Electrophysiological studies in mild idiopathic carpal tunnel syndrome

Many techniques have been reported to improve the diagnosis of carpal tunnel syndrome (CTS), but there is no agreement on the diagnostic yield of these different methods. We used an electrophysiological protocol including the assessment of the orthodromic sensory conduction velocity of the median nerve along the carpal tunnel, comparison of median and ulnar sensory conduction between the ring finger and wrist, short segment incremental median sensory nerve conduction across the carpal tunnel recording from the III digit ('inching test'), the study of the refractory period of transmission (RPT) and calculation of the distoproximal ratio obtained by dividing the nerve conduction velocity in the median nerve between the third digit and the palm and between the palm and wrist in 41 patients with mild CTS (75 symptomatic hands) and in 45 control subjects. The distoproximal ratio calculation was the most sensitive technique (81%), but was also the least specific. The 'inching test', even though less sensitive, had the advantage of localising focal abnormalities of the median nerve along the carpal tunnel. RPT was abnormal in patients with recent symptoms. Combining the different techniques, an overall sensitivity of 92% was reached, 11% higher than the yield of the single best test suggesting that a multimodal approach could be useful. The best procedure for electrodiagnosis of mild CTS was to combine the median/ulnar comparison test with calculation of the disto-proximal ratio.