[Somatosensory evoked potentials: normal values and methodological problems in various recording technics]

In order to optimize the method of examination, somatosensory evoked potentials (SSEP) were studied in 27 healthy subjects, as recorded during stimulation of the median nerve at the wrist. The study shows that recordings with surface electrodes produced better results than with needle electrodes, owing to lesser disturbance from muscle artifacts. The SSEPs of each subject were recorded taking a fixed position of the active electrode placed over the contralateral sensory "hand field", and a reference electrode positioned for the first recording mid-frontally and for the second recording at the contralateral ear or vice versa. The first 3 positive and the first 3 negative peaks were studied. Exact identification of each peak was more often possible when recordings were made with the reference electrode placed mid-frontally than placed at the contralateral er. Normal values were evaluated for the latency P15, P25, P45, N30, N35 and N55, for the amplitudes P15-N20, N20-P25, P25-N35, N35-P45 and P45-N55 and for the latency intervals P15-N20, N20-N35, N35-N55, P15-P25 and P25-P45, with the reference electrode in different positions. In addition, the side difference for each subject was determined. Statistical analysis (T-test for independent variables) revealed a significant difference, depending on the position of the reference electrode: with the reference electrode placed at the contralateral ear, the latency P15 was shorter, the amplitude P15-N20 was increased and the amplitude N20-P25 was decreased as compared with a reference electrode placed mid-frontally.(ABSTRACT TRUNCATED AT 250 WORDS)