De novo aneurysm formation on middle cerebral artery branches adjacent to the anastomotic site of superficial temporal artery-middle cerebral artery bypass surgery in two patients: technical case report.

OBJECTIVE: Aneurysm formation is a complication of superficial temporal artery-middle cerebral artery bypass surgery occurring as pseudoaneurysms caused by technical failure, but also as true aneurysms discovered after long-term follow-up.

CLINICAL PRESENTATION: A 53-year-old woman presented with a left internal carotid artery cavernous aneurysm manifesting as double vision. Superficial temporal artery-middle cerebral artery bypass, internal trapping of the internal carotid artery, and embolization were performed. Three years later, angiography disclosed a distal middle cerebral artery aneurysm. A 70-year-old man who had undergone right superficial temporal artery-middle cerebral artery bypass after internal carotid artery occlusion died of subarachnoid hemorrhage from a ruptured anterior spinal artery aneurysm 21 years later. Angiography and postmortem examination revealed de novo aneurysm formation on a middle cerebral artery branch adjoining the anastomotic site. Both patients had hypertension and multiplicity of aneurysms.

INTERPRETATION: Both cases were de novo true aneurysms caused by hemodynamic stress because of saccular to fusiform shape, location extending to the middle cerebral artery, high perfusion pressure, projection along the hemodynamic stress, and presence of common risk factors.

CONCLUSION: Bypass surgery is increasingly performed in patients with complicated aneurysms if sacrifice or temporary occlusion of any major vessel is required. Therefore, de novo aneurysm formation may not be rare in patients with risk factors such as hypertension or multiple aneurysms. Extended follow-up examination is required in such patients.