Navigation-guided clipping of a de novo aneurysm associated with superficial temporal artery-middle cerebral artery bypass combined with indirect pial synangiosis in a patient with moyamoya disease.

De novo aneurysms associated with superficial temporal artery (STA)-middle cerebral artery (MCA) bypass are an extremely rare complication of direct revascularization surgery for moyamoya disease (MMD). The basic pathology of MMD includes fragility of the intracranial arterial wall characterized by medial layer thinness and waving of the internal elastic lamina. However, the incidence of newly formed aneurysms at the site of anastomosis currently remains unknown. Among 317 consecutive direct/indirect combined revascularization surgeries performed for MMD, we encountered a 52-year-old woman manifesting a de novo aneurysm adjacent to the site of anastomosis 11 years after successful STA-MCA bypass with encephalo-duro-myo-synangiosis (EDMS). Although the patient remained asymptomatic, the aneurysm gradually increased in diameter to more than 6 mm with the formation of a daughter sac, and a computational fluid dynamic study revealed low wall shear stress at the aneurysm dome. The patient underwent microsurgical clipping of the aneurysm using a neuro-navigation system that permitted the minimally invasive dissection of the temporal muscle flap used for EDMS at the site of the aneurysm without affecting pial synangiosis. The aneurysm was successfully occluded using a titanium clip without complications. The postoperative course was uneventful, and the patient was discharged without neurological deficits. De novo aneurysms associated with STA-MCA bypass for MMD may be safely treated with microsurgical clipping, even in cases initially managed by a combined revascularization procedure that includes complex pial synangiosis. We recommend the application of the neuro-navigation system for the maximum preservation of pial synangiosis during this procedure.