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%, p < 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%, p < 0.001) and cortical silent period duration (p = 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, p < 0.001), suggesting a shift toward cortical excitation. Increased excitation correlated with upper motor neuron signs (R 2 = 0.235, p = 0.016) and greater functional disability as reflected by a correlation with the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised score (R 2 = 0.335, p = 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.

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