Median sensory nonresponders in carpal tunnel syndrome workup

To determine an optimal strategy when, in the course of performing the Combined Sensory Index (CSI) or routine median sensory evaluation of carpal tunnel syndrome (CTS), an unobtainable median sensory response is encountered.

Prospectively assessed collected data for optimal electrodiagnostic strategies in the setting of an absent median sensory response.

Electrodiagnostic laboratory of a Veterans Affairs medical center.

Two hundred forty consecutive outpatient veterans referred to the laboratory with CTS-compatible symptoms were evaluated with the CSI. Of these, 62 were identified as having 1 or more unobtainable median CSI components.

Performed tests to determine latencies across the palm and to the thumb and the ring finger; motor comparison studies of median thenar and ulnar hypothenar motor latencies and second lumbrical-interosseous latency differences.

Prevalence of latency test absence, prevalence of electrophysiologically confirmed CTS, and CSI component response rate.

When the transpalmar response was absent, subsequent median-ulnar to digit 4 testing and median-radial to digit 1 testing yielded responses in 6% and 8% of cases, respectively. When the median-ulnar to digit 4 response was absent, subsequent transpalmar and median-radial to digit 1 testing yielded responses in 39% and 32% of cases, respectively. The second lumbrical-interosseous comparison was always obtainable and the standard median thenar response was obtainable in 95% of cases; the motor comparison studies confirmed a median mononeuropathy in 98% and 85% of the cases, respectively.

When initially a median CSI component is absent, subsequent median sensory studies are often unobtainable. Proceeding directly to motor comparison studies is efficient and provides the requisite electrodiagnostic information.