The difference in the diaphragmatic physiological measures between inspiratory and expiratory phases in ALS

We aimed to clarify the differences in static and dynamic diaphragm parameters between the expiratory and inspiratory phases in amyotrophic lateral sclerosis (ALS).

Twenty patients with early-stage ALS and 16 healthy controls were enrolled in the study. We measured the amplitudes of compound muscle action potential (phCMAP) by electrical stimulation of the phrenic nerve and the zone of apposition wall thickness of the diaphragm (DT) using ultrasonography. We analyzed the differences in phCMAP (∆phCMAP) and DT (∆DT) between the end-inspiratory and end-expiratory phases and their correlation with forced vital capacity (FVC).

The ΔphCMAP (mean 129.7 ± SD 204.7 µV) and ∆DT (0.80 ± 0.88 cm) in patients were significantly smaller than those in controls (348.6 ± 247.7 µV, p = 0.0003 and 1.89 ± 1.10 cm, p = 0.0002, respectively). Although ∆DT was significantly correlated with FVC, we found no correlation between ∆phCMAP and FVC. The phCMAP was paradoxically smaller during inspiration than during expiration in 35% of patients but in none of the controls.

Dynamic parameters of the diaphragm were abnormal in early-stage ALS. The paradoxical reduction in phCMAP during inspiration may reflect early respiratory dysfunction. Assessment of dynamic abnormalities of the diaphragm may provide helpful information for respiratory management in patients with ALS.