Diaphragmatic CMAP amplitude from phrenic nerve stimulation predicts functional decline in ALS

To evaluate phrenic nerve motor amplitude (PhrenicAmp) as an independent predictor of functional decline in amyotrophic lateral sclerosis (ALS). We also assessed both PhrenicAmp and forced vital capacity (FVC) as predictors of functional loss in patients with bulbar dysfunction.

We included consecutive ALS patients with PhrenicAmp and FVC at baseline. Participants were evaluated with the revised ALS Functional Rating Scale (ALSFRS-R) at inclusion and at, at least, one subsequent follow-up visit. The outcome measure of functional decline was the percentage reduction in ALSFRS-R from baseline. Bulbar dysfunction was defined by the presence of any relevant symptom on the ALSFRS-R bulbar sub-score. Correlations and mixed-effects regressions were used to study the relationship between functional decline and both PhrenicAmp and FVC baseline evaluations.

A total of 249 ALS patients were included; 64.2% of these had bulbar dysfunction. At inclusion, significant correlations were found between PhrenicAmp and FVC (p < 0.001), as well as between each respiratory measure and ALSFRS-R (all p < 0.001). The functional decline at first (median 3 months) and second (median 6 months) follow-up visits was significantly correlated with baseline values of both respiratory evaluations (all p < 0.01) in the entire ALS population, but only with baseline PhrenicAmp (all p < 0.05) in bulbar dysfunction cases. Regression analysis revealed that PhrenicAmp (all p < 0.05), but not FVC, was a significant independent predictor of functional decline in ALS patients and in those with bulbar dysfunction.

Baseline PhrenicAmp is an independent predictor of functional decline in ALS, whether or not bulbar dysfunction is present.