Single motor unit estimation of the cutaneous silent period in ALS

Objective: Recent evidence indicated that amyotrophic lateral sclerosis (ALS) also impairs spinal circuits, including those mediating cutaneous silent period (CSP). However, most studies utilised surface electromyography (sEMG), which needs more resolution to pinpoint changes at the single motoneuron level. We aimed to investigate CSP properties using single motor unit discharges in ALS. Methods: In mild and severe ALS patients and controls, CSP was recorded in the first dorsal interosseus and analysed using the discharge rate method, which accurately shows the inhibitory postsynaptic potentials (IPSPs) profile. Results: Our findings confirmed that the CSP latency was prolonged only in severe ALS patients. Moreover, the CSP duration was similar in each group, but late-stage ALS patients tend to have a longer CSP duration. The discharge rate method revealed a significantly longer duration (up to 150 ms) than the duration detected using sEMG. Strikingly, the motoneuron discharge rate – IPSP duration inverse relationship is lost in ALS patients, indicating a possible impairment in the motoneuron integrative properties. Conclusions: Our data support previous findings of prolonged latency, presented input–output modifications of motoneurons, and revealed the entire course of the CSP, representing a much stronger inhibitory event than previously thought. Significance: Motoneuron integrative property modification assessed by CSP could be a new biomarker for ALS.