Hemifacial Spasms with Unusual Neurovascular Compression Type: Arterial Cisternal Segment Offender.

(1) Background: Hemifacial spasm (HFS) is most commonly caused by neurovascular compression at the root exit zone (REZ) of the facial nerve; however, isolated compression along the distal cisternal segment is uncommon and remains poorly characterized. This study aimed to analyze the clinical features, intraoperative neurophysiological patterns, and surgical outcomes of patients with HFS caused by cisternal segment arterial compression. (2) Methods: Among 874 patients who underwent microvascular decompression (MVD) for HFS, 18 (2.1%) were identified as having isolated neurovascular conflict at the cisternal segment, all involving the anterior inferior cerebellar artery (AICA). Clinical characteristics, offender location, intraoperative monitoring results including lateral spread response (LSR), brainstem auditory evoked potentials, and postoperative outcomes were retrospectively evaluated. A standardized Teflon interposition technique was used in all cases. (3) Results: Postoperatively, 83.3% of patients experienced immediate spasm relief, and at the latest available follow-up, 94.4% achieved significant improvement without severe complications. (4) Conclusions: Although rare, cisternal segment arterial compression can produce typical HFS and should be considered when REZ compression is unclear or when intraoperative neuromonitoring does not respond as expected. Microvascular decompression using Teflon interposition is a safe and effective treatment option for this anatomically challenging offender location.