Epispinal stimulation of the spinal cord: a proposed method of operative monitoring

By monitoring spinal cord function during surgical procedures in and around the spinal cord, vital information can be obtained regarding the integrity of this structure. However, the techniques currently used have several limitations. This paper describes a system of monitoring in the cat, whereby, using electrodes placed outside the spine for stimulation of the cord, reproducible responses can be recorded from a midline electrode tapped into the calvarium overlying the sensorimotor cortex. The levels of activation of the spinal cord achieved by direct stimulation of the cord through epidural electrodes and by the epispinal method were found to be similar. By studying the effects of various lesions on the epispinal evoked response, components in the recorded response were identified that were carried in all three funiculi of the cord; the changes produced by these lesions in the epispinal evoked response were immediate. In addition, the sensitivities of the epispinal evoked response and of the somatosensory evoked response to the lesions were compared. When the dorsal blood supply was interrupted, it became evident that, unlike the somatosensory evoked response, which monitors only the dorsal blood supply, the epispinal evoked response monitors both the dorsal and ventral blood supply. By evaluating the recorded response obtained using this method of monitoring, it was possible to identify the particular quadrant of spinal cord injury.