Somatosensory evoked potentials from posterior tibial nerve and lumbo-sacral dermatomes

Techniques for recording the somatosensory evoked potential following stimulation of the skin of L5 and S1 dermatomes are described and validated. Normal data and their range are given for 54 subjects (108 legs). The latency of the peak of the first positive wave (P40) can be predicted from the subject's height from the regression formulae: P40 latency in msec = height in metres X 23.7 + 8.6 for the L5 dermatome. P40 latency in msec = height in metres X 24.5 + 8.7 for the S1 dermatome. P40 latency in msec = height in metres X 15.0 + 14.6 for the posterior tibial nerve. The standard deviations are 2.90 for L5; 2.95 for S1 and 1.60 for the posterior tibial nerve. Age and sex of the subjects had no significant effect. The data will have value when dermatomal somatosensory evoked potentials are used to investigate radiculopathies.