High-density surface electromyography to assess motor unit firing rate in Charcot-Marie-Tooth disease type 1A patients.

OBJECTIVE

The aim of this study was to elucidate the characteristics of the motor unit (MU) firing rate in Charcot-Marie-Tooth disease type 1A (CMT1A) patients and its longitudinal change using high-density surface-electromyography (surface-EMG) and MU decomposition analysis.

METHODS

Nineteen patients with CMT1A and 21 force-matched healthy controls prospectively underwent surface-EMG recording of the vastus lateralis muscle during ramp-up and sustained contractions on performing isometric knee extension. After decomposition analysis, instantaneous firing rates (IFRs) of individually identified MUs were calculated. In CMT1A patients, follow-up measurements were performed one year after the baseline. Comparison of IFRs and clinical variables between CMT1A patients and controls at the baseline and between the baseline and after one year in CMT1A patients was performed.

RESULTS

Mean IFRs of MUs were lower in CMT1A patients than in controls. This was true at various force levels in ramp-up contractions (p < 0.01. e.g., 10.3 (CMT1A patients) vs. 12.2 (controls) pulses-per-second (pps) at 22.5-27.5% of maximal voluntary contraction (MVC) in MUs recruited at <7.5% of MVC) and at any time-point during sustained contractions (p < 0.001. e.g., 8.0 vs. 9.3 pps, respectively, at 10-20 seconds). In CMT1A patients, mean IFRs at 0-10 seconds of sustained contraction were significantly decreased over one year (from 8.06 to 7.52 pps; p = 0.027), whereas the disease severity score and MVC of knee extension did not change over time.

CONCLUSION

CMT1A patients had a lower individual MU firing rate.

SIGNIFICANCE: The MU firing rate is a potential short-term biomarker of axonal damage in CMT1A patients.