Comparative diagnostic sensitivities of orthodromic or antidromic sensory inching test in mild carpal tunnel syndrome.
To compare the reliability, sensitivity, and specificity of the "inching test" (IT) or "centimetric test," performed orthodromically (OIT) and antidromically (AIT). Incremental palmar study of the sensory fibers of the median nerve was evaluated over 10cm across the wrist in 20 patients with mild carpal tunnel syndrome (CTS) and in 20 controls. Mild CTS was defined as clinical features of CTS with normal electrophysiologic findings by standard methods. The CTS patients were preselected with abnormal orthodromic median-ulnar latency difference of the fourth digit (mean .66+/-.21ms; nl < .40ms). In controls, the mean conduction delay per centimeter (CD/cm) was .192ms for OIT and .191ms for AIT; the mean maximum conduction delay per centimeter (MCD/cm) was .250+/-.032ms for OIT and .344+/-.10ms for AIT. MCD/cm was located inside the carpal tunnel in 85% of patients (OIT) versus 80% for AIT. No MCD/cm was greater than .32ms (OIT) or .60ms (AIT). With corresponding pathologic thresholds of .36ms (mean + 3.4 standard deviation [SD]) for OIT and .64ms (mean + 2.6 SD) for AIT, IT was abnormal in 20 patients (100%) with OIT compared with only 4 patients (20%) with AIT. The orthodromic method was superior to the antidromic method in controls and in patients (chi2 = 23; p = 1.8 x 10(-6)). These findings suggest that orthodromic IT should be used when standard electrodiagnostic tests fail to reveal median nerve sensory abnormality in persons with mild CTS.