Advanced MEPs and SSEPs

What is the generator for these obligate peaks?

N20 P23

Somatosensory cortex

What is the generator for these obligate peaks?

N18

Upper brainstem/ thalamus CPc

What is the generator for these obligate peaks?

P14

Caudal medulla

What is the generator for these obligate peaks?

N13

upper cervical spinal cord Cv

What is the generator for these obligate peaks?

N9

Nerve AP Erb’s Point

What is the generator for these obligate peaks?

P37 N45

Somatosensory cortex

What is the generator for these obligate peaks?

N34

upper brainstem/ thalamus

What is the generator for these obligate peaks?

P31

caudal medulla

What is the generator for these obligate peaks?

N9

Nerve AP Pop Fossa

SSEP alert criteria

50% Decrease in amplitude 10% increase in latency

Dorsal column pathway

1. Stimulation at extremity.

2. Travels into dorsal horn into dorsal column

3. Travels up dorsal horn, fasciculus grascilis for lower extremities (medial) or fasciculus cuneatus for upper extremities (lateral)

4. Synapses at the medulla

5. Decussates at medulla

6. Synapse at thalamus

7. Synapse at cortex

What sensory info does the dorsal column pathway conduct?

1. Fine touch

2. Vibration

3. Proprioception

Which SSEP wave forms will be most affected by anesthesia?

Corticals (N20/P23)

Which SSEP wave forms will be least affected by anesthesia?

Peripheral nerve N9

What is the depolarizing pole for SSEP stimulation?

Cathode

What are dermatomal SSEPs?

stimulate one specific dermatome at a time with the goal of increasing the sensitivity and specificity of spinal nerve root monitoring

What are the prominent dermatomes of the arms

C4- collar
C5 - Tricep
T2 - Bicep

C6 & T1 - Forearm

What are the prominent dermatomes of the hands

C6- thumb
C7- middle and index finger
C8- Pinky and Ring finger

What are the prominent dermatomes of the legs

L1 - psoas

L2 - Upper quad

L3 - Lower quad

L4 - Front of lower leg

L5 - Toes

S1 - heel

S2 - back of legs

S3 - glutes

What factors can prolong SSEP latencies?

Anesthesia (Inhalational agents)
Temperature (Hypothermia)
Blood pressure (Hypotension)
Spinal cord injury
Demyelination
Stroke
Peripheral neuropathy
Neurodegenerative disorders

What is antidromic?

An impulse in an axon traveling in the opposite of the normal/natural direction (from axon terminal toward soma)
Stim spinal cord records peripheral nerves.

What is orthodromic?

An impulse in an axon traveling in the normal/ natural direction (from axon terminal toward soma)
Stim nerve record spinal cord
Stim spinal cord record from brain

How is dorsal column mapping performed?

(1) antidromic pathway technique
(2) orthodromic pathway techniques

Why is dorsal column mapping performed?

Dorsal column mapping is done with intramedullary tumors. It is used to find the dorsal median septum for myelotomy

What are the three methods of dorsal column mapping?

Antidromic: stim spinal cord with probe or grid record from peripheral nerve (PTN) stim along the gracilis distal to the tumor moving medial to lateral until no response

What is the goal of phase reversal cortical mapping?

To determine where the central sulcus is located to help better determine where the sensory and motor cortex are located.

How is phase reversal cortical mapping performed?

Occurs at the cortex with a change in recording location from posterior to anterior across the central sulcus following upper limb stimulation.

What are the steps of the auditory pathway?

1. Sound waves (pressure waves) enters external auditory canal

2. Ear drum moves back in forth, pressure waves causes negative waves into inner canal and vice versa

3. Ossicles starts to vibrate with oval window open and closing to cochlea

4. Pressure change causes wave changes of fluid in cochlea

5. Hair cells move and deflect to wave changes, causes depolarization for auditory nerve

6. Travels into brainstem, bypassing medulla into pons

7. Synapse at cochlear nucleus at pons

8. Decussates

9. Synapse at superior olivary complex

10. Becomes lateral lemniscus and synapses at inferior colliculus (thalamus)

11. Travels to cortex

BAEP WAVE 1

distal acoustic nerve, recorded by ipsilateral (Ai)

BAEP WAVE 2

Proximal acoustic nerve, recorded by Cz

BAEP WAVE 3

cochlear nucleus synapse (post pons medulla junction) recorded by Cz

BAEP WAVE 4

Superior olive complex, recorded by Cz

BAEP WAVE 5

lateral lemniscus Inf Colliculus, recorded by Cz

What wave should be missing from a Ac-Ai recording?

Wave 1

Review the optic pathway

Transmits visual information from the retina to the brain
Leaves orbit via the optic canal and form the optic chiasm

What is the primary waveform of interest with flash VEPs

N2 P2

What do VEPs monitor?

