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Peak Training + Board Prep > tcMEP > Flashcards

tcMEP Flashcards

1
Q

Transcranial Motor Evoked Potentials (tcMEPs) are stimulated in the ______ and recorded in the _________.

A

Stimulated in the central nervous system (CNS) and recorded in the peripheral nervous system (PNS)

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2
Q

An anode is placed at the C4 scalp location and cathode is placed at the C3 location for tcMEP stimulation. What side of the body will muscle responses be elicited from?

A

On the left side of the body

(Remember that tcMEP stimulation is anodal stimulation meaning the anode delivers the stimulus.) You will need to understand this and also consider the tcMEP pathway to answer correctly.

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3
Q

In order to elicit a muscle response to the tcMEP stimulation, the alpha motor neuron must undergo temporal summation, requiring what kind of stimulation technique?

A

A multipulse train stimulus

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4
Q

What is temporal summation?

A

Temporal summation occurs when multiple subthreshold EPSPs from one neuron occur close enough in time to combine and trigger an action potential at the axon hillock.

(We need to cause temporal summation from the alpha motor neuron in order to record reliable CMAP responses - and we do this by using a multipulse train stimulus.)

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5
Q

Proper tcMEP stimulation results in what type of recorded data?

A

Compound Muscle Action Potential (CMAP) responses recorded from muscle groups selected.

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6
Q

You are stimulating tcMEPs and want to record from the LEFT side of the body. What is the best scalp location for your anode?

C3, C4, CP4, or CP3?

A

C4

Technologist must have awareness of anodal stimulation, knowledge of the 10-20 system, and tcMEP pathway in order to answer correctly.

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7
Q

Give a few examples of what tcMEP data can be used to assess intraoperatively

A
  • Functional integrity of the corticospinal tracts within the spinal cord (spinal cord monitoring)
  • Perfusion of the internal capsule (brain)
  • Functional integrity of nerve roots (nerve root monitoring)
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8
Q

What is alert criteria for tcMEP monitoring?

A
  • Amplitude decrease 50-70%

and/or

  • Loss of CMAP response

and/or

  • Need to increase stimulation intensity by 100V or more*

(*Given anesthesia is appropriate for monitoring)

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9
Q

What is the most common post-operative injury seen in cervical procedures?

A

C5 nerve root palsy

clinically, patient shows a loss of strength in deltoid or biceps brachii

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10
Q

During an anterior cervical discectomy and fusion procedure, the C5 nerve root is stretched due to
over-retraction. You report transient EMG activity, and the activity is quiet after only a moment. How
can you confirm conduction of the C5 nerve root?

A

tcMEP CMAP response recorded from the deltoid muscle can verify C5 nerve root conduction

(Linking muscles in practice will inhibit your ability to isolate injury. For example, a MEP recording channel of Deltoid-Bicep. In order to isolate nerve root changes appropriately, do not link muscles)

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11
Q

You observe a sudden loss of lower extremity tcMEP responses during a scoliosis correction. What surgical interventions are appropriate given the data observed?

A
  • Reversal of last surgical maneuver
  • Remove rods and/or screws
  • Increase the mean arterial pressure (MAP) above 90mmHg
  • Recommendation for steroids

This is a spinal cord injury protocol

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12
Q

During an L4-S1 Posterior Spinal Fusion you observe a persistent >75% amplitude decrease in the tibialis anterior. What postoperative injury is most likely?

A

Foot Drop / Loss of dorsiflexion

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13
Q

We request a bite block be placed prior to tcMEP stimulation in order to prevent what type of injury?

A

Tongue laceration

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14
Q

tcMEP monitoring can be used to assess what?

A
  • cortical perfusion
  • the anterior portion of the spinal cord
  • ventral nerve roots
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15
Q

You are stimulating tcMEPs and want to record from the RIGHT side of the body. What is the best scalp location for your anode?

C3, C4, FZ, or CZ?

A

C3

Technologist must have awareness of anodal stimulation, knowledge of 10-20 system, and tcMEP pathway in order to answer correctly.

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16
Q

What are d-wave responses?

