H reflex latency as an adequate predictor of the spinal evoked potential latency

By stimulation of the posterior tibial nerve, somatosensory responses can be recorded at the level of the popliteal fossa, thoraco-lumbar spinal region and scalp. The thoraco-lumbar spinal potential is frequently difficult to record. Therefore several methods to estimate its latency were used and the results were compared with the real latency of the N24 recorded at spinal level in a group of healthy volunteers. A formula using the Hoffmann reflex latency (H-M interval) proved to be the most adequate for this purpose. By knowing the spinal potential latency, either through direct recording or through estimation by the formula, determination of a central conduction time, the N24-P40 interval, is possible. Normative values for this parameter are given.