Residual latency: new applications of an old technique.

Median and ulnar nerve conduction velocity was studied using residual latency techniques in a control population and in a population with neuropathy. Midpalmar stimulation sites were used to determine the residual latency of the distal short segment. The residual latency index (RLI) was the short segment residual latency divided by the residual latency for the entire terminal nerve segment. While it was 0.61 in the control population, it increased to 0.69 in the patients with ulnar neuropathy distal to the wrist and decreased to 0.36 in patients with neuropathy of the median nerve at the carpal tunnel. In five cases of median neuropathy at the carpal tunnel, muscle biopsy specimens revealed evidence consistent with neuropathic processes. The RLI is an accurate and relevant method of evaluating the sensory and motor conduction of very small nerve segments.