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.