Frequency-dependent conduction block in carpal tunnel syndrome.

Frequency-dependent conduction block (FDB) across segments of demyelination in response to high-frequency nerve stimulation has been well demonstrated in animals and has been explored in humans. However, attempts to demonstrate this phenomenon in sensory fibers involved in entrapment neuropathies have been unsuccessful. Therefore, we investigated the effects of high-frequency nerve stimulation in the median motor nerve in patients with carpal tunnel syndrome (CTS) with moderate to severely increased distal motor terminal latencies (MTL). As a group, the mean decrease in negative peak amplitude (npAmp) during 20 stimuli at 30-HZ frequency was significantly greater in CTS subjects (-11.3%) than in controls (+7.9%). The degree of FDB was greater when MTL was more prolonged (i.e., -4.9% at 5.0 ms and -25.3% at 9.4 ms) and FDB was more pronounced at higher stimulation frequencies (20 and 30 HZ). Our results suggest that the safety margin for impulse transmission is impaired in the motor axons of patients with a focal demyelinating lesion. These findings may correlate with the observation of weakness in the absence of conduction block in patients with entrapment neuropathies.