Segmental v dermatomal somatosensory-evoked potentials. Normal intertrial variation and side-to-side comparison.

Although segmental and dermatomal somatosensory evoked potentials have been used in the diagnosis of lumbosacral radiculopathy, the unilateral or bilateral normal intertrial variation of these responses has not been investigated. Furthermore, there exists little consensus on how many sequential trials for a single nerve or dermatome are optimal and how to use the data generated from these trials. Without investigation of these basic parameters, determination of what constitutes a normal somatosensory-evoked potential (SEP) is difficult. In 29 normal subjects, the ipsilateral intertrial variations, arithmetic mean side-to-side differences and maximum potential side-to-side differences with stimulation of the superficial peroneal sensory nerve, sural nerve and L5 and S1 dermatomes with respect to P1 and N1 latencies and peak-to-peak amplitudes were investigated. Considerable ipsilateral intertrial variation was observed and side-to-side comparisons revealed a further increase in this inherent variation regarding the above measured parameters. The maximum potential side-to-side differences were even more remarkable than the mean side-to-side differences. For these results and others presented, a method of evaluating SEP parameters whereby the arithmetic mean of two sequential trials for both latency and amplitude is presented that attempts to minimize the normal, inherent variation. We also suggest an additional parameter with which to evaluate SEPs: the maximum side-to-side latency difference. Only through the use of a consistent methodology can segmental or dermatomal SEPs be utilized in a reliable diagnostic manner.