The use of high-density surface electromyography in amyotrophic lateral sclerosis: a scoping review

Amyotrophic lateral sclerosis (ALS) is characterised by progressive degeneration of motor neurons, resulting in muscle weakness and atrophy. This neuronal loss is partially compensated for by the collateral sprouting of surviving motor neurons, leading to the formation of enlarged motor units (MUs). These MU adaptations, together with hyperexcitability and altered descending messages from the brain, lead to altered characteristics of the MU action potential shape and discharge pattern, that can be captured using high-density surface electromyography (HDsEMG). The aim of this review is to survey all available literature, investigating how HDsEMG has been used in ALS, and highlight differences in methods and outcomes to allow comparison between studies. A systematic literature search was conducted using four databases (PubMed, Scopus, IEEE Xplore, and Academic Search Ultimate) to identify studies employing HDsEMG in individuals diagnosed with ALS. Eligible studies were reviewed to examine experimental protocols, hardware and software configurations and reported outcome measures. Out of 168 identified articles, 26 were included in this review. High heterogeneity was observed in recording methods, analysis, and reporting strategies. Based on measurable features of MU behaviour and morphology, the outcomes reported in the studies were grouped into five main categories: fasciculations, MU properties, MU discharge characteristics, multiple discharges and number of MUs. HDsEMG represents a promising non-invasive technique that allows for repeated, longitudinal measurements as well as the detection of multiple MUs and their individual analysis, the potential of which has not been fully explored. HDsEMG has a strong potential for clinical use in ALS, but its application should first be based on a clear understanding of disease pathophysiology. The findings of this review highlight the urgent need for a consensus on standardised protocols and reporting practices for the application of HDsEMG in ALS research, along with the development of methods that can sensitively indicate disease-specific physiological changes to improve comparability, reproducibility. This understanding will improve how HDsEMG findings are interpreted and support the translation of HDsEMG into a diagnostic tool.