Epidural and subpial cortico-spinal potentials evoked by transcutaneous motor cortex stimulation during spinal cord surgery.

Cortico-spinal potentials (CSPs) to transcutaneous motor cortex stimulation were recorded epidurally and subpially in 8 patients undergoing spinal cord surgery. In 6 patients without cortico-spinal tract involvement, a CSP consisted of a direct (D) wave, which was followed by indirect (I) waves at a higher stimulus intensity. The D wave appeared at 25-50% of the maximum stimulus intensity. With increasing stimulus its amplitude levelled off between 10 and 30 microV when recorded epidurally. At high stimulus a prominent indirect potential (I' wave) appeared which followed the D wave after 2.2-3.6 msec. The D and I' wave conduction velocities calculated in 3 cases were identical -65, 70 and 80 m/sec. Subpial records had larger amplitudes than epidural ones but were of the same configuration. In 2 patients with spastic paraparesis the CSP changes were detected below the site of the cord lesion.