Venous compression of the facial nerve root exit zone by a transverse pontine vein: Intraoperative findings and quantitative lateral spread response monitoring.

Background: Hemifacial spasm (HFS) is most commonly caused by arterial compression at the root exit zone (REZ) of the facial nerve. In contrast, isolated venous compression in the absence of arterial involvement is uncommon, accounting for 0.4-5.5% of cases in surgical series, and poses diagnostic challenges due to the subtle appearance of venous structures on conventional neuroimaging. This rarity highlights the importance of recognizing pure venous neurovascular conflict as a distinct, albeit underdiagnosed, and etiology of HFS. Case Description: We report a 69-year-old woman with a 5-year history of refractory right-sided HFS. Although preoperative magnetic resonance imaging suggested possible arterial contact, retrosigmoid exploration revealed compression of the facial nerve REZ by a transverse pontine vein coursing along the middle cerebellar peduncle, with no arterial offender identified. Microvascular decompression was performed through careful mobilization and transposition of the offending vein using Teflon interposition. Intraoperative quantitative lateral spread response (LSR) monitoring showed marked attenuation of response amplitudes following decompression. The patient experienced immediate and complete resolution of symptoms postoperatively. Conclusion: This case illustrates that pure venous compression by a transverse pontine vein is a rare but surgically treatable cause of HFS that can be misinterpreted as arterial conflict on imaging. A successful outcome was achieved through meticulous intraoperative identification and vein-preserving decompression, combined with quantitative LSR monitoring, which proved valuable even without complete response disappearance. Thorough exposure of the REZ, including endoscopic inspection when indicated, remains important to identify such atypical neurovascular conflicts.