What range of stimulus intensities should we apply to elicit abnormal muscle response in microvascular decompression for hemifacial spasm?

BACKGROUND: Abnormal muscle response (AMR) has been considered as a predictor of the prognosis after microvascular decompression (MVD) for hemifacial spasm (HFS). However, its predictive value has not always been satisfactory. The objective of this work was to confirm an optimal range of stimulus intensities to elicit AMR in surgery.

METHODS: Seventy-two consecutive patients with primary HFS treated by MVD were retrospectively included in this study. A wide range of stimulus intensities from 1 to 100 mA was applied in AMR monitoring. The AMR-elicited threshold value was quantitatively traced throughout all surgical procedures. The relationship between clinical outcomes and electrophysiological findings was analyzed.

RESULTS: Of the 72 patients, 44 were immediately cured and 24 were delayed cured; the remaining 4 were proved not to be cured in their follow-up periods. The patterns of AMR-elicited threshold changes were categorized into five types, which could only be discriminated with a wide range of stimulus intensities. The constituent ratio of the patterns was significantly different (P < 0.001) among the clinical outcomes.

CONCLUSIONS: Some patterns of AMR changes might have been ignored if we had only applied a narrow range of stimulus intensities (1-30 mA) to judge whether AMR disappeared or not. Thus, a wide range of stimulus intensities (1-100 mA) to trace the AMR-elicited threshold values was proposed for a more precise prediction.