What are somatosensory and motor evoked potentials?

What are somatosensory and motor evoked potentials?

The electroencephalogram (EEG) is the measurement of the spontaneous activity of the brain. Evoked potentials are a measurement of the electrical responses “evoked” by a stimulus to the nervous system. Evoked potential moni-toring is utilized in patients felt to be at high risk for spinal cord injury from surgical trauma, operative position, or impairment of blood supply.

The most commonly utilized evoked potentials are those produced by stimulation of the sensory system: somatosen-sory evoked potentials (SSEPs). In this technique a peripheral nerve (e.g., posterior tibial or median) is stimulated and the neural response measured. A characteristic pattern of peaks and valleys is obtained which can be used to follow the response at various points along the stimulated tract (Figure 21.1). This technology intermittently determines the integrity of the posterior spinal sensory pathways and large peripheral nerves. Significant changes in the SSEPs may consist of alterations in amplitude, latency, and/or morphology compared with baseline waveforms. Changes in SSEP waves that indicate possible disruption of the sensory pathway include a decrease in the amplitude of the waveform and an increase in the latency of those waves.

Motor evoked potentials (MEPs) are electrical impulses measured in the peripheral nerves and muscles in response to stimulation of the cortex or spinal cord. MEPs may be used in conjunction with SSEPs when there is a possibility of injury to the motor pathways in the anterior spinal cord. Two basic techniques for stimulating the cerebral motor cortex are direct transcutaneous electrical stimulation and transcutaneous magnetic stimulation. Addition of MEPs to SSEP monitoring may increase detection of potential injury by as much as 10%. MEPs may also be useful in the detection of spinal cord ischemia during aortic cross-clamping for thoraco-abdominal aneurysm repair.