Sympathetic skin response in idiopathic and diabetic carpal tunnel syndrome

Background In carpal tunnel syndrome (CTS), certain changes were expected in sympathetic skin response (SSR) because median nerve carries postganglionic unmyelinated fibres. Purpose To investigate the median and ulnar SSR in idiopathic and diabetic CTS without autonomic dysfunction in hands and to find possible relations with electrophysiological features of these diseases. Patients and methods SSRs were elicited by electrical stimulation on the supraorbital nerve and recorded from the median and ulnar territories in the hand from 20 diabetic patients with only CTS (29 hands), 24 idiopathic CTS patients (42 hands) and 13 normal subjects (26 hands). Hands with ulnar neuropathy at the wrist without symptoms and normal hands of unilateral CTS were excluded. In addition to classical parameters and comparative methods, SSR waveform changes and percentile method was used in finding abnormality. Results Median SSRs had significant delayed latency compared to ulnar latency in both CTS patients but this was not important clinically (1.38/1.37 s for idiopatic CTS; 1.43/1.36 s for diabetic CTS). Median and ulnar SSR amplitude, area, median/ulnar latency difference, amplitude and area ratio were compared and only median/ulnar latency difference and median/ulnar latency ratio were found different between the three groups. Four idiopathic CTS hands were outside of the limits or absent (9.5%). SSR waveforms were significantly different from normal subjects in CTS patients. P type SSR replaced M type in idiopathic CTS and N type in diabetic CTS. Conclusions Findings regarding SSR parameters suggest that unmyelinated C fibers were affected in CTS. These values were not useful because they were too small. Data on SSR were not normally distributed and the percentile method seems to be more convenient for finding any abnormality in clinical practice. Also, SSR waveform analysis could give us valuable data and should add to the SSR examination parameters.