CNIM Motor Evoked Potentials (MEP) 1 — Questions and Answers
Question 1: Transcranial electrical motor evoked potentials (tcMEPs) primarily monitor the integrity of which neural pathway?
- Dorsal column-medial lemniscal pathway
- Corticospinal tract (Correct answer)
- Spinothalamic tract
- Vestibulospinal tract
Correct answer: Corticospinal tract
tcMEPs monitor the corticospinal tract (anterior and lateral funiculi of the spinal cord), which is responsible for voluntary motor control.
Question 2: The D-wave in tcMEP monitoring is generated by:
- Muscle action potentials
- Direct activation of corticospinal tract axons at the cortex (Correct answer)
- Interneuronal activation in the spinal cord
- Peripheral nerve compound action potentials
Correct answer: Direct activation of corticospinal tract axons at the cortex
The D-wave results from direct (D) electrical activation of corticospinal axons at the initial segment, producing a fast-conducting descending volley recorded epidurally.
Question 3: What is the standard alarm criterion for tcMEP amplitude change during intraoperative monitoring?
- 10% amplitude decrease
- 25% amplitude decrease
- 50% amplitude decrease or complete loss (Correct answer)
- 75% amplitude decrease
Correct answer: 50% amplitude decrease or complete loss
A 50% or greater decrease in tcMEP amplitude from baseline or complete loss is the widely accepted alarm criterion requiring surgical team notification.
Question 4: Which muscle group is most commonly recorded for lower extremity tcMEP monitoring?
- Tibialis anterior and abductor hallucis (Correct answer)
- Gastrocnemius only
- Vastus lateralis only
- Iliopsoas
Correct answer: Tibialis anterior and abductor hallucis
The tibialis anterior and abductor hallucis are the most commonly recorded lower extremity muscles for tcMEP monitoring due to their reliable cortical representation.
Question 5: Why is total intravenous anesthesia (TIVA) preferred over volatile anesthetics for tcMEP monitoring?
- TIVA is cheaper
- Volatile agents cause dose-dependent suppression of tcMEP amplitudes (Correct answer)
- TIVA eliminates all muscle artifact
- Volatile agents increase seizure risk from transcranial stimulation
Correct answer: Volatile agents cause dose-dependent suppression of tcMEP amplitudes
Volatile anesthetic agents cause dose-dependent suppression of tcMEP amplitudes by inhibiting corticospinal tract activity and synaptic transmission in the spinal cord interneurons.
Question 6: Neuromuscular blockade (NMB) should be avoided or minimized during tcMEP monitoring because:
- NMB increases stimulus artifact
- Muscle relaxants reduce or abolish compound muscle action potential (CMAP) responses recorded from muscles (Correct answer)
- NMB causes cortical spreading depression
- Muscle relaxants interfere with cortical stimulation
Correct answer: Muscle relaxants reduce or abolish compound muscle action potential (CMAP) responses recorded from muscles
Neuromuscular blockade prevents neuromuscular junction transmission, reducing or abolishing the CMAPs recorded from muscles that constitute the tcMEP response.
CNIM Motor Evoked Potentials (MEP) 1
Transcranial electrical motor evoked potentials (tcMEPs) primarily monitor the integrity of which neural pathway?