Advancing Evidence Quality and Reproducibility in Bypass in Neurosurgery Research: A Systematic Review and Reporting Guideline for Enhanced Comparability

Background and Objectives: Bypass surgery is an important option for treating complex intracranial aneurysms (IA) and Moyamoya disease (MMD), particularly when conventional endovascular or microsurgical approaches are insufficient. Despite advances in surgical techniques and imaging, the literature on bypass procedures remains heterogeneous, with methodological limitations that hinder reproducibility and comparability. We aimed to evaluate the current evidence on bypass surgery for IA and MMD, assess the quality and robustness of existing studies, and propose a Reporting Guideline to improve transparency, comparability, and methodological rigor in future research. Methods: Following PRISMA guidelines, we searched PubMed, Embase, Cochrane Library, and Web of Science databases. Eligible studies were observational or randomized; reported clinical and/or surgical outcomes of Bypass for IAs or MMD; included a sample size of ≥3 patients; and were published in English between January 1, 2000, and January 1, 2025. Studies were assessed with a focus on key domains: (1) Assessment of how studies reported on the baseline characteristics of the patient sample; (2) Assessment and reporting on imaging methods and aneurysm characteristics; (3) Reporting on pivotal concepts definitions; (4) Reporting on Neurosurgeon(s) and staff characteristics; (5) Reporting on anesthetic protocol; (6) Reporting on antiaggregant and anticoagulation therapy; (7) Reporting on Surgical Details; (8) Assessing and Reporting Clinical and Surgical Outcomes; (9) Reporting Retreatment Details. Results: Thirty studies comprising 2,667 patients were included. Our assessment revealed substantial reporting gaps across all domains pertinent to bypass – imaging and aneurysm characterization, key definitions, neurosurgeon/staff, anesthetic protocol, procedure details, antiaggregant/anticoagulation, outcomes and adverse events, and retreatment. A Bypass Reporting Guideline was developed, structured across 10 domains with a pragmatic checklist emphasizing technical description and patency verification, antithrombotic management, and standardized clinical and radiologic outcome reporting. Fig. 1–3 Bypass Reporting Guideline Checklist. Conclusion: This systematic review of 30 studies on bypass surgery for intracranial aneurysm (IA) and moyamoya disease (MMD) highlighted significant limitations in methodological and reporting quality, which hinder the advancement of evidence levels and the reliability of cumulative findings in the current literature. The proposed Reporting Guideline emerges as a solution, offering a structured framework to address these gaps and improve transparency, reproducibility, and comparability in future research. By promoting standardized reporting, the guideline has the potential to strengthen the evidence base, facilitate the development of cumulative knowledge, and ultimately enhance both understanding and patient care in the context of IA and MMD. Publication History Article published online: 27 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany