Selective Muscle Involvement in Amyotrophic Lateral Sclerosis: Evidence Inferred from the Point of Motor Unit Firing Rates.

Introduction: The aim of the study is to determine the role of upper motor neuron (UMN) or lower motor neuron (LMN) dysfunction as the primary initiator of distal-proximal and lateral-medial gradients of muscle involvement in amyotrophic lateral sclerosis (ALS).

Methods: Concentric needle electromyography recordings were performed in deltoid, abductor digiti minimi, and first dorsal interosseous (FDI) muscles in patients with ALS and controls during slight voluntary contraction needed to activate two motor units (MU). Five motor unit potential (MUP) pairs were recorded from each muscle. Motor unit potential analyses were performed offline using Multi-MUP analysis program. Quantitative MUP parameters, MU firing rate (FR), FR variability (FRV), and mean consecutive difference (MCD) were calculated. Motor-evoked potentials and the triple stimulation technique (TST) were performed to evaluate UMN involvement.

Results: Twenty patients with ALS along with 20 age and sex-matched healthy volunteers were enrolled. Quantitative MUP parameters compatible with denervation and reinnervation were found in patients with ALS, who also showed higher FR, FRV, and MCD values, most prominently in FDI. First dorsal interosseous FRV was lower in patients with abnormal central motor conduction time (CMCT). Firing rate and FRV were negatively correlated with CMCT, but not with TST.

Conclusion: Distal limb muscles, particularly FDI, revealed more prominent FR abnormalities in patients with ALS in parallel with the distal-proximal and lateral-medial gradients of the selective muscle involvement pattern which seems predominantly to be correlated with LMN dysfunction. Reduced FRV may be associated with the presence of UMN dysfunction in ALS.