Intraoperative neurophysiological monitoring in spinal cord surgery

Abstract

Knowing the functional integrity of the spinal cord during surgery is an intriguing concept that was first probed by orthopaedic surgeons three decades ago. Sensory-evoked potentials (SEPs) were available then, but with, from today’s perspective, a rather primitive technology. Furthermore, SEPs reflect the functional integrity of the sensory pathways. Information about the more important motor pathways was only indirect. This may be acceptable when external cord compression is the expected mechanism of injury, and it has indeed been shown to be effective in an extensive retrospective study of scoliosis surgery.1 The resection of lesions within the substance of the cord is more complex. It carries a risk of selective damage to the motor tract, which may not be reflected by SEP changes,2 and SEPs can even be recordable in paralysed individuals prior to surgery.3 Moter-evoked potential (MEP) monitoring is based on the cumulative understanding of the motor system acquired since the 1950s,4,5 when a small but essential fibre population in the corticospinal tract was identified and found to give rise to a recordable travelling wave, then termed the D-wave. After the development of transcranial electrical motor cortex stimulation in humans,6 this knowledge was applied in the operating room.7,8 Muscle recording techniques9 were hampered by the effects of general anaesthesia on the α-motor neurons. This was resolved by the multipulse stimulation technique.10 Thus, two techniques to monitor the functional integrity of the motor system are now available: the D-wave and muscle MEPs. The practical application of these in various types of spine and spinal cord surgery were refined during the 1990s.3,11–14 More recently, very strong evidence for the benefit of MEP monitoring for spinal cord surgery was reported