Microvascular decompression for hemifacial spasm associated with large fenestration of the vertebral artery: illustrative case.

backgroundHemifacial spasm (HFS) associated with a large fenestration of the vertebral artery (VA) is extremely rare. Here, the authors report a case of HFS caused by this vascular anomaly and discuss its embryological background and clinical implications. OBSERVATIONS: A man in his 30s presented with a 7-year history of right-sided HFS. MRI demonstrated vascular compression at the root exit zone of the facial nerve. MR angiography revealed an aberrant artery running parallel to the right VA, forming a large fenestration. Three-dimensional CT angiography demonstrated a common trunk giving rise to the anterior inferior cerebellar artery and posterior inferior cerebellar artery. The offending artery was identified as the common trunk.. Microvascular decompression with real-time abnormal muscle response monitoring achieved sufficient arterial transposition, with careful preservation of the brainstem perforators. The patient experienced complete resolution of spasms postoperatively, without complications. LESSONS: A large VA fenestration may represent a persistent primitive lateral basilovertebral anastomosis. Successful surgical treatment requires a precise preoperative understanding of the complex vascular architecture and consideration of the limitations imposed by perforating branches during arterial transposition. https://thejns.org/doi/10.3171/CASE25777.