Overview of MEP:
Motor Evoked Potentials (MEP) are electrical responses recorded either from muscles, or from axons of the descending motor tract, in response to electrical or magnetic stimulation of nervous system structures that govern movement. While there are many different methods for stimulation and recording, the most common approach involves transcranial electrical stimulation of the motor pathway, using subdermal needle electrodes positioned in the scalp above primary motor cortex.
As the volley of electrical activity travels along the corticospinal tract on the way to the muscles, it can be recorded from an sub/epidural electrode situated on the dorsomedial surface of the spinal cord. This response is called a “D-wave” because it is the result of direct activation of motor fibers in the brain. D-wave recordings are invasive and typically reserved only for tumors of the spine (intramedullary or extramedullary/intradural).
Once the electrical activity reaches the periphery, intramuscular needle electrodes are used to record muscle contractions, which are called Compound Muscle Action Potentials (CMAPs). These “myogenic” MEPs are complex and polyphasic.
MEP in IONM
Like the SSEP, the D-wave is an averaged response; however, the number of presentations required to record a well-formed, reliable D-wave is usually less than 10. So, even though it is an averaged response, it usually takes just a couple of seconds to record a D-wave. The D-wave is not a real time measure of motor function, but it is very close. Muscle MEPs are not averaged. Rather, the CMAP is a single recording following stimulus delivery. This is a real time measure of motor function, but movement may limit the frequency with which you can test motor function, so MEPs really represent a snapshot of motor function during a very small window of time.
Myogenic MEPs are highly sensitive to the effects of anesthesia, particularly inhalational agents, because they require depolarization at multiple synapses throughout the nervous system, including local interneurons of the cerebral cortex, and α-motoneurons of the spinal cord. D-waves are stable, reliable and highly resistant to anesthetic effects because they are mediated asynaptically.
Changes in MEPs may be indicative of evolving injury somewhere along the pathway between stimulating and recording electrodes. In order to detect changes in MEPs, the responses must be quantified. When we record MEPs, we look at several variables, which are described below. Note that the descriptions are highly simplified.
The MEP amplitude is measured peak-to-peak. Amplitude is measured in μV. While D-wave amplitude is quite stable, myogenic MEPs amplitudes tend to be quite variable. Neural transmission is altered by low temperatures, nerve compression, nerve injury, etc. So, decreases in amplitude, can be indicative of evolving injury.
Because myogenic MEP amplitudes tend to vary, some practitioners don’t really worry about changes in amplitude at all. Rather, they are mostly concerned about whether or not the MEP is present at all. So, if a MEP disappears, it can be indicative of evolving injury.
The threshold is operationally defined as the minimum voltage necessary to trigger a CMAP with amplitude > 0 μV. Threshold is measured in volts. If significant increases in voltage are required to generate MEPs, then this can be indicative of evolving injury.
Why we use MEPs in surgery:
1. Map motor cortex:
When eloquent cortex is at risk for injury during tumor resection surgery, direct stimulation of motor cortex is a critical step toward preserving motor function. Positive results identify regions that provoke movement and should be spared. Negative results identify regions that are “silent” and can be used as an access route toward tumor resection, for example.
2. Map corticospinal and corticobulbar tracts:
Similar to motor cortex mapping, subcortical, brainstem and spinal motor tracts can be mapped and preserved.
3. Monitor corticospinal and corticobulbar tracts:
MEPs recorded from muscles in the arms and legs are used to monitor the corticospinal tract as it traverses the internal capsule, peduncles and pyramids before synapsing on α-motoneurons in the ventral gray matter of the spinal cord. MEPs recorded from muscles in the head and neck are used to monitor the corticobulbar tract as it traverses the internal capsule and peduncles before synapsing on neurons in the cranial nerve motor nuclei of the brainstem. MEPs are particularly useful during a wide variety of brain and spine surgeries, as they detect ischemia and iatrogenic injury these pathways, and particularly around the terminal neurons in the brainstem and spinal motor neuron nuclei.
