Estimation of the brain and spinal cord conduction time in man by means of the somatosensory evoked potentials and F and H responses.

Somatosensory evoked potential (SSEP) recordings from the scalp were performed in 17 healthy subjects. In seven of these SSEP was also recorded at the level of the second lumbar spine. In the other ten F and H responses and the corresponding M responses were studied. By means of the SSEP recordings at the level of the second lumbar spine and the F- and H-responses, the conduction time in the brain and spinal cord, that is central latency, was calculated and the following results were obtained: 16.0 ms with standard deviation (SD) +/- 1.1 ms (by means of SSEPs), 9.5 +/- 2.4 ms (by means of F response) and 13.1 +/- 1.5 ms (by means of H response). Of the three methods used the H response method seems to be the best for clinical purposes: it is easy to perform and statistically it is more stable than the F response recording; moreover the recording can be performed reliably even in persons with thick back muscles and subcutaneous fat, unlike the evoked potential procedure which only with difficulty shows detectable responses at the lumbosacral levels in such persons. Three patients are presented to illustrate the technique; in one of these the recording evoked potentials from the epidural space were recorded.