Carpal tunnel syndrome: comparison of the compound muscle action potentials recorded at the thenar region from ulnar and median nerve stimulation.

Thenar muscles are primarily innervated by the median nerve. However, compound muscle action potentials (CMAPs) evoked by ulnar nerve stimulation can be recorded at the thenar region due to proximity of some ulnar-innervated muscles, and from volume conduction events. This study was to determine if loss of thenar muscle mass from carpal tunnel syndrome (CTS) could alter the size of ulnar CMAPs obtained at the thenar region, because of changes in the physical surroundings and electrical conductivity. Supramaximal CMAPs were recorded over the thenar eminence to electrical stimulation of the ulnar nerve at the wrist and median nerve at the palm in 102 hands with CTS. Needle EMG was done in the thenar muscles. Severity of needle EMG abnormality was negatively correlated with median-evoked CMAP amplitude (r = -0.76), but not with ulnar-evoked CMAP amplitude (r = -0.12). There was no correlation between the absolute amplitudes of the median and ulnar CMAPs (r = -0.13). Needle EMG abnormality had modest negative correlation (r = -0.43) with median/ulnar CMAP amplitude ratio. Mean median/ulnar CMAP amplitude ratios for normal EMG and for mild, moderate, and severe needle EMG abnormalities were 3.72, 3.31, 1.56, and 0.37, respectively. The absolute amplitude of the ulnar CMAP recorded at the thenar area does not seem to be influenced significantly by the degree of thenar muscle loss (atrophy) from median nerve pathology. However, if the median/ulnar CMAP amplitude ratio falls below 0.5, the study suggests severe loss of motor units in the thenar muscles.