Functional Assessment of Hemifacial Spasm Using a Whole-Face Surface Electromyography Electrode Array

Hemifacial spasm (HFS) is a refractory neuromuscular disorder that primarily affects facial movement. Electromyography (EMG) is one common-used clinical evaluation technique for HFS, which can help support the effective treatment. Nonetheless, due to its invasiveness and low-resolution issues, the application of EMG is limited. This study aimed to evaluate the facial muscle activities in HFS, by employing a whole-face surface EMG (sEMG) technique. Twenty patients with HFS were recruited in this study. Whole-face sEMG electrodes were used to record the muscle activities of the frontalis, orbicularis oculi, orbicularis oris, mentalis, and midfacial muscles during different facial movements. The root mean square (RMS) and median frequency (MDF) features from sEMG signals were computed for analysis. Especially, the sEMG topographic map was applied to analyze the coordinated activities of midfacial muscles, by using stretchable sEMG electrode arrays. Results demonstrated that the RMS of the affected side's frontalis was significantly higher than the unaffected side ( ${P} \lt 0.001$ ), while no significant difference was observed in MDF ( ${P} \gt 0.05$ ). In the RMS topographic map, the center of gravity in the horizontal direction (CoGx) shifted significantly toward the midline on the affected side during the teeth-showing task ( ${P} \lt 0.05$ ) and the entropy feature on the affected side was significantly lower than that on the unaffected side during the cheek-showing task ( ${P} \lt 0.05$ ). These findings indicate the compensatory mechanisms in the facial nerve distribution areas of HFS and provide an effective evaluation tool for the objective quantification of HFS severity and abnormal co-activation patterns.