Science News: Detecting Motor Unit Abnormalities in Amyotrophic Lateral Sclerosis Using High-Density Surface EMG

Submitted by: Eman Tawfik, MD
Edited by: Nakul Katyal, MD

Nishikawa Y, Holobar A, Watanabe K, et al. Detecting motor unit abnormalities in amyotrophic lateral sclerosis using high-density surface EMG. Clin Neurophysiol. 2022;142:262-272. doi:10.1016/j.clinph.2022.06.016

Summary: In this study, the authors evaluated motor unit (MU) abnormalities in patients with amyotrophic lateral sclerosis (ALS) using quantitative non-invasive high density-surface EMG.

The inclusion criteria included patients with ALS diagnosed using the Awaji criteria and the needle EMG study showed motor unit action potentials (MUAPs) in their vastus lateralis. The control group included age-matched healthy volunteers. Patients with lower limb injury, dementia, myositis, spinal muscular atrophy, and dystonia were also included. The disease severity was graded using the ALS Functional Rating Scale-Revised (ALSFRS-R).

The high density surface EMG was recorded from the vastus lateralis 10 minutes after submaximal ramp-up contraction task. The EMG signal was recorded from the weaker side in ALS patients and from the dominant side in the control group while performing the ramp and hold contraction task (ramp up to 30% maximum volume contraction (MVC) in 15 s, sustained contraction for 15 s, and ramp down in 15 s). The number of motor units, mean firing rate, recruitment threshold (= the force level (%MVC) at the first firing of each MU, coefficient of variation of the MU firing rate (= the ratio of standard deviations and the mean values of the MU firing rates), MU firing rate at recruitment, and motoneurons excitability were calculated.

In ALS patients, the number of MUs were significantly lower and the mean firing rate, recruitment threshold, coefficient of variation of the MU firing rate, MU firing rate at recruitment, and motoneurons excitability were significantly higher, compared to the controls. Moreover, the number of MU, MU firing rate, recruitment threshold, and MU firing rate at recruitment were significantly correlated with disease severity. The findings denote compensatory increase in motorneuron excitability at recruitment secondary to neurodegeneration.

Comments: This article is interesting for several reasons: it demonstrates the pattern of firing rate abnormality in ALS patients and the feasibility of the using non-invasive surface EMG in detection of this abnormality, and highlights the relation between the firing rate abnormality and disease severity which may help in disease prognostication. The methodology is well-conducted, and the study has a practical implication.