The role of forearm mixed nerve conduction study in the evaluation of proximal conduction slowing in carpal tunnel syndrome

ObjectiveA decrease of forearm median motor conduction velocity (CV) is a common electrophysiological finding in carpal tunnel syndrome (CTS), ascribed to two possible mechanisms: either conduction block or slowing of the fastest myelinating fibers in the carpal tunnel, or retrograde axonal atrophy (RAA) with retrograde conduction slowing (RCS). We hope to utilize both direct and derived forearm median mixed nerve conduction studies to clarify the mechanism of the decrease of forearm median motor CV in CTS.MethodsSeventy-five CTS patients and 75 age-matched control subjects received conventional motor and sensory nerve conduction studies of median and ulnar nerves and forearm median mixed nerve conduction techniques. First, direct measurement of forearm median mixed conduction velocity (Forearm mixed CV) and nerve action potential amplitude (Forearm mixed amplitude) was determined with recording at elbow and stimulation at wrist. Then, stimulating electrode was placed over palm and recording at elbow and then at wrist to calculate the derived Forearm mixed CV. Electrophysiological parameters, including direct Forearm mixed CV and amplitude and derived Forearm mixed CV, were compared between CTS patients and controls.ResultsCTS patients had significantly prolonged wrist–palm sensory and motor conduction, significantly decreased forearm median motor CV, and normal ulnar nerve conduction. The direct Forearm mixed amplitude was significantly decreased in CTS patients. The direct Forearm mixed CV was similar in CTS patients and controls, but there was a significant decrease in derived Forearm mixed CV in CTS group. The difference between direct and derived Forearm mixed CV was significantly greater in the CTS, suggesting that direct and derived Forearm mixed CV represent CV from different nerve fibers, one passing outside carpal tunnel without undergoing RAA or the other through the carpal tunnel with occurrence of RAA.ConclusionA decrease of direct Forearm mixed amplitude really occurs in CTS, implying that RAA and RCS will develop over proximal median nerve at distal nerve injury and the decreased forearm median motor CV is best ascribed to RAA and RCS. Furthermore, in CTS, the direct Forearm mixed CV measures the CV from undamaged nerve fibers without passing through carpal tunnel, resulting in the misinterpretation of the cause of proximal conduction slowing secondary to conduction block or slowing over the wrist.SignificanceWe provide a direct evidence of the occurrence of RAA and RCS that would explain the cause of proximal median nerve conduction slowing. However, the clinical significance of RAA and RCS is uncertain.