A case of middle cerebral artery large circumferential aneurysm treated with bypass-assisted trapping surgery.

M1 large circumferential aneurysms are clinically challenging because they cannot be treated by simple neck clipping and they may involve the lenticulostriate arteries (LSAs). Although some reports have described endovascular and direct surgical treatment of these aneurysms, the optimal treatment approach remains uncertain. We report a case involving a ruptured large M1 circumferential aneurysm that was treated with bypass-assisted trapping surgery and showed favorable outcomes. The patient was a 47-year-old man presenting with subarachnoid hemorrhage. Digital subtraction angiography revealed a large circumferential aneurysm in the right middle cerebral artery M1 segment with involvement of the lateral and medial LSAs. We successfully performed trapping surgery with the assistance of a superficial temporal artery (STA)-M2 bypass while preserving the medial and lateral LSAs. Although left hemiparesis caused by medial LSA thrombosis appeared in the early postoperative period, the patient showed good recovery from symptoms with rehabilitation and could independently perform daily activities at the five-month follow-up. The treatment of M1 large circumferential aneurysms should involve considerations for prevention of rebleeding, blood supply to the distal area, and preservation of perforating arteries. The treatment strategy for this challenging aneurysm should be planned based on the patient's condition and individual anatomy.