Utility of Motor Evoked Potentials for Intraoperative Nerve Root Monitoring

There is no entirely satisfactory way to monitor nerve root integrity during spinal surgery. In particular, standard free-running electromyography carries a high false-positive rate and some false-negative rate of injury. Stimulated electromyography to direct root stimulation can only be done intermittently, and roots are often inaccessible. This article reviews to what extent muscle motor evoked potential (MEP) monitoring might help. It presents background considerations, describes MEP methodology, and summarizes relevant experimental animal and clinical studies. Based on current evidence, root compromise can cause myotomal MEP deterioration that in some cases may be reversible. However, because of radicular overlap, limited sampling, confounding factors, and response variability, the effects range from no appreciable change to variable degrees of amplitude reduction to disappearance and some false-positive and false-negative results should be expected. For root monitoring, multichannel MEP recordings should span adjacent myotomes and avoid mixed myotome derivations. Only amplitude reduction warning criteria have been studied, but no percentage cutoff consensus has emerged, and this approach is troubled by response variability. There is some evidence that MEPs might reduce false electromyographic results. In conclusion, muscle MEPs could compliment electromyography but seem unlikely to completely solve the problem of nerve root monitoring.