Provocative motor nerve conduction testing in presumptive carpal tunnel syndrome unconfirmed by traditional electrodiagnostic testing.

In this controlled prospective study, 22 consecutive surgical candidates with clinically diagnosed CTS and negative findings on median nerve-sensory and motor-conduction velocity tests in both hands were reexamined with a protocol incorporating 5 specific positions of the wrist. Four of the 5 positions represented maximum physiologic ranges of motion for the patient. These positions were neutral (unstressed), extension, flexion, radial deviation, and ulnar deviation. Motor latency was recorded in each of the 5 positions using otherwise standard technique. The least latency value in the test sequence was subtracted from the greatest to yield a value called differential latency. Thirty-two control studies were obtained on both hands of 16 normal volunteers and were used to establish a control differential latency, which was seen to have a mean of .13 ms. A 2 standard deviation z value of .11 ms was calculated, giving an upper limit of normal (control) differential latency of .24 ms. Preoperative studies yielded an average differential latency of .44 ms, with 20 of 22 patients having differential latency values of greater than .24 ms. Evaluations of these same patients 3 months after surgery showed differential latency values within the same range as those of the control group. Simple modification of standard nerve testing techniques to include positional variation increased the yield of positive test results in 20 of 22 patients with CTS whose electrodiagnostic tests otherwise produced negative findings.