Forearm mixed nerve conduction velocity: questionable role in the evaluation of retrograde axonal atrophy in carpal tunnel syndrome.

The objective of this study was to determine whether forearm mixed nerve conduction velocity (Fmix) reflects the real conduction velocity of forearm motor nerve (Fmot) and forearm sensory nerve (Fsen) fibers passing through the carpal tunnel. Forearm mixed nerve conduction velocity is presumed to be indicative of the conduction velocity of the median nerve over the forearm. Therefore, Fmix is used widely to assess the causes of slowing forearm conduction velocity in carpal tunnel syndrome. However, some authors claim that Fmix comes chiefly from the undamaged fibers in carpal tunnel syndrome, and thus cannot replace Fmot or Fsen in the evaluation of retrograde axonal atrophy. Patients with clinical symptoms and signs of carpal tunnel syndrome confirmed with standard electrodiagnosis were included. Age-matched volunteers served as control subjects. Conduction velocities across the wrist and over the forearm were measured, including those of the wrist sensory (Wsen), wrist motor (Wmot), and wrist mixed nerves (Wmix); and forearm mixed (Fmix), forearm motor (Fmot), and forearm sensory nerves (Fsen). The authors compared and correlated Wsen, Wmot, and Wmix; and Fmix, Fmot, and Fsen respectively. The mean values of Wsen, Wmot, Wmix, Fmix, Fmot, and Fsen of the control subjects less those of corresponding conduction velocity of carpal tunnel syndrome patients were designated Wsen N, Wmot N, Wmix N, Fmix N, Fmot N, and Fsen N respectively and were compared and correlated again. Wrist motor nerve conduction velocity, Wsen, and Wmix were significantly lower in carpal tunnel syndrome patients, and Fmot and Fsen but not Fmix were reduced significantly when compared with control subjects. Mean wrist sensory nerve conduction velocity, Wmot N, and Wmix N; and Fsen N and Fmot N showed good correlation except for Fmix N, suggesting that Fmix reflects the conduction velocity of undamaged fibers in carpal tunnel syndrome. Forearm mixed nerve conduction velocity cannot replace Fmot or Fsen in the assessment of retrograde axonal atrophy in carpal tunnel syndrome. In the disease state, Fmix possibly represents the conduction velocity of the palmar cutaneous branch.