Quantification of Laryngeal Electromyography Combined with Laryngeal Motor Conduction Study for Evaluating Laryngeal Nerve Injury

OBJECTIVES: To quantify characteristics of laryngeal electromyography with the laryngeal motor conduction study (LMCS) for evaluating laryngeal nerve injury. METHODS: This study included 809 patients with unilateral vocal fold paralysis. Quantitative parameters were measured on abnormal spontaneous activity, motor unit potential (MUP), recruitment patterns, and synkinesis of the affected thyroarytenoid (TA), posterior cricoid (PCA), and cricothyroid (CT) muscles. The proportion of recorded compound muscle action potential in LMCS was calculated to further divide muscles into LMCS+ and LMCS- groups, between which cut-off values for recruitment patterns were determined. RESULTS: In laryngeal muscles, amplitudes of fibrillation potentials were 90-100 μV and durations were 2.2-2.4 milliseconds, which were of 80-90 μV and durations of 4.8-5.0 milliseconds in positive sharp waves. Amplitudes of abnormal MUP showed increased amplitude compared to the normal, with prolonged durations of 6.16 ± 1.02 milliseconds, 6.57 ± 1.32 milliseconds, and 6.19 ± 1.19 milliseconds (P < 0.01). The amplitude and turns of abnormal recruitment were significantly smaller than those of normal laryngeal muscles (P < 0.01), with turns in the mixed and simple pattern being 30% and 20-25% of the full interference pattern, respectively. In mixed patterns, the LMCS- group exhibited smaller amplitudes, with cut-off values at 298.50 μV, 355.00 μV and 379.50 μV for TA, PCA and CT muscles. CONCLUSIONS: Amplitude and duration are critical parameters for identifying abnormal spontaneous activity and MUP in laryngeal muscles, with duration being of higher specificity. Combined with LMCS-derived cut-off values for amplitude, the reduced turns can help distinguish recruitment patterns to assess the degree of laryngeal nerve injury.