Lower motor neuron involvement examined by quantitative electromyography in amyotrophic lateral sclerosis

The diagnosis of amyotrophic lateral sclerosis (ALS) includes demonstration of lower motor neuron (LMN) and upper motor neuron (UMN) involvement of bulbar and spinal muscles. Electromyography (EMG) is essential to confirm LMN affection in weak muscles, and to demonstrate changes in clinically non-involved muscles. The aim of the study was to determine the relative importance of ongoing (active) denervation, fasciculations, chronic partial denervation with reinnervation at weak effort and loss of motor units at maximal voluntary contraction (MVC) in ALS.

EMG was carried out in weak and non-weak muscles in 220 patients suspected of ALS using concentric needle electrodes. Denervation activity and fasciculations in 966 muscles was quantified, the mean durations and amplitudes of motor unit potentials (MUPs) were compared to controls in 745 muscles, and the amplitudes and recruitment patterns at maximal voluntary effort were measured in 939 muscles. Twenty-five percent of patients had clinical involvement of 1 region, 42% of 2 regions and 33% of 3 regions. Clinically 65% had UMN involvement. Eighty-six percent of the patients had died on follow-up.

Denervation activity occurred in 72% of weak muscles but in only 45% of non-weak muscles. Fasciculations occurred in 56% of weak muscles and in 65% of non-weak muscles. MUPs showed reinnervation in 87-91% of weak and non-weak muscles and in 44% of muscles neurogenic MUPs occurred in the absence of denervation activity. In patients with clinical involvement of 1 region, combined EMG criteria increased the number of affected regions in 93%, and in 40% of patients with clinical involvement of 2 regions EMG increased the number of involved regions.

Quantitative EMG confirmed widespread LMN involvement in patients with early ALS including clinically non-involved regions. These findings suggest that the maintenance of force is due to compensatory reinnervation in early disease and that this capacity may decline at later stages of ALS.

These findings support a recent consensus report (the Awaji criteria) that EMG should have equivalent weight to clinical manifestations to indicate LMN involvement. The findings strongly indicate that spontaneous activity is insufficient to show LMN involvement in non-affected muscles at early stages of disease, and that analysis of MUPs are needed to document the distribution of LMN involvement.