Dermatomal somatosensory evoked potentials: cervical, thoracic, and lumbosacral levels.

Somatosensory evoked potentials were recorded at the scalp to stimulation of the skin at C4, C5, C6, C7, C8, T2, T4, T6, T8, T10, T12, L2, L3, L4, L5, and S1 dermatomes and of the tibial nerve. Stimulation and recording techniques are described. Data were obtained from 41 normal subjects, 25 of which had all 16 dermatomes studied. Wave form descriptions include both typical and atypical presentations. Descriptive statistics for latency, amplitude, left to right comparisons, and level to level comparisons are given. Scalp response latencies for distal extremity dermatomes were well correlated with height but not with vertebral column length, whereas latencies for thoracic dermatomes were not well correlated with either height or vertebral column length. Since scalp response amplitude data were not normally distributed, they were logarithmically transformed and minimum and maximum limits for 1 S.D., 2 S.D., and 3 S.D. derived. Left/right amplitude ratios were similarly treated. Level to level comparisons were achieved with a Z score concordance analysis, which showed that the response values at one level can be used to predict the response values at another level.