The Prognosis Of Microvascular Decompression For Hemifacial Spasm And The Pathogenesis Of Hemifacial Spasm Were Evaluated By Electrophysiological Monitoring

OBJECTIVE: Microvascular decompression(MVD)represents an effective treatment for hemifacial spasm(HFS).The aim of this study was to explore the application value of abnormal muscle response intraoperative monitoring as a prognostic indicator for the outcome of MVD,and to explore the origin and mechanism of abnormal muscle response(AMR).HFS is a cranial nerve hyperactivity disorder characterized by unique neurophysiological features,although the underlying pathophysiology remains disputed.In this study,the authors compared the effects of sevoflurane on facial nerve motor evoked potentials(FNMEP)from the spasm and nonspasm sides of patients who were undergoing MVD to test the hypothesis that hemifacial spasm is a central elevation of facial motor neuron excitability.To determine whether the monitoring of FNMEP during MVD for HFS might be useful for predicting the postoperative clinical course and final outcomes.METHODS:Part 1:Between June 2012 and March 2016,a total of 124 patients underwent MVD for HFS with intraoperative monitoring at Suzhou Kowloon Hospital,Shanghai Jiaotong University School of Medcine.Patients were divided into two groups on the basis of disappearance or persistence of AMR after operation.Immediated outcome and clinical outcome during follow-up were assessed.Data analysis was performed to investigate the effect of intraoperative abnormal muscle response monitoring on efficacy of MVD.Part 2: Between Apirl 2016 and March 2018,a total of 60 patients underwent MVD for HFS with intraoperative monitoring at Suzhou Kowloon Hospital,Shanghai Jiaotong University School of Medcine.Patients were underwent both AMR and FNMEP monitoring during MVD and were administered total intravenous anesthesia with or without additional sevoflurane,an inhaled anesthetic known to centrally suppress motor evoked potentials.By studying the effect of sevoflurane on AMR monitoring,we can infer the formation mechanism and source of AMR.Part 3:Between April 2018 and March 2019,a total of 60 patients underwent MVD for HFS with intraoperative monitoring at Suzhou Kowloon Hospital,Shanghai Jiaotong University School of Medcine.FNMEP monitoring were elicited in all the patients and administered total intravenous anesthesia with or without additional sevoflurane.The activation threshold voltage and mean amplitudes of the FNMEP from both sides of the face were compared.By studying the effect of sevoflurane on the FNMEP monitoring,it was verified that the FNMEP originates from the facial nerve motor neuron.Immediated outcome and clinical outcome during follow-up were assessed.Data analysis was performed to investigate the effect of intraoperative FNMEP monitoring on efficacy of MVD.RESULTS:Part 1:A complete elimination of AMR was observed in 92 of 132 patients.It occured in 12 cases during craniospinal fluid drainage,in 23 cases during dissection of arachnoid membrane,and in 57 cases during implantation of Teflon.At completion of dura closure,30 cases still had AMR persistence.There was no significant difference in preoperative clinical characteristics as well as postoperative complications between the two groups.The patients had significantly higher spasm relief in immediate postoperative period when AMR disappeared during the procedure.There is no difference in spasm relief at follow-up period between the two groups.Part 2:AMR was observed during intraoperative monitoring in all of the 60 patients.There was no significant difference in amplitude of AMR during administered total intravenous anesthesia with or without additional sevoflurane.A complete elimination of AMR was observed in 44 of 60 patients.A tatol of 16 patietns with persistence of AMR were underwent facial nerve combing.AMR disappeared in 14 out of 16 paitents,but the amplitude of facial nerve motor evoked potentials did not change significantly.There was no significant difference in immediated outcome as well as clinical outcome during follow-up between the two groups.Part 3:We successfully recorded FNMEP in all patients from spasm-side,and 29 of 30 from control-side.The mean amplitudes of FNMEP were 207.2±62.1 ?V on the HFS side and 180.2±55.0 ?V on the control side.The threshold voltage that elicited these responses was significantly lower on the spasm side than on the control side(140.3 ± 26.8 VS177.0 ± 23.2 V;,respectively).The effects of sevoflurane on FNMEP responses were compared between the HFS and control sides after the administration of intravenous anesthesia.Sevoflurane predictably significantly suppressed FNMEP amplitude at both 0.5 and 1.0 MAC.CONCLUSION: AMR is an invaluable tool in MVD for identifying the offending vessel and adequate decompression.The AMR-related findings may help to predict whether HFS disappears immediately after surgery or delayed resolution.Persistence of AMR correlated dose not always correlate with poor outcome.An AMR results from lateral spread of impulses between motor axons at the site of vascular compression rather than at the facial nucleus.This study demonstrated that facial nerve combing was a safe and effective treatment for HFS in AMR persistence cases after MVD.Facial motor neuron excitability was significantly greater on the spasm side than on the nonspasm side.Motor neuron excitability and the actions of sevoflurane are both the result of physiological processes with the central nervous system.The excitability within the facial motor neuron pool is a result of chronic antidromic signaling produced by the rhythmic pulsing on facial nerve axons.