The Diagnostic Potential of Axon Excitability Is Consistent Across Hand Muscles in Amyotrophic Lateral Sclerosis

Recent work suggests that nerve excitability testing has diagnostic potential in amyotrophic lateral sclerosis (ALS). The diagnostic performance of nerve excitability across hand muscles is currently unknown. This study aimed to assess if muscles of the so-called split hand (abductor pollicis brevis [APB], first dorsal interosseous [FDI], and abductor digiti minimi [ADM]) manifest differences in diagnostic performance.

We prospectively recruited 60 consecutive patients investigated for ALS. Nerve excitability, motor unit number and size (MScanFit), needle electromyography (EMG), and standard clinical data were collected. ALS and non-ALS groups were compared using t tests, area under receiver operating characteristic curves (AUROC), and multivariate modeling.

Forty-eight patients completed testing of all three muscles, 25 were diagnosed with ALS. The most prominent nerve excitability changes were in superexcitability (APB p = 0.001, FDI p = 0.0001, ADM p = 0.002). Diagnostic performance with superexcitability was similar across the three muscles (p > 0.05). Reductions in motor unit number were observed in ALS patients. Changes in excitability were evident without loss of motor units, most frequently in APB (40% of recordings). Improvements to the AUROC were obtained using combined excitability/motor unit parameters from APB/FDI (AUROC 0.97, p = 0.01 vs. FDI superexcitability alone). Combined excitability and motor unit modeling outperformed detection of EMG abnormalities.

Disturbances to nerve excitability are similar across the split hand muscles at the time of ALS diagnosis. These occurred prior to motor unit loss and traditional EMG changes. Combining excitability and motor unit parameters in the lateral hand can identify early pathology and potentially lead to earlier diagnosis.