4. Monitor nerve roots:
When the appropriate muscles are selected for recording, MEPs can be particularly useful for detecting evolving injury to cervical and lumbar spinal nerve roots.
5. Monitor peripheral nerves (positioning):
Compression or stretch of peripheral nerve, usually as a result of patient malpositionng during surgery, can result in attenuation or obliteration of MEPs recorded from muscles innervated by the nerve. The most common sites of compression or stretch include the ulnar notch, brachial plexus, spiral groove, femoral nerve and fibular head. When the appropriate nerve is selected, MEPs are particularly sensitive to detecting evolving injuries, allowing the surgical team to intervene by optimizing the patient’s position. Left untreated, malpositioning can result in permanent neurologic deficit.
6. Monitor peripheral nerves (vascular):
Vascular occlusion can result in decreased perfusion to a limb, and ultimately permanent neurologic injury. Perhaps the most common situation in which this occurs is during anterior lumber interbody fusion (ALIF) in which 1) retractors are place on the iliac artery/vein, or 2) there is risk to injury of the iliac artery/vein. While surgeons usually place a pulse oximitry monitor on the great toe, MEPs can be particularly sensitive to detecting vascular insufficiency as well. Thus, they serve as a useful adjunct.
Benefits of D-wave Recordings During Surgery:
- Negates use of wake-up test.
- Nearly real-time measure of corticospinal tract function.
- Informative regarding long-term prognosis for motor functions.
- Unlike SSEPs, directly measures corticospinal pathway.
- Requires low stimulation, which rarely produces movement.
- Not markedly affected by inhalational anesthetics.
- Unaffected by neuromuscular blockade.
Limitations of D-wave Recordings During Surgery:
- Invasive – requires laminotomy.
- May be impossible to record above C2 or below T10.
- May be absent following radiation.
- Can have false negative results if electrode moves from midline.
Benefits of Muscle MEP Recordings During Surgery:
- Relatively non-invasive.
- Negates use of wake-up test.
- Stimulated EMG is a gold standard for evaluating nerve function.
- High sensitivity (100%) and specificity (100%)
- Can be done in patients where SSEPs not obtainable.
- Unlike SSEPs, directly measures corticospinal pathway.
- More sensitive to ischemia than SSEP.
- More sensitive to nerve injury than SSEPs.
Limitations of Muscle MEP Recordings During Surgery:
- Can cause patient movement.
- Standards for use vary.
- No consensus for alert criteria.
- Neuromuscular blockade is contraindicated.
- Inhalational anesthetics are contraindicated.
- Precedex is contraindicated.
- Reportedly contraindicated in patient with history of seizures.
- Risk of oral trauma (requires placement of soft bite guards to prevent tongue bite).
Surgical procedures for which MEPs are indicated:
- Spine (extradural, decompression/fusion):
- Spinal cord stimulator placement for intractable pain.
- Spinal Cord (intradural):
- Extramedulalry tumor.
- Intramedullary tumor, syrinx.
- Cauda equina tumor, tethered cord.
- Intracranial (supratentorial):
- Long tract motor/sensory monitoring.
- Motor/sensory cortex and internal capsule mapping.
- Language mapping.
- Motor cortex stimulator placement for intractable pain.
- Intracranial (infratentorial):
- Brain stem (CPA, acoustic neuroma, cranial nerves, etc).
- Intracranial Vascular:
- Cerebral aneurysm clipping.
- Repair of arteriovenous malformation (AVM).
- Cerebral aneurysm coiling (interventional radiology; IR).
- Aorta (ascending, descending, arch aneurysm/repair).
- Peripheral Nervous System – Neurosurgery:
- Brachial Plexus.
- Lumbosacral Plexus.
- Individual Peripheral Nerves.
- Peripheral Nervous System – Orthopedic Surgery:
- Shoulder arthroplasty.
- First rib resection, thoracic outlet syndrome.
- Hip arthroplasty.
- Pelvis fixation.
- Other extremities.
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