From the Research
Motor evoked potentials (MEPs) are a reliable and effective neurophysiological technique for monitoring motor tract function, with high sensitivity and specificity, as demonstrated in a 2017 study published in the Journal of Neurosurgery: Spine 1. The technique involves applying transcranial magnetic stimulation (TMS) or electrical stimulation to the motor cortex and recording the resulting muscle responses using surface electromyography (EMG) electrodes. For standard MEP assessment, stimulation is typically delivered at 110-120% of the resting motor threshold, with recording electrodes placed on target muscles such as the abductor pollicis brevis for upper limbs and tibialis anterior for lower limbs. Normative data varies by age, sex, and specific muscles tested, but generally, normal MEP latencies range from 19-24 ms for upper limbs and 27-35 ms for lower limbs in adults. Amplitude values typically range from 1-10 mV, with side-to-side differences less than 50%. Central motor conduction time (CMCT), calculated by subtracting peripheral conduction time from total latency, normally ranges from 5-8 ms for upper limbs and 10-15 ms for lower limbs. When performing MEPs, patients should be in a relaxed state for resting measurements or maintaining slight muscle contraction (10-20% of maximum) for facilitated recordings. Temperature should be monitored as it affects conduction velocity, with cooler extremities showing prolonged latencies. MEPs are valuable for diagnosing and monitoring conditions affecting the corticospinal tract, including multiple sclerosis, amyotrophic lateral sclerosis, and for intraoperative monitoring during spinal and brain surgeries to prevent neurological damage, as supported by a 2017 study published in Frontiers in Surgery 2 and a 2010 review published in Spine 3. Some key considerations when using MEPs include:
- The importance of proper patient positioning and relaxation to ensure accurate recordings
- The need for careful monitoring of temperature and other factors that can affect conduction velocity
- The use of MEPs in conjunction with other monitoring techniques, such as somatosensory evoked potentials (SSEPs) and electromyography (EMG), to provide a comprehensive assessment of neural function
- The potential for MEPs to detect and predict neurological deficits, as demonstrated in a 2008 study published in Spine 4. Overall, MEPs are a powerful tool for monitoring motor tract function and preventing neurological damage during surgical procedures, and should be used in conjunction with other monitoring techniques and careful patient management to ensure optimal outcomes.