The impact of spasticity on diaphragm contraction: Electrophysiological assessment

We aimed to evaluate diaphragm spasticity by measuring diaphragm compound muscle action potentials (CMAPs) to phrenic nerve stimulation at end-expiration (exp) and at full-inspiration (insp) in amyotrophic lateral sclerosis (ALS), primary lateral sclerosis (PLS) and aged-matched controls. We also compared diaphragmatic responses of ALS patients with and without spasticity.Diaphragm CMAPs were recorded from 111 ALS patients, 15 PLS patients and 36 controls. Percentage of change (%insp-exp) was calculated for each neurophysiological measure. Clinical evaluation included: functional ALS scale, spasticity and forced vital capacity.Diaphragmatic exp and insp CMAPs in ALS patients had longer latency, lower peak-to-peak amplitude and smaller negative-peak area (all p < 0.05). ANCOVA analysis for %insp-exp differences across groups, taking into account end-expiration values, revealed a group effect for peak-to-peak amplitude (all p < 0.001) and negative-peak area (all p < 0.01). For both measures, the change in ALS and PLS patients was smaller than controls (all p < 0.05). Among ALS patients, those without spasticity (74%) had longer latency, lower peak-to-peak amplitude and smaller negative-peak area (all p < 0.05).Upper motor neuron involvement changes physiological variability of diaphragmatic CMAPs, likely due to decreased muscle shortening and mobility.Spasticity impacts on diaphragm electrophysiology, with potential implications in respiratory function.