Angiographic characteristics of improper watershed shift after STA-MCA bypass in a patient with moyamoya disease: illustrative case

BACKGROUND: In patients with moyamoya disease (MMD) who receive superficial temporal artery (STA) to middle cerebral artery (MCA) bypass, hypoperfusion remote from the anastomosis site rarely occurs. Watershed shift due to direct bypass has been proposed as the mechanism; however, no report has confirmed this phenomenon using angiography. OBSERVATIONS: A 48-year-old man presented with transient weakness in his left arm. Angiography revealed severe bilateral stenosis of the MCAs and moyamoya vessels. The right anterior cerebral artery (ACA) had short stenosis at A2 but ample blood supply to the cortical area of the right ACA and MCA regions. The patient was diagnosed with MMD and received a single STA-MCA bypass. The next day, he had difficulty communicating, and a cerebral infarction away from the anastomosis site was identified. Perfusion examination revealed hyperperfusion around the direct bypass and hypoperfusion away from the anastomosis site. Angiography revealed bypass patency; however, the original anterograde flow of the right ACA decreased significantly at the stenosed point, indicating an improper watershed shift. LESSONS: STA-MCA bypass for patients with MMD can cause an improper watershed shift decreasing cerebral flow. Donor flow should be prepared based on each angiographic characteristic, and the risk of the improper watershed shift should be considered.