A Case of a Non-giant Intracranial Aneurysm with Spontaneous Occlusion Directly Observed during Clipping Surgery

A man in his 60s was referred to our hospital due to a large basilar artery aneurysm incidentally detected 12 years before. Cerebral angiography revealed the basilar artery aneurysm and another 2 aneurysms at the left middle cerebral artery. Coil embolization was performed for the basilar artery aneurysm, and the middle cerebral artery aneurysms were to be monitored because of their small size. The middle cerebral artery aneurysms had enlarged on magnetic resonance imaging over 8 years, and therefore we recommended surgery. Clipping was performed 4 years after the recommendation and intraoperative findings revealed that there were 3 aneurysms at the left middle cerebral artery, one of which closest to the M2 superior trunk harbored an entirely sclerosed dome wherein no blood inflow was observed. Rather than simple growth of the 2 middle cerebral artery aneurysms initially found 12 years before, it appeared that one of the 2 middle cerebral artery aneurysms had spontaneously occluded, and another had been newly formed and enlarged. Spontaneous occlusion of small, unruptured saccular, intracranial aneurysms is rare, and just 10 cases have been reported in the literature. To the best of our knowledge, this is the first report in which the spontaneous occlusion of such an aneurysm was directly observed during microscopic surgery.