Estimation of the conduction velocity of sympathetic sudomotor C fibres in healthy subjects: study of sympathetic skin reflex.

The aim of this study was to establish a simple method for estimating the conduction velocity (CV) of post-ganglionic sympathetic sudomotor C fibres (SSFCV) in the upper and lower limbs by simultaneously measuring the sympathetic skin reflex (SSR) in two distant sites. Fifty healthy volunteers were studied. SSRs were recorded with standard surface electrodes applied to both proximal (axilla and crural line) and distal sites for each limb (hand and foot). The CV of the efferent branch of the SSR was calculated by dividing the difference in the latencies of the response from two recording sites by the distance between the sites (axilla-hand for upper limb; crural line-foot for lower limb). Day-to-day reproducibility and intra-individual variability of the SSFCV were calculated. For the upper limbs, the SSFCV in the axilla-hand tract was 2.0+/-0.3 m/sec (range 1.6-2.4 m/sec). For the lower limb, the SSFCV in the crural line-foot tract was 1.4+/-0.4 m/sec (range 1.2-1.6 m/sec). Mean intra-individual variability of the SSFCV for the upper and lower limbs was 0.11 and 0.09, respectively. The coefficient of variation of the SSFCV for the upper and lower limbs was 5.1% and 5.4%, respectively. Our data show that this simple and non-invasive method can reliably be used to measure the CV of the sympathetic sudomotor fibres, in suitable temperature conditions, and may be useful when investigating the physiological functions of peripheral nerves in patients with peripheral neuropathies.