Somatosensory evoked potentials during spinal surgery.

A L T H O U G H anaesthetists routinely use a myriad of monitors to assess the integrity of many organ systems, we do not commonly monitor the nervous system. During many operative procedures nervous tissue is placed at considerable risk of injury yet, for the most part, continues to go unmonitored. One of the most common operative procedures which places nervous tissue at risk is spinal surgery, in particular spinal instrumentation such as that for scoliosis. The devastating consequences of a permanent spinal cord injury are obvious. Neurological complications from spinal surgery are not rare the incidence is approximately 1.6%. 4 In the past the only relatively reliable means of assessing spinal cord integrity intraoperatively was the Stagnara Wake-up Test. This test required that the patient be awakened during the surgical procedure, usually at the point of maximal risk of injury, and asked to move his or her limbs. This is not without risk, since an awake patient may cause endotracheal tube or intravenous displacement, and there may be an increased risk of venous air embolism if the patient is allowed to breath spontaneously. From the patient's perspective the prospect of being awake during surgery may be quite distressing. The wake-up is not 100% reliable and, thus, the search for a reliable, relatively non-invasive spinal cord monitor that will allow us to oversee the spinal cord safely yet have the patient remain anaesthetized throughout has led to the use of somatosensory evoked potentials (SSEP). Somatosensory evoked potentials are a representation of the electrophysiological responses to stimulation of an afferent pathway measured over the somatosensory cortex. They reflect the net result of neuronal activity from the peripheral nerves through the spinal cord to the brain. The scientific merit and the clinical intraoperative utility of SSEP have been topics of debate since the introduction of this technique into the operating room more than twenty years ago) The intraoperative use of evoked potentials appears to be increasing in North America, stimulated in part by the position paper put forth by the Scoliosis Research Society in 1992 which endorsed the routine use of SSEP during scoliosis instrumentation surgery for preventing spinal cord injury. 2 Whether it is the litigious climate we find ourselves in or the belief that SSEP monitoring will truly benefit patients undergoing spinal surgery SSEPs will likely become a common component of OR monitoring equipment in the foreseeable future. The intraoperative use of SSEP is not limited to spinal surgery as it is also used as a neuromonitor in carotid artery, neurovascular, acetabular and surgery involving aortic cross-clamping. Although SSEP monitoring may also be useful during intraand extracranial vascular surgery, acetabular procedures and surgery involving aortic cross-clamping, this discussion is restricted to the use of SSEP monitoring during spinal surgery. Several important questions must be answered. Can SSEP be reliably recorded in the OR without placing the patient at an increased risk of morbidity direcdy related to the monitoring? Do intraoperative SSEP changes correlate with postoperative neural deficits? Can intraoperative SSEP changes be used to prevent neural injury from occurring? Do the benefits warrant the additional expense? Is the monitor relatively easy to use? The answers to these questions now appear to be yes. In the current issue of this journal Dr. Manninen reviewed one centre's experience with SSEP monitoring during spinal surgery over 18 months. The results are