Efficacy of a Modified Superficial Temporal Artery–Middle Cerebral Artery Bypass Using Superficial Temporal Artery Side-Branch Donors in Adult Moyamoya Disease: A Technical Note

<b>Background:</b> Adult moyamoya disease (MMD) is a progressive steno-occlusive cerebrovascular disorder for which surgical revascularization is the primary treatment. The standard direct superficial temporal artery-middle cerebral artery (STA-MCA) bypass uses the frontal and/or parietal branch of the STA as the donor. However, in some patients, conventional STA-MCA bypass may be suboptimal because of a large mismatch in caliber between the STA branch and the recipient artery, increasing the risk of cerebral hyperperfusion. This study aimed to investigate the impact of a modified STA-MCA bypass on MMD treatment. <b>Methods:</b> We retrospectively reviewed adult cases of MMD at our institution (2012-2025) for patients who underwent modified direct STA-MCA bypass using a small side branch of the STA as the donor artery. Surgical techniques and clinical outcomes of these cases were analyzed descriptively. <b>Results:</b> Five cases (five hemispheres in four patients) underwent side-branch STA-MCA bypass. All procedures were completed successfully, with 100% graft patency confirmed by intraoperative indocyanine green angiography, and a mild increase in cerebral blood flow confirmed by postoperative single-photon emission computed tomography. No patients developed postoperative cerebral hyperperfusion syndrome or wound healing complications. Clinically, all patients experienced a stable or improved neurological status, with no reported new ischemic or hemorrhagic events during follow-up. <b>Conclusions:</b> In this small feasibility series, the side-branch STA-MCA bypass was technically feasible and safe, with no cerebral hyperperfusion syndrome observed. Any risk-mitigating effect on hyperperfusion remains theoretical and requires confirmation in comparative studies.