Co-stimulation of ulnar nerve affects median nerve distal latency in carpal tunnel syndrome: A quantitative measurement

Objective Median nerve motor conduction studies should be performed with supramaximal stimulation. However, overstimulation at the wrist can cause co-stimulation of the ulnar nerve which results in depolarization of ulnar innervated muscles in the thenar space and changes in the CMAP distal latency and amplitude. In testing carpal tunnel syndrome (CTS), this is a common technical error for both the beginners and experienced electromyographers. This study reports the normal ulnar-thenar CMAP distal latency relevant to the electrodiagnostic evaluation for CTS. Methods Median and ulnar nerve motor nerve conduction studies were performed in 112 normal subjects on 224 sides, and in 16 individuals with CTS on 18 sides. In a subset of 10 healthy subjects, the median nerve was stimulated with sequentially increasing currents to obtain 30%, 50%, 70%, and 100% of maximum CMAP amplitude, and with 120%, 150% and 200% supra-maximal stimulation. CMAP characteristics were measured for all stimulation currents. In CTS subjects, after obtaining the median-APB CMAP, the currents were sequentially increased until ulnar co-stimulation was obtained with changes of CMAP latency and waveform. Results In normal subjects, the ulnar-thenar CMAP distal latency was 3.17±0.25 ms, and amplitude 6.60±1.07 mV; the median-APB latency was 3.45±0.31 ms and the amplitude was 6.47±1.08 mV. In normal subjects, the increased stimulation intensity reduced the median- APB distal latency by 0.1 to 0.3 ms. There was no further change in latency with up to 200% supra-maximal stimulation. In CTS patients, with increased intensity of stimulation resulting in co-stimulation of ulnar nerve, the median-thenar distal latency reduced by 0.5-8.7 ms. Conclusions During median-APB testing with increasing intensity of stimulation, a decrease in latency of over 0.3 ms with a shortened latency in the range of 2.9 to 4.1 ms might be suggestive of probable co-stimulation of the ulnar nerve. Key words: Carpal tunnel syndrome; Ulnar nerve co-stimulation; Ulnar-thenar distal latency