Assess the visual pathway activated with LED light goggles that initiates depolarization of the optic nerve then we send action potential and record response.
VEPs can detect blindness in the optic nerve but not in the visual cortex

Review the corticospinal tract pathway

1. Stimulation for primary motor cortex

2. Pyramidal cells travel down, with Betz cells being the largest ones

3. Majority 80-90% decussate at medulla

4. Travels down cortical spinal tracts either anterior or lateral

5. Synapse at ventral horn

6. Alpha motor neuron travels to muscle

7. Synapse at muscle

Trapezius

C3-C4

CN 10, Accessory

What is the nerve root and nerve for Deltoid

C4-C5

Axillary Nerve

What is the nerve root and nerve for Biceps

C5-C6

Musculocutaneous

What is the nerve root and nerve for Triceps

C6-C7

Radial

What is the nerve root and nerve for Flexor carpi radialis (FCR)

C6-C7

Median

Intercostals

T2-T5
Intercostal

Abdominorectus

Upper: T6-T9
Lower T10-T12
Thoracoabdominal

Illiopsoas

L1-L2

Lumbar plexus

Quads

L2-L4

Femoral

Tibialis anterior

L4-L5

Sciatic

Gastroc

S1-S2

Sciatic

Abductor hallucis

S1-S2

Sciatic

External Anal Sphincter

S3-S4

Pudendal

Abductor digiti minimi

C8-T1
Ulnar

Be able to describe multipulse stimulation

A multipulse is a way to create temporal summation by stimulating multiple times to create I waves

Why is multipulse stimulation necessary

To record MEPs for patients under anesthesia, to create temporal summation

When is multipulse stimulation not necessary

When the patient is awake or you are recording from the spinal cord

How did the pioneers developing MEPs first propose that they be performed?

Stimulate the spinal cord above the surgical site and record from the peripheral nerve in the leg (PTN) (NMEPs)

Stimulate the spinal cord above the surgical site and record from the peripheral nerve in the leg (PTN)
What was wrong with this approach?

They were actually monitoring the somatosensory pathway in the reverse direction (antidromic)

How did they find out NMEP was not recording motor responses?

Dorsal Rhizotomy abolished the MEP response
Collision studies confirmed that the potentials were somatosensory (NMEP would collide with orthodromic sensory stimulation at PTN)
False negative reports appeared

When and why are D waves recorded?

D waves are recorded in procedures that involve intramedullary spinal cord tumors (IMSCT)

Where is the stim site for Dwaves

At the surgical site on the spinal cord, one rostral (control) and one caudal (active)

List of pros and cons of D waves versus myotomal recordings of tcMEPs

Pros:

1. Relatively unaffected by anesthetics

2. No need for multipulse train

3. less to no patient movement

4. Thresholds for eliciting D waves fluctuate very little

5. Fluctuation is indicative of pathological chang

Cons:

1. Does not reflect function below corticospinal tracts

2. Invasive electrode placement

3. No differentiation of laterality

What are the two main methods of cortical motor mapping? Compare and contrast

1. Plainfield method: stimulate the cortex with a bipolar probe for 4-6 seconds. High risk for seizures

2. Taniguchi technique: Stimulate with grid and probe (dcMEP). Seizures more rare

What obligate peaks are recorded by this active electrode?

CPc for UN

N20, P23, N13/P14, N18

What obligate peaks are recorded by this active electrode?

CPi for UN

N13/P14, N18

What obligate peaks are recorded by this active electrode?

Cz for UN

N13/P14, N18

What obligate peaks are recorded by this active electrode?

FPz for UN

N13/P14, N18

What obligate peaks are recorded by this active electrode?

C5 for UN

N13/P14

What obligate peaks are recorded by this active electrode?

CPc for PTN

N22, P31

What obligate peaks are recorded by this active electrode?

CPi for PTN

N34, P37, N22, P31

What obligate peaks are recorded by this active electrode?

Cz for PTN

N22, P31

What obligate peaks are recorded by this active electrode?

FPz for PTN

N22, P31

Betz cells

largest pyramidal neurons found in the motor cortex that travel down to the ventral horn of the lumbar spine

Which SSEP waveform is more affect by anesthetics?

Subcortical (N13/P14, N18)

Antidromic dorsal mapping technique

stimulate the spinal cord with a bipolar probe or grid electrode and record from a peripheral nerve

Orthodromic dorsal column mapping

stimulate at the peripheral nerve, record from CNS (either spinal cord or brain)

During dorsal column mapping, the location on the cord that causes the smallest response is likely the _______________________, the ideal location for the ____________

dorsal median septum

myelotomy

dorsal median septum

line of separation between the two gracile fasciculi of the dorsal columns of the spinal cord

What is the dorsal median septum

Neurologically silent structure ideal for myelotomy. In between the two fasciculus gracilis

What is the antidromic technique for dorsal column mapping?

Stimulate along gracilis distal to tumor with either bipolar or electrode grid. Looking for least responsive area. Recording from PTN or popfossa

What are the two orthodromic techniques for dorsal column recording?

1. Placing a grid on the dorsal column to identify the dorsal median septum, which would be the point between the 2 maximum amplitude recordings. Stim from the PTN and record from the spinal cord

2. Left-to-right phase reversal. Stim from electrode strip or a bipolar on dorsal column and record using C3-C4 montage. Notate where the phase reversal occurs, and dorsal median septum is between where it reverses.

What is conductive hearing?

Steps of the auditory pathway prior to cochlea

What is sensorineural hearing?

Steps of the auditory pathway from cochlea onward

What is interpeak latency I-III?

Auditory nerve. Increase of latency is due to anything with auditory nerve, Caudal brainstem

What is interpeak latency III-V?

III-V rostral brainstem

What are the montages for BAEPs?

A1 - Cz

A2 - Cz

What is the expected change with conductive hearing loss?

Increased absolute latency, but IPL stays the same

What is the expected cahnge with sensorineural hearing loss?

Increase of IPL