Beyond the observation of all or nothing: The clinical significance of the pre-decompression instability of abnormal muscle response in Microvascular decompression for Hemifacial spasm

Background: In microvascular decompression (MVD) surgery, abnormal muscle response (AMR) monitoring was utilized to confirm sufficient decompression. However, the AMR seems to contain more information that could improve surgical results. Method: Patients' records of HFS treated with MVD under AMR monitoring, from January 2018 to December 2019 in our centre, were retrospectively reviewed. MVD procedures were performed via a suboccipital retro-sigmoid approach, and AMR monitoring was performed. Pre-Decompression Instability (PDI) of AMR before the final decompression, including amplitude inconsistency and waveform chaos, was inspected and notified to the surgeon. Result: 165 cases were found with full follow-up data. In these cases, PDI was recognized in 144 cases. And in the remaining 21 cases, the AMR disappeared abruptly or continued to exist to the end of the MVD surgery. When PDI appeared, the rate of electrophysiological relief was significantly higher (91.7 % vs 66.7 %, P = 0.001). In cases with PDI appearance during MVD procedure, the rate of neurological dysfunction was lower (13.2 % vs 38.1 %, P = 0.004). The relief rate the PDI group tended to be higher without statistical significance. Conclusion: The appearance and observation of pre-decompression instability of AMR monitoring made a positive impact on the surgical outcomes of MVD surgeries. The advent of PDI indicates that the key step of the procedure has arrived. The timely notification of the PDI advent improved the surgical outcomes of MVD surgery by increasing the electrophysiological relief rate, reducing the incidence of neurological dysfunction, and possible elevation of the relief rate. Therefore, continuous intra-operative communication between the surgeon and electrophysiological monitoring staff should be encouraged.