Motor Evoked Potentials (MEP) in Neuromonitoring
MEP stands for Motor Evoked Potentials, which are electrical signals recorded from muscles or nerves after stimulation of the motor cortex, used to assess the functional integrity of descending motor pathways during surgery. 1
How MEPs Are Generated
MEPs are performed by stimulating the motor cortex through either:
- High-voltage short-duration electrical stimulus, or
- Magnetic induction 1
The responses are then recorded at different levels:
- Neurogenic MEPs: responses recorded at peripheral nerves 1
- Myogenic MEPs: large biphasic responses recorded over the muscle belly 1
Clinical Purpose and Applications
MEPs monitor the anterior motor column of the spinal cord, which is the pathway most vulnerable to ischemic injury during surgical procedures. 1 This makes MEPs superior to somatosensory evoked potentials (SSEPs) for detecting motor pathway compromise, as SSEPs only monitor the posterior and lateral columns and can miss isolated anterior column injuries. 1, 2
MEPs are routinely used in:
- Thoracic and thoracoabdominal aortic surgery to detect spinal cord ischemia 1
- Spinal fusion procedures for degenerative lumbar disease 1
- Cervical spine surgery for myelopathy and radiculopathy 1
- Brain tumor surgery near the motor cortex 3, 4
- Brainstem surgery 4, 5
Key Technical Considerations
The amplitude of MEP responses is proportional to the number of motor neurons being stimulated, making these signals highly sensitive to:
A critical pitfall is "anesthetic fade": the voltage threshold required to generate a 50 microV amplitude signal increases progressively during surgery, directly proportional to anesthetic exposure duration. 1 This phenomenon must be recognized to avoid false-positive interpretations.
Interpretation and Clinical Thresholds
A reduction in MEP amplitude of greater than 50% from baseline, or complete loss of MEPs, indicates impending neurological injury. 6 More recent evidence suggests setting alarm criteria at 70-80% amplitude loss. 7
When MEP changes occur during surgery, immediate interventions may include:
- Reimplantation of critical segmental arteries 1
- Optimization of hemodynamics and perfusion pressure 1
- Repositioning of surgical clamps 1
- In some cases, aborting the procedure 1
Advantages Over SSEP Monitoring
MEP monitoring demonstrates superior sensitivity compared to SSEP monitoring alone. In one study of descending aortic reconstruction, 29% of patients showed MEP evidence of spinal cord ischemia compared to only 7% with SSEP changes. 1 The routine use of multimodal monitoring including MEPs has contributed to reducing neurologic complication rates from historical levels of 23% down to 2-6% in thoracic aortic surgery. 1
Important Caveats
False-positive MEP alerts can lead to unnecessary surgical modifications or procedure abandonment. 1 In cervical spine surgery, substantial reliance on MEP alerts alone may lead to surgeon behavior that could be detrimental to patient welfare. 1
MEPs require careful anesthetic management: total intravenous anesthesia is typically necessary, and muscle relaxants must be avoided or carefully titrated to maintain recordable signals. 1, 3