Pial collateral shift after unilateral bypass in bilateral moyamoya disease: illustrative cases.

backgroundBilateral revascularization surgery is commonly performed in patients with bilateral moyamoya disease (MMD), even when ischemic symptoms or strokes are confined to one hemisphere. However, in select cases, a unilateral bypass may suffice to revascularize both hemispheres due to the development of post-revascularization pial collateral shift. This report aims to introduce the concept of pial collateral shift and presents 3 cases of bilateral MMD in which unilateral revascularization alone was deemed sufficient. OBSERVATIONS: Three adult patients with ischemic MMD exhibited bilateral type III vascular insufficiency on single-photon emission CT (SPECT) imaging after Diamox challenge. Each underwent unilateral revascularization of the symptomatic hemisphere. Postoperatively, cerebral angiography and SPECT with Diamox were performed. The angiograms revealed a phenomenon the authors term "pial collateral shift," an adaptive redistribution of pial collateral flow following cerebral revascularization, specifically an increase in pial collateral flow toward the non-bypassed hemisphere in those 3 cases. Postoperative SPECT with Diamox demonstrated normalization of cerebrovascular reserve in both hemispheres, eliminating the need for contralateral revascularization. LESSONS: Pial collateral shift observed on the postoperative angiogram suggests a fully augmentative bypass. Patients with bilateral MMD should be reassessed after the initial bypass, as bilateral revascularization may not be necessary in all cases. https://thejns.org/doi/10.3171/CASE25229.