Imbalance of cortical facilitatory and inhibitory circuits underlies hyperexcitability in ALS.

Abstract

To determine the relative contribution of inhibitory and facilitatory circuits in the development of cortical hyperexcitability in amyotrophic lateral sclerosis (ALS). In this cross-sectional study, cortical excitability was assessed in 27 patients with ALS, and results compared to 25 healthy controls. In addition, a novel neurophysiologic measure of cortical function, short-interval intracortical facilitation (SICF), was assessed reflecting activity of the facilitatory circuits. There was a significant increase in SICF (ALS -18.51 ± 1.56%, controls -8.52 ± 1.21%, <i>p</i> < 0.001) in patients with ALS that was accompanied by a reduction of short-interval intracortical inhibition (ALS 3.94 ± 1.29%, controls 14.23 ± 1.18%, <i>p</i> < 0.001) and cortical silent period duration (<i>p</i> = 0.034). The index of excitation, a biomarker reflecting the contribution of inhibitory and facilitatory circuit activity, was significantly increased in patients with ALS (82.79 ± 6.01%) compared to controls (36.15 ± 3.44, <i>p</i> < 0.001), suggesting a shift toward cortical excitation. Increased excitation correlated with upper motor neuron signs (<i>R</i> ² = 0.235, <i>p</i> = 0.016) and greater functional disability as reflected by a correlation with the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised score (<i>R</i> ² = 0.335, <i>p</i> = 0.002). The present study established that cortical hyperexcitability is a key contributor to ALS pathophysiology, mediated through dysfunction of inhibitory and facilitatory intracortical circuits. Therapies aimed at restoring the cortical inhibitory imbalance provide novel avenues for future therapeutic targets.