Changes in electrical threshold in human peripheral neuropathy.

A new method for analysis of excitability changes in human peripheral neuropathy is described. The range of the electrical threshold in a nerve is estimated in relative units from the ratio (S90-S10)/S10, where S10 and S90 are the stimulus voltages for 10% and 90% of maximal compound muscle potential. The ratio is increased if there is a fraction of high threshold fibres in the nerve; it is not affected by extraneural factors (e.g. distance to the nerve). The ratio was 0.16 +/- 0.03 in normals and significantly higher in subjects with chronic renal insufficiency or diabetes mellitus. Some of these had normal nerve conduction velocity. The ratio was markedly increased in the median nerve at the wrist in subjects with the carpal tunnel syndrome, which indicated that there are retrograde changes in the entrapped nerve fibres.