Sensory conduction of the median nerve at the carpal tunnel for eight 
consecutive 1 cm segments of the nerve was evaluated in 217 hands of 153 of our 
patients with carpal tunnel syndrome. Impairment was found to be highly focal 
and often confined to a single 1 cm segment of the nerve. The section of the 
nerve at or just distal to the distal margin of the carpal tunnel was affected 
most frequently, the section within the tunnel was affected less often, and the 
section proximal to the tunnel at the level of the mid-carpal and radio-carpal 
joints was affected least. The greatest contrast between frequencies of slowing 
at adjacent segments occurred at the proximal and distal margins of the carpal 
tunnel. The distribution of the nerve impairment was similar between the sexes; 
however, among the men the segment affected most frequently was located 1 cm 
distal to the segment affected most frequently among the women. The general 
pattern of slowing which we found does not substantiate some commonly-held 
opinions about the aetiology of carpal tunnel syndrome.