CNIM Motor Evoked Potentials (MEP) 2 — Questions and Answers
Question 1: Which tcMEP stimulation parameter is most commonly adjusted to optimize responses intraoperatively?
- Pulse duration only
- Stimulation frequency
- Stimulus intensity (voltage or current) (Correct answer)
- Number of channels
Correct answer: Stimulus intensity (voltage or current)
Stimulus intensity is most commonly adjusted to achieve supramaximal activation of the corticospinal tract while avoiding excessive patient movement or patient safety concerns.
Question 2: Multi-pulse (train) stimulation is used for tcMEPs rather than single pulses because:
- Single pulses produce excessive patient movement
- Multiple pulses summate to overcome the higher threshold of the spinal motor neurons under anesthesia (Correct answer)
- Train stimulation reduces seizure risk
- Single pulses cannot penetrate the skull
Correct answer: Multiple pulses summate to overcome the higher threshold of the spinal motor neurons under anesthesia
Under anesthesia, motor neurons have elevated thresholds, requiring temporal summation from a train of stimuli to reliably generate descending volleys sufficient to activate muscles.
Question 3: An alarm for complete tcMEP loss in all four limbs simultaneously during thoracic spine surgery most likely indicates:
- Spinal cord injury at the surgical level
- A systemic event such as hypotension, anesthetic bolus, or neuromuscular blockade administration (Correct answer)
- Electrode displacement in one channel
- Normal intraoperative variability
Correct answer: A systemic event such as hypotension, anesthetic bolus, or neuromuscular blockade administration
Simultaneous four-limb tcMEP loss suggests a systemic cause (blood pressure drop, NMB given, anesthetic change) rather than a focal surgical injury, which typically affects specific limbs.
Question 4: The C3/C4 electrode positions are used for tcMEP stimulation to preferentially activate:
- Lower extremity motor cortex
- Upper extremity motor cortex (Correct answer)
- Visual cortex
- Somatosensory cortex
Correct answer: Upper extremity motor cortex
C3 and C4 electrode positions overlie the hand and upper extremity representation of the motor cortex, making them ideal for activating upper extremity corticospinal projections.
Question 5: Relative contraindications to transcranial electrical MEP monitoring include:
- Spinal stenosis
- Cortical electrodes placed subdurally
- Intracranial metal clips, cochlear implants, or cardiac pacemakers (Correct answer)
- Use of propofol anesthesia
Correct answer: Intracranial metal clips, cochlear implants, or cardiac pacemakers
Intracranial metal implants, cochlear implants, and certain cardiac pacemakers are contraindications because transcranial electrical stimulation can induce currents in these devices or cause tissue heating.
Question 6: Which surgical procedure most commonly uses both SSEP and MEP monitoring together?
- Hip arthroplasty
- Thoracic and cervical spinal cord surgery (Correct answer)
- Peripheral nerve repair
- Cranial bone flap elevation
Correct answer: Thoracic and cervical spinal cord surgery
Thoracic and cervical spinal cord surgeries use combined SSEP and MEP monitoring because the two modalities provide complementary assessment of both dorsal and ventral spinal cord function.
CNIM Motor Evoked Potentials (MEP) 2
Which tcMEP stimulation parameter is most commonly adjusted to optimize responses intraoperatively?