Orthodromic study of the sensory fibers innervating the fourth finger.

Fourth finger stimulation has been used to obtain the compound nerve action potential (CNAP) of the median and ulnar nerves by a single cutaneous bipolar recording electrode placed in some specific sites of the upper limb. In normal subjects, the response was a combination of both action potentials which could be seen as separated peaks only when the recording was made at midarm with the elbow flexed at 90 degrees. This finding is mainly attributed to the longitudinal sliding of nerves according to the joint movements. In patients with a carpal tunnel syndrome, there were a striking separation between the responses of both nerves in wrist recording. This finding allows this technique to be applied in the clinical inspection of median nerve entrapment at wrist, demonstrating graphically the delay on the median nerve action potential with regard to that of the ulnar nerve.