as SAH of unknown etiology. However, it is very rate that the angiographically occult aneurysm is located on the peripheral branch of the distal MCA as it was in the present case. Exploratory surgery should be undertaken when aneurysms are suspected to be the cause of bleeding in those patients diagnosed as SAH of unknown etiology. And it is important to investigate not only the main trunk of the cerebral arteries but also the peripheral branches as extenively as possible in the exploratory surgery.
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