Coactivation of the ulnar nerve in motor tests for carpal tunnel syndrome.

Coactivation of the ulnar nerve at the wrist can be a source of error in tests for carpal tunnel syndrome (CTS). We compared the effects of coactivation in two tests for CTS: the abductor pollicis brevis-distal motor latency (APB-DML) and lumbrical-interosseus-distal motor latency difference (2LI-DML). We studied 33 hands of 25 consecutive patients referred for suspected CTS. In severe CTS, when selective supramaximal stimulation of the median nerve was impossible, all APB-compound muscle action potentials (CMAP) showed abnormalities, indicating volume conduction of ulnar muscle activation. 2LI-DML in these hands led to falsely normal test results, as two identical CMAP were obtained after median and ulnar stimulation. In less severe CTS, warning signs indicating coactivation were observed in APB-DML virtually as often as in 2LI-DML. Undetected coactivation was more likely to be associated with false normal test results in 2LI-DML than in APB-DML.