Hemifacial Spasm: Intraoperative Identification for Adequacy of Microvascular Decompression

Background : Delayed effect of microvascular decompression (MVD) in hemifacial spasm (HFS) is not uncommon one unlike MVD in trigeminal neuralgia (TN) with immediate pain relief. Final evaluation of MVD effect in HFS is possible at least one year after surgery. Intraoperative monitoring (IOM) plays a great role in identification of main offending vessels and gives confidence to the surgeon in adequacy of facial nerve (FN) decompression, saving patient from unnecessary revision surgery. Objective. To evaluate sufficiency of FN decompression using IOM with abnormal muscle response/lateral spread response (AMR/LSR) registration. Practical application of new waveform for IOM in HFS based on “sympathetic bridge hypothesis” (Zheng X., Li S. et al. 2012). Methods : Between 2008 and 2013, fourteen patients were underwent MVD with preoperative and intraoperative AMR/LSR registration. Additionally “Z-L response” (Zheng X. et al. 2012) was registered from facial muscles in stimulation of offending vessels in 2 patients. Three patients had concomitant TN, two patients had posterior fossa meningioma with secondary HFS. Intraoperative identification of compressing vessels was matched with IOM responses. Assessment of the clinical results was performed with four-grade scale (excellent, good, fair, poor). Results : AMR/LSR was recorded in all patients before MVD (mean latency 12.6ms). AMR disappeared after dura opening in two cases, so it could not be used for monitoring. AMR disappeared after offending vessel transposition and FN decompression in 11 patients. IOM played guiding and confirming role in these cases assisting in offenders identification. In one patient AMR did not disappeared in spite of FN decompression and IOM was inconclusive. This patient was not spasm free and he was reoperated in 4 months. Additionally to AMR, “Z-L response” was registered in two patients and disappeared after MVD. In one year, 9 patients (64%) had excellent result, 4 (29%) had good result (including two patients with AMR disappearance after dura opening), 1 patient had poor functional outcome. In general, MVD results correlated with IOM data. Immediate spasm relief was observed in 9 cases, delayed effect – in 4 patients. Conclusions : IOM in HFS surgical management is an important point in supporting of sufficient FN decompression, improvement of treatment outcomes and patient safety. Further investigations of «sympathetic bridge hypothesis» with associated IOM techniques are required.