Investigation of motor unit firing and recruitment properties in proximal and distal muscles using QEMG, MUP analysis and TST in patients with ALS

Introduction: It is well known that clinical and EMG abnormalities are more prominent in distal limb muscles and thenar muscles than hypothenar ones in patients with amyotrophic lateral sclerosis (ALS). The pathophysiology of this selective involvement is unknown. Methods: Needle electromyography was recorded from five different insertion points in abductor digiti minimi (ADM), first dorsal interosseus (FDI) and deltoid muscles of 20 patients with ALS and 20 controls during voluntarily contraction needed to contract two different motor units. Quantitative motor unit action potential (MUAP) analysis, firing rate (FR), interpotential interval variability (vIPI), FR variability (FRV) and FR difference between two MUAPs were calculated. Triple stimulation technique was performed to quantify upper motor neuron involvement. Results: Quantitative MUAP parameteres were different in ALS patients than controls, most prominently in distal muscles, compatible with denervation and reinnervation as electrophysiologic parameters of lower motor neuron (LMN) impairment. ALS patients showed higher MÜP FR and MCD values than controls and the most prominent muscle was FDI. Although vIPI did not differ between groups, MCD was higher in ALS than controls, especially in distal muscles. Between patients with normal and abnormal TST, MUP quantitative and FR parameters did not show any difference. In correlation analysis of ALS; FR, FRV and MCD revealed positive correlation with each other, whereas FR correlated with LMN markers negatively but not correlated with TST. Conclusion: Our findings showed more prominent abnormalities in quantitative MUrP and FR parameters in distal limb and tenar muscles of ALS. This selective impairment seemed to be related with LMN involvement rather than UMN. Keywords: Amyotrophic lateral sclerosis, motor unit, firing rate, upper motor neuron, triple stimulation technique.