Does retrograde axonal atrophy really occur in carpal tunnel syndrome patients with normal forearm conduction velocity?

Objective: The cause of decreased median forearm motor conduction velocity (FMCV) in carpal tunnel syndrome (CTS) is best ascribed to retrograde axonal atrophy (RAA); however, the relationships between the occurrence of RAA and electrophysiological or clinical severity remains controversial. We attempt to determine whether RAA really occurs in CTS patients with normal median FMCV and to investigate any relationships between RAA and severity of compression at the wrist. Methods: Consecutive CTS patients were enrolled and age-matched volunteers served as controls. We performed conventional nerve conduction studies (NCS) and measured median and ulnar distal motor latencies (DML), FMCV, compound muscle action potential (CMAP) amplitudes, distal sensory latencies (DSL), and sensory nerve action potential (SNAP) amplitudes. Furthermore, palmar median stimulation was done to calculate the wrist-palm motor conduction velocity (W-P MCV). Patients included for analysis should have normal FMCV and needle examination. We compared each electrodiagnostic parameters between the patient group and controls. Results: The mean ± SD of the W-P MCV for patients and controls were 33.26 ± 6.74 and 52.14 ± 5.85 m/s and those of median FMCV were 55.26 ± 3.56 and 57.82 ± 3.9 m/s, respectively. There was a significant reduction in the W-P MCV (36.2%, P<0.00001), significant decrease in the median FMCV (4.43%, P<0.00001) and SNAP amplitudes, and an increase of the DML and DSL in the patient group (P<0.00001) compared to the controls; however, there were no differences in median and ulnar CMAP amplitudes, ulnar FMCV and DML between the controls and patients. Conclusions: RAA and relatively slowed median FMCV do occur in CTS patients with normal median FMCV, regardless of severity of clinical manifestations and electrophysiological abnormalities. Significance: This article provides new information for research of the electrophysiological changes of the proximal nerve part at distal injury.