Disappearance of Lateral Spread Response: Reliability as a Prognostic Marker in Hemifacial Spasm.

Objective: Intraoperative neuromonitoring (IONM) using lateral spread response (LSR) has been mostly studied as a tool in optimizing the benefits of microvascular decompression (MVD) in cases of hemifacial spasm (HFS). Although evidence suggests its utility as a prognostic marker, our experience with the same remains otherwise. The significance of the time of disappearance of LSR is also less studied as a factor determining the outcome. This is a pilot study involving a series of patients operated on for HFS using MVD. The prime objectives of the study are: (1) to review LSR in HFS as a prognosticator of outcome and (2) to study the significance of time of disappearance of LSR and its correlation with outcome. Materials and Methods: Patients operated on for HFS with MVD under IONM guidance, between August 2022 and June 2024, were observed in the postoperative period in three phases-immediate, early, and late. The outcomes were divided into-complete, partial or no improvement, and recurrence. The results were studied against the corresponding findings of LSR in those cases. Results: A total of six patients were studied. Of these, three were males and three were females. Intraoperatively, all the patients showed complete loss of LSR after separating the compressing vessel from affected facial nerve. In the immediate postoperative phase, two out of six patients had incomplete improvement in symptoms which later improved in one of the patients in early postoperative phase. The remaining one patient who had partial improvement of symptoms had increased symptoms within 3 months of follow-up. Conclusion: Although LSR monitoring indicates adequate facial nerve decompression, in our experience, LSR disappearance was not congruent with the absence of symptoms in postoperative phase of HFS. The time of disappearance of the LSR waveform intraoperatively has a significant role in determining the outcome.