A

Responses recorded directly from the spinal cord in response to stimulation at the motor cortex of the brain.

This is similar to tcMEP functionally, but instead of recording from muscles we record from the spinal cord. Why do this? D-waves are asynaptic responses which are resilient to the effects of anesthesia. D-waves can be particularly helpful in monitoring intramedullary spinal cord tumors where tcMEP alone may not yield reliable results.

17
Q

D-wave responses are:

A
  • recorded directly from the spinal cord
  • recorded after stimulation of the motor cortex of the brain
  • asynaptic responses, which are resilient to anesthetic effects
18
Q

Medical literature cites significant safety concerns surrounding use of tcMEP. Name a few of these safety concerns.

A
  • Patient history of seizure disorder
  • Patient has implanted pacemaker
  • Activation of biting muscles during stimulation / risk of tongue laceration
19
Q

A large number of motor units firing synchronously results in a:

A

CMAP

Compound Muscle Action Potential

20
Q

Describe the tcMEP Corticospinal Pathway

A

Originates in primary motor cortex (frontal lobe) and is a descending pathway

Decussates in the caudal medulla

Synapses in ventral horn with alpha motor neuron

Alpha motor neuron synapses at neuromuscular junction

CMAP response is recorded from muscle

21
Q

During a T5 - S1 Posterior Spinal Fusion procedure, you observe a 75% amplitude decrease of the
tcMEP CMAP recorded from the Left Tibialis Anterior muscle. All other tcMEP CMAPs from bilateral
UE, abdominal, and lower extremity muscle groups are robust and unchanged from baseline. What is
the appropriate to communicate to the Surgeon?

A

Inform the Surgeon that a left lower extremity nerve root (possibly L5) may have experienced injury or ischemia.

22
Q

Which of the following stimulation techniques is most likely to elicit a seizure?

  • D-waves for spinal cord monitoring
  • Phase reversal mapping for central sulcus identification
  • 60Hz direct cortical stimulation for cortical mapping (Penfield method)
  • Fast frequency pulse-train direct cortical stimulation for motor cortex mapping (Taniguchi method)
A

60Hz direct cortical stimulation for cortical mapping (Penfield method)

Cold saline or cold ringer’s solution is made available during this procedure in order to stop a seizure should it occur

23
Q

Why do we use a train of multiple stimulus pulses for tcMEP monitoring?

A

The alpha motor neuron requires temporal summation to reach activation threshold

24
Q

Transection of the right spinal cord at the T12 level will result in loss of tcMEP potentials where?

A

All muscle recordings below T12 level on the right side.

25
Q

During tcMEP, when the anode is placed at C4, responses can be expected on what side of the body?

A

Left side

26
Q

Following the corticospinal pathway from the motor cortex of the brain to the site of muscle activation, the first synapse occurs at the _____________ and the second synapse occurs at the _____________.

A

1st synapse: Alpha motor neuron

2nd synapse: Neuromuscular junction

27
Q

Give 3 reasons why CMAP responses to tcMEP stimulation require a multi-pulse technique

A
  • multiple pulses are required to overcome anesthetic effects in the cortex
  • multiple pulses induce temporal summation at the alpha motor neuron
  • multiple pulses mimic the D and I wave volleys that descend the spinal cord in awake patients
28
Q

Identify the alert criteria for D-Wave monitoring that correlates with long term muscle weakness

A

50% decrease in amplitude of D-Wave

29
Q

What is the primary advantage of D-wave recording?

A

Reliable prediction of long term motor function

30
Q

How many pulses are necessary for D Wave stimulation?

A

1 pulse

31
Q

D waves are recorded from which of the following electrodes?

  • Epidural electrodes over the spinal cord
  • Subdural electrodes over the motor strip
  • Surface electrodes over peripheral nerves
  • Needle electrodes in distal extremity muscles
A

Epidural electrodes over the spinal cord

32
Q

Transcranial motor evoked potentials are bet elicited when using which of the following?

  • A train of stimuli
  • Complete neuromuscular blockade
  • Halogenated agents
  • Stimulus rates above 20/sec
A

A train of stimuli

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