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Module 8: Evoked Potentials (ICETAP)

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on 11 January 2015

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Transcript of Module 8: Evoked Potentials (ICETAP)

Module 8: Evoked Potentials
Different Types of Intraoperative Neuromonitoring (IOM)
SSEP (Somatosensory Evoked Potentials)
BAEP (Brainstem Auditory Evoked Potentials)
VEP (Visual Evoked Potentials)
TcMEP (Transcranial Motor Evoked Potentials)
cMEP (Cortical Motor Evoked Potentials)
eMEP (Epidural Motor Evoked Potentials) or “D wave”
EMG (triggered or non-triggered)
Cranial Nerve Monitoring (EMG of V, VII, IX, X, XI, or XII)

EEG: spontaneous, nonspecific, large signal (50µV)

Evoked potentials: specific stimulus, specific pathway, small signal (0.1-20µV)

EEG vs. Evoked Potentials
Sensory Evoked Potentials
Rely on signal amplification, filtration, conversion, and averaging
SSEP: 50% reduction in amplitude or 10% increase in latency considered significant
BAEP: 1msec increase in latency considered significant
Evoked potentials of cortical origin (cortical SSEP, VEP) are most vulnerable to anesthetic effects
Brainstem potentials (subcortical SSEP, BAEP) are least vulnerable to anesthetic effects

Specific Drug Effects on Sensory Evoked Potentials
What are Somatosensory Evoked Potentials (SSEPs)?
Electrical signals generated by the nervous system in response to somatosensory stimuli
A series of waves that reflect sequential activation of neural structures along somatosensory pathways
Typical stimulation sites:
median n., ulnar n., common peroneal n., posterior tibial n.
Typical recording electrode sites:
scalp, cervical spine, peripheral nerve proximal to stimulation site

The Somatosensory Pathway
SSEP Monitoring: Pros
The “gold standard” monitor
Anesthetic effects relatively well understood
Can use neuromuscular blockade
High sensitivity (0.063% false negatives)
High negative predictive value (99.93%)

SSEP Monitoring: Cons
Requires time-consuming averaging of signals (delayed results)
Monitors only the dorsal columns (compromise of other parallel sensory pathways, e.g., anterior spinothalamic tracts, can go undetected)
Limited/no recording in the presence of peripheral neuropathies
Very sensitive to electrical noise (requires amplification of the signal:noise ratio)

Sensory axon (peripheral nerve)
 Dorsal root ganglion

 Dorsal columns (gracile fasciculus, cuneate fasciculus)
 Medulla (gracile nucleus, cuneate nucleus)
 Cross-over in medulla to form medial lemniscus
 Thalamus (VPL nucleus)
Primary somatosensory cortex

Dorsal columns/lemniscal system are the major structures monitored by SSEPs

SSEP Monitoring: Upper Extremities
Median nerve (C6-T1)
Ulnar nerve (C8-T1)  preferred for assessing lower cervical spinal cord because median nerve stimulation may remain unchanged in the presence of C7-C8 dorsal column injury

SSEP Monitoring: Lower Extremities
Common peroneal nerve
Posterior tibial nerve: advantages  larger signal, less variable, less patient movement produced, ankle usually more accessible than knee; disadvantages  difficult to stimulate (especially in the setting of peripheral neuropathy where long nerves are more affected)

SSEP: Technical Considerations
Monophasic square pulses, 0.1-0.3 msec, constant current stimulation
Stimulus intensity high enough to produce consistent muscle twitch
Stimulus rate: 3-6 per second, avoid exact harmonic rates of 60 Hz (e.g., 5/sec will contaminate averaged SSEPs with artifact from 60 Hz line frequency every 12 stimulations)
Amplification of signal (x100,000)
Filtering of signal (30-3000Hz): there are small amplitude components that are very high and very low frequency; filtering too wide a bandwidth noisy SSEPs; filtering too narrow attenuate either high or low frequency components and distort SSEPs
Conversion of signal: analog digital
Averaging of signal (SSEPs not visible independently, averaging extracts them from other “noise” [ECG, EMG, EEG])
Polarity relative, no uniform convention

SSEP: Measured Parameters
Peak latencies (easiest to measure)
Interpeak latencies (delineate pathology along pathways)

SSEP: Recording Sites
Cortical: international 10-20 system
CZ-FZ: lower extremities on sensory homunculus
C3-FZ: right upper extremity on sensory homunculus
C4-FZ: left upper extremity on sensory homunculus
We often use C3’ (1 cm behind C3), C4’ (1 cm behind C4), and CZ’ (1 cm behind CZ) for more robust signals
Spine: cervical/thoracic/lumbar
Upper extremities: Erbs point (angle between clavicular head of SCM and clavicle)
Lower extremities: iliac crest

SSEP: Typical Upper Extremity Montage (Median Nerve)
Channel 4: Cortex
Channel 3: Cortex
Channel 2: Cervical spine (C5)
Channel 1: Erbs point

SSEP: Typical Lower Extremity Montage (Posterior Tibial Nerve)
Channel 4: Cortex
Channel 3: Cortex
Channel 2: Cervical spine (C5)
Channel 1: Iliac crest

SSEP: Spinal Components
Smaller amplitude than cortical signals (cervical > thoracolumbar), but more resistant to depression by anesthetics
Latency difference between extremity/cervical signals and cortical signals measures central sensory conduction (referred to as “central conduction time”)

SSEP: Terminology
Components are named by their polarity and typical peak latency
Example: N20 (negative deflection at 20 msec post-stimulus)
Normal latencies (peripheral and central) vary with patient stature
Polarity is relative
Example: N20 (median nerve) can be read as P20 if stimulating and recording electrodes are reversed

SSEP: Median Nerve (N20)
N9 (Erbs point), N13 (cervical spine), P14, N18, N20 (cortical)
Routinely measured interpeak latencies: N9-N20, N9-P14 (brachial plexus  lower brain stem), P14-N20 (lower brain stem  primary somatosensory cortex)

SLP [stationary lumbar potential] (iliac crest), P31 (cervical spine), P37 (cortical)
Routinely measured interpeak latencies: SLP-P37 (lumbar spinal cord  primary somatosensory cortex), SLP-P31 (lumbar spinal cord  brain stem), P31-P37 (brain stem  primary somatosensory cortex)

Sensory Homunculus
Let's review some neuroscience!
SSEP: Pro vs Con
SSEP: Basics
SSEP: Lower Extremity
SSEP: Upper Extremity
Content by: John Bebawy
Prezi by: Andrew Park

Other evoked potentials to come later!
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