Sympathetic reflex latencies and conduction velocities in patients with polyneuropathy

(1) Micro-electrode recordings of multi-unit sympathetic activity were attempted in skin or muscle branches of the peroneal nerve at the fibular head and the median nerve at the elbow in 41 patients with polyneuropathy of different causes. An indirect measure of sympathetic conduction velocity was obtained by determining the latency of either of two sympathetic reflexes. For skin nerve sympathetic activity (SSA) reflex responses to electrical skin stimuli were used and for muscle nerve sympathetic activity (MSA) reflex inhibition caused by the arterial pulse wave. The skin sympathetic function was also evaluated by measuring changes in skin resistance and finger/toe pulse plethysmograms. Motor conduction velocities were measured with surface electrodes. (2) In muscle (but not in skin) nerve fascicles afferent mass activity in myelinated fibres was often weak or absent. (3) There was a significant relationship between symptoms of autonomic impairment and impaired skin resistance and/or plethysmographic responses. There was also a relationship between impairment of these responses and failure to detect SSA. (4) Failure to find sympathetic activity occurred in 60% of diabetic patients but only in 27% of the whole material. When found, sympathetic activity had normal appearance and sympathetic reflex latencies were normal irrespective of degree of slowing of motor conduction velocity. (5) The results suggest that in polyneuropathy conduction velocities of post-ganglionic sympathetic fibres are normal as long as the fibres conduct. Degeneration of sympathetic fibres may be especially common in diabetic neuropathy.