O120 Sensory conduction slowing does not precede motor slowing in the development of mild carpal tunnel syndrome

Objectives To study changes in median nerve conduction across the wrist in workers exposed to short-term hand-intensive work. Methods Eleven males (mean age 35.7 years (SD: 10.2)) skinned minks during an intensive 22 days’ season. Median and ulnar nerve conduction studies were performed pre-, mid-, end-, and post-season. Age-corrected reference values were used. Results Changes from pre- to end-season were seen for mean median nerve distal motor latency (DML) (0.41 ms; CI: 0.27–0.56), antidromic sensory nerve conduction velocity (SNCV) to digit 2 (−6.3 m/s; −10.2 to −2.5) and digit 3 (−6.2 m/s; −10.6 to −1.9). Normalisation to SDs ( Z -scores) revealed that motor and sensory mean changes were of equal magnitude: DML 1.22 (0.85–1.59); SNCV(dig2) 1.31 (0.61–2.10); SNCV(dig3) 1.23 (0.47–1.99). There were no changes in ulnar nerve parameters. At end-season, five workers fulfilled criteria for carpal tunnel syndrome (CTS). At post-season 3–6 weeks after end of exposure, all parameters had reverted to pre-season levels. Discussion Our data contradict the common assumption that sensory changes precede motor changes in CTS. This assumption may erroneously rely on the use of absolute fixed borders for abnormality instead of age-corrected and normalised reference values. Conclusions Conduction slowing in the development of mild CTS follows similar courses in motor and sensory fibres. The quick recovery of mild sub-acute CTS suggests another mechanism of conduction slowing than demyelination. Significance Severity scales and guidelines for CTS examination should be reviewed. Elaborated reference values may provide new insight and improve diagnostic workup.