A method of calculating spinal cord transit time from potentials evoked by tibial nerve stimulation in normal subjects and in patients with spinal cord disease.

Somatosensory potentials were evoked by stimulation of the tibial nerve at the ankle and recorded over the spine and scalp in 16 normal subjects and 26 patients with known or suspected spinal cord disease, with the aim of developing a method of measuring spinal sensory conduction velocity using a tolerable number of stimuli, applied unilaterally to alert subjects. In normal subjects N21 was consistently recorded over L1 vertebra and in most subjects a complex, N27/N29/P33, was recorded over the cervical spine referred to the vertex. Constant latencies at different spinal levels and, in one subject, comparison with the latency of the ascending volley indicate that the complex was not derived from the spinal cord but from more rostral structures, and therefore only transit time, rather than velocity, could be measured. In patients with clinically definite multiple sclerosis, even with minimal clinical signs, the N27/N29/P33 complex was always abnormal. Abnormalities in this and other forms of spinal cord disease were commonly absence or distortion of the complex, prolonged transit time being rare. The clinical value of the method is limited by the very low amplitude of the responses.