Direct cerebral revascularization remains a central therapeutic approach for Moyamoya disease (MMD) and Moyamoya syndrome (MMS). However, existing outcome data are largely derived from East Asian populations, with limited evidence characterizing North American cohorts. This study presents the experience of a Canadian university hospital with superficial temporal artery–middle cerebral artery (STA–MCA) bypass for MMD and MMS, focusing on perioperative management, surgical outcomes and graft durability.

A retrospective case series was conducted, including nine patients with MMD/MMS who underwent STA–MCA bypass between December 2021 and March 2026 at the Hôpital du Sacré-Coeur de Montréal. Data on demographics, clinical presentation, perioperative care and follow-up outcomes were analyzed. Primary outcomes included bypass graft patency and ischemic or hemorrhagic events. Secondary outcomes included functional status assessed via the modified Rankin Scale and perioperative complications.

All patients (n = 9) demonstrated sustained graft patency, with a median clinical follow-up of 16 months and a median radiological follow-up of 14 months. Eight patients (89%) experienced stable functional outcomes, while one (11%) experienced significant neurological decline due to postoperative complications. No ischemic events were observed during follow-up; however, one patient experienced a subdural hematoma in the immediate postoperative period, necessitating surgical intervention.

Direct STA–MCA bypass is a viable revascularization strategy for MMD/MMS, achieving high graft patency and preventing recurrent ischemic events. It can be safely established in a Canadian center. However, the potential for complications, particularly in high-risk patients, highlights the critical importance of meticulous perioperative management and careful patient selection.

Moyamoya disease (MMD) is characterized by progressive carotid fork steno-occlusion and the development of “puff-of-smoke” collaterals on angiography. However, a subset of patients present with similar vascular changes but lack these hallmark collaterals, complicating both diagnosis and management. This “smokeless” phenotype, associated with ring finger protein 213 (RNF213) gene variants, challenges the traditional description of MMD. We describe a series of such patients who responded favorably to revascularization.

In this ambispective observational study, we evaluated 12 patients with carotid fork steno-occlusive disease but without “puff-of-smoke” collaterals. Clinical, radiological and genetic assessments were assessed. Structural modeling of RNF213 protein variants was conducted through 3D homology modeling, validated via Ramachandran plots and further refined with COOT and PyMOL. Functional insights were derived through ConSurf analysis.

Of the 12 patients, 9 carried the RNF213 p.R4810K variant, 1 harboured a novel variant, 1 had both p.R4810K and a novel variant and 1 had p.R4859K. Initial misclassification as intracranial atherosclerosis or vasculitis led to inappropriate treatment. Following genetic confirmation, 9 patients underwent revascularization, with no stroke recurrence and a favorable clinical outcome. Structural modeling revealed minimal functional impact for the Val1529Met variant, whereas other variants significantly disrupted RNF213 stability and functionality.

“Smokeless moyamoya,” characterized by carotid fork steno-occlusion without typical angiographic collaterals, represents a distinct clinical phenotype responsive to revascularization. RNF213 genetic screening enhances diagnostic precision, reshaping traditional paradigms and supporting tailored therapeutic approaches.