[Somatosensory evoked potentials for the diagnosis of carpal tunnel syndrome]

The middle latency somatosensory evoked potential (SEP), in particular the N60 of median nerve SEP, can be recorded in patients with severe peripheral neuropathy even in the absence of a recordable compound muscle action potential (CMAP). However, clinical utilization of N60 is hampered by its inter-trial variabilities that depend on the level of consciousness, habituation, stimulus rate and/or other unknown factors. We have now obtained a stable N60 SEP when the subject stays awake and alert using a slow stimulus rate. We have applied this method to 17 hands of 15 patients with severe carpal tunnel syndrome (CaTS) whose CMAP were not recordable. The stimuli were applied to two sites: one was 2 cm proximal to the distal crease at the wrist and the other was on the middle finger. SEP were recorded from a central electrode contralateral to the side of stimulation and referenced to the bilateral ear. In normal subjects, the estimated conduction velocities between the distal crease and the middle finger measured by the N60 latency difference ranged from 45.1 to 89.2 m/sec with a mean velocity of 67.4 +/- 13.0 m/sec. In 12 CaTS hands, the stimulation of the fingers resulted in a significantly delayed N60 (8 hands) or no response (4 hands). The estimated conduction velocity in CaTS ranged from 9.3 to 21.2 m/sec with mean velocity of 15.6 +/- 4.2 m/sec. In order to localize the point of maximal conduction delay in the CaTS, a further 17 hands were examined by stimulation of the median nerve at multiple sites across the carpal tunnel.(ABSTRACT TRUNCATED AT 250 WORDS)