The senile hand: Age effects on intrinsic hand muscle CMAP amplitudes influence split-hand index calculations

Age can affect hand muscles non-uniformly. We investigated the influence of age on the compound muscle action potential (CMAP) amplitude of the hand muscles and the derived split-hand index (SHI).

We studied 244 subjects investigated for suspected myasthenia gravis but without neuromuscular disorders. Abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) CMAPs were obtained by supramaximal stimulation at the wrist, recording with surface electrodes while checking the best recording site. We applied Tukey's HSD and Kruskal-Wallis one-way analysis of variance for comparing age groups defined by median and interquantile ranges (IQRs). Spearman's rank correlation coefficient and linear regression were used for testing age-dependence of measurements.

Median age was 61.5 y (first IQR, 44.5; third IQR, 72.0; range 18-89). Age and neurophysiological measurements were similar between genders. APB_CMAP , FDI_CMAP , ADM_CMAP , and SHI were correlated with age (P < .001). Median and cutoff values were significantly different between age groups. APB_CMAP , FDI_CMAP , and ADM_CMAP decreased by 0.8/0.7/0.3 mV/y, respectively, and SHI decreased 0.15/y.

The CMAP amplitudes of hand muscles and derived SHI were strongly age-dependent, although this effect was less in ADM. This represents a physiological phenomenon. Future studies using the SHI should consider age effects.