Hemifacial Spasm Caused by a Penetrating Arterial Offender

Abstract Hemifacial spasm (HFS) is a neurovascular compression disorder in which an arterial loop irritates the facial nerve at its root exit zone (REZ). Penetrating arterial offenders and mixed double-compression patterns involving both the REZ and the cisternal segment of the facial nerve are exceptionally rare, and the optimal extent of decompression in such situations remains unclear. We report a 49-year-old man with right HFS associated with an anterior inferior cerebellar artery loop compressing the REZ and penetrating the cisternal portion of the facial nerve. Decompression of the REZ and the inferior aspect of the penetrating segment was performed under intraoperative neurophysiological monitoring. Further superior dissection was avoided when A-train activity appeared, and the operation resulted in complete resolution of HFS without facial nerve palsy. This case illustrates a rare penetrating, mixed proximal–cisternal double-compression pattern and highlights the role of intraoperative neurophysiological monitoring in tailoring the extent of decompression. In such configurations, decompression should balance lateral spread response resolution with avoidance of A-train activity to preserve facial nerve function.