Background: Treating cerebral aneurysms in the M4 segment of the middle cerebral artery (MCA) is challenging because they are small and are buried in the brain parenchyma. Case Description: A right-handed Asian woman in her 80s was referred to our hospital with a chief complaint of altered level of consciousness. On admission, her consciousness level on the Glasgow Coma Scale was 7 (E1V1M5), and a computed tomography (CT) scan showed subarachnoid hemorrhage with intracerebral hematoma in the left temporal lobe. Subsequent 3-dimensional CT angiography showed an aneurysm in the M4 segment of the left MCA. The aneurysm of the patient was clipped safely and effectively because of the navigation system in combination with intraoperative angiography. The navigation system was especially useful for estimating the proximal part of the parent artery at the brain surface, whereas intraoperative angiography was especially useful for confirming that the proximal portion of the parent artery identified by the navigation system was correct. Conclusions: We emphasize the importance of choosing the modality of intraoperative imaging according to each characteristic when treating M4 segment aneurysms.
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