Spinal Evoked Potentials: Data and Experience

Peripheral Recording To monitor spinal cord function, peripheral input/output relationships _must be analyzed. Peripheral nerves are, therefore, important sites for stimulation and recording. As is typical for EPs, selection of electrodes and their use is essentially the same for stimulation and recording. There are several types of electrodes used for peripheral nerve stimulation or recording; 1) noninvasive disc electrodes, cuff Velcro electrodes, subcutaneous needle electrodes, or 2) invasive mono or multipolar hook or cuff style electrodes (fig.l). Using noninvasive superficial electrodes always requires that contact impedance between the electrodes and skin be kept as low as possible; usually by application of conductive paste or by moistening the skin. With acute invasive hook electrodes, it is important to protect the nerve against drying and hypothermia, usually by immersion in a warm mineral oil bath. Properly constructed cuff style electrodes are usually better than hook electrodes since they do not require an electrode holder, there is larger contact with the nerve, and the problems with nerve drying, hypothermia or mechanical damage are minimized since the wound is closed after implantation. In addition, cuff style electrodes are suitable for chronic experiments lasting for days or weeks. However, it is important to choose a cuff electrode with the proper internal diameter and to use minimal force to secure the electrode in order to prevent peripheral nerve compression and subsequent nerve ischemia. EPs recorded from peripheral nerve can be evoked by stimulation of the distal parts of the nerve or its sensory field, or by stimulation of the proximal part of the nerve, spinal structures such as spinal ganglia, spinal roots, spinal tracts, or su-praspinal and cerebral structures (usually sensory or motor cortex) [2]. We have used peripheral EP recordings with simultaneous spinal somatosen-sory EP (SEP) recording to evaluate the function of rabbit spinal interneurons injured by spinal cord ischemia and treated by epidural cooling [3J. EPs were evoked by bipolar stimulation of a sciatic nerve. Identical bipolar recording J shaped hook electrodes were placed symmetrically on the contralateral sciatic nerve. Thus, sensory pathways were stimulated on one side and the EP recorded on the contralateral side. We found a clear correlation between neurological status, his-tological damage, recovery of ChAT and AChE activity, and recovery of peripherally recorded motor EPs with respect to the intraspinal temperature during spinal cord ischemia (fig.2).