[Efferent spinal evoked potentials by transcranial magnetic stimulation in dog]

We have investigated the utility of transcranial magnetic stimulation for spinal cord monitoring, experimentally in dog. Experiment 1. Spinal cord potentials evoked by transcranial magnetic stimulation (TCMS) were recorded in the epidural space, using a bipolar mapping spinal electrode, and compared with potentials by transcranial electrical stimulation (TCES). Experiment 2. The effect of lateral compression on the spinal cord was observed, recording from 3 bipolar electrodes attached to the bilateral and dorsal midline epidural space. Experiment 3. The potentials evoked by TCMS were compared with the afferent potentials evoked by spinal electrical stimulation after lateral compression of the spinal cord. Efferent spinal cord potentials evoked by TCMS consisted of a 1 st component of high amplitude and short latency and a later polyphasic component with prolonged latency. The 1 st component was stable, and not affected by intravenous anesthesia. The potentials evoked by TCMS were similar to the potentials evoked by mild TCES, and the mean latency of the 1 st component by TCMS was very similar to that by mild TCES. After releasing the lateral compression, the amplitude recorded from the contralateral side of the spinal cord showed better recovery than that from the compression side. In experiment 3, for both potentials after good recovery, the latency was almost completely normalized, but the amplitude gradually decreased after two days. When there was relatively good recovery in the potentials by magnetic stimulation, the hind limbs resumed normal function. However when there was poor recovery in the potentials, the hind limbs showed a poor functional recovery. From these findings, we concluded that the potentials evoked by TCMS seem to reflect the function of the posterolateral portion of the spinal cord. These potentials were therefore a good indicator for monitoring the motor function of the spinal cord.