Localization of frequency-dependent conduction block in carpal tunnel syndrome.

Frequency-dependent conduction block (FDB) has been demonstrated in subjects with carpal tunnel syndrome (CTS) when the median nerve was stimulated at 30-Hz frequency proximal to the carpal tunnel (CT). However, it was still questionable whether FDB occurred within the region of the CT or more distally. High-frequency nerve stimulation (HFNS) was performed in controls and in CTS subjects while stimulating the median nerve proximal and distal to the carpal tunnel. The degree of FDB was measured by comparing the 20th thenar CMAP with the 1st following a train of 20 stimuli at 30-Hz. FDB occurred across the CT in 11/12 studies (91.7%) and across the distal portion of the palmar aponeurosis-distal terminal motor branches in 5 of the 11 studies (41.7%). These results suggest that the safety margin for impulse transmission can be impaired distal to the presumed site of injury in CTS.