Current status of motor evoked potentials

INTRODUCTION

Surgery on the spine and the thoracic aorta impose great risks to the spinal cord. Intraoperative somatosensory evoked potential (SEP) records electrical potentials along the somatosensory pathway in response to stimulation of peripheral nerves (usually the median or the posterior tibial nerves) and indicate the integrity of the spinal cord. The principles and utility of SEP have been previously reviewed.[1] [2] [3] In general, spinal cord injury should be suspected when there is a 50% or more decrease in SEP amplitude and/or 10% or more increase in latency. In a well-known multicentre survey conducted by the Scoliosis Research Society (n = 51263 scoliosis repair), SEP monitoring alone detected neurological injury in 77% of the cases with a specificity of >98%.[4] However, the real concern of SEP lies with the incidence of false negative cases (0.13–25%), where patients develop new post-operative deficits in the absence of intraoperative SEP change. Apart from the technical and pharmacological problems that may interfere with SEP recording, it is now recognised that isolated injury to the anterior corticospinal tract can go undetected with unchanged SEP recordings.[5] [6] [7] Intraoperative ‘wake-up’ test assesses spinal cord motor functions and has been a useful adjunct to SEP during spinal instrumentation for many years.[8] However, wake-up test cannot be repeated frequently during the whole course of surgery and cannot be applied to ‘uncooperative’ patients. Motor evoked potentials (MEPs) have been developed to overcome the limitations of SEP by monitoring the descending motor pathways in the anterior and lateral part of the spinal cord.

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