Exclusive electrophysiological motor involvement in carpal tunnel syndrome.

OBJECTIVE

To define the frequency of exclusive electrophysiological motor involvement in carpal tunnel syndrome (CTS).

METHODS

We reviewed the electrophysiological studies of 2727 consecutive hands with typical symptoms and signs of CTS and at least one abnormal test of the following: median distal motor latency (DML), digit two sensory conduction velocity (D2-SCV), segmental D2-SCV from wrist to palm, median-ulnar sensory latency difference from ring finger stimulation.

RESULTS

Thirty-one hands (1.2%) had prolonged median DML ( > 4.4 ms) with normal SCV ( > 48 m/s). In 17 of 31 hands, segmental D2-SCV from wrist to palm or median-ulnar latency difference from ring finger stimulation were also performed with normal results in 8 hands, demonstrating a true exclusive electrophysiological motor involvement.

CONCLUSIONS

In CTS, exclusive electrophysiological involvement of median motor fibers is rare. It may be related to preferential compression of the intraneural motor fascicles clumped superficially in the most volar-radial nerve quadrant or, more probably, to the fact that the recurrent thenar branch may exit the carpal tunnel through a separate ligamentous tunnel within the transverse carpal ligament where it may be preferentially or selectively compressed.