SSEPs Flashcards
Name the muscle activated with stim of the MN at the wrist
Thenar muscle group : Abductor Pollicis Brevis (APB)
Name the muscle activated with stim of the UN at the wrist
Hypothenar group : Abductor Digiti Minimi (ADM)
Define Orthodromic
The direction a signal normally travels
e.g. SSEPs afferent (up), tcMEPs efferent (down)
Define Antidromic
The direction opposite to which a signal normally travels
What happens when Stimulus Intensity is reduced?
Amplitude decreases
Most pronounced in Erb’s Point and Pop Fossa (peripheral responses)
What happens when Patient’s body temperature drops?
Latency increases, amplitude stays the same
What happens to an the Erb’s Point response during ischemia (positional, BP cuff, weighting of the head, taping of the shoulders)?
Both latency and amplitude decrease
What is the function of Erb’s Point/Pop Fossa according to the guidelines?
To verify adequate stimulation
What type of montage is best for recording subcortical or far field SSEP responses?
Referential
What type of montage is best for recording SSEP cortical responses?
Bipolar
Which peak is most susceptible to anesthesia effects?
N20
What time does EP to N20 define?
the conduction time between the brachial plexus and the primary sensory cortex
(aka central conduction)
What time does the EP latency define?
Peripheral conduction
What does EP to P14 define?
The conduction time between the brachial plexus and the lower brainstem
(central conduction)
What does P14 to N20 define?
The conduction time between the lower brainstem and the cortex
(central conduction)
Define Lumbar Potential
Stationary/non-propagated potential reflecting mainly postsynaptic activity in the lumbar cord from internuncial neurons (relay neurons)
What metal is best for Epidural Spine electrodes?
Platinum iridium-guards against neurotoxicity
Alternate electrodes are stainless steel or platinum
What is the most common technical problem with epidural electrodes?
Shunting from fluid over the electrode
What type of stimulation is best for SSEPs?
Constant current
What does an increase in stimulus intensity do to the SSEP?
Does not change latency, increases amplitude to a maximum
What is asynchronous stimulation?
Stimulation with delay
Define false positive data
significant changes in data, but no deficits post-operatively
Define false negative data
No changes in data, but patient has post-operative deficits
During upper extremity SSEP recording, the Erb’s Point electrode records data from where?
Brachial Plexus
Hypothermia of the lower extremities will result in
prolonged peripheral conduction time
What’s the most common injury during an ALIF?
left iliac artery injury
What are the dorsal column tracts for Lower Extremity SSEPs?
Fasciculus Gracilis, Nucleus Gracilis
Think “G” for Ground (Which is LOW)
What are the dorsal column tracts for Upper Extremity SSEPs?
Fasciculus Cuneatus, Nucleus Cuneatus
Think “C” for Clouds (which are UP)
What is the lumbar potential and which artery supplies the area?
Lower Extremity SSEP Obligate Waveform
(not commonly recorded in OR)
Stationary, post-synaptic potential generated by the Spinal Cord, Near Field Potential
Supplied by the Artery of Adamkiewicz
What is N34 and which artery supplies the area?
Lower Extremity SSEP Obligate Waveform
Subcortical Response, generated by the Brainstem/Thalamus, Far Field Potential
Supplied by Basilar Artery
What is the P37 and which artery supplies the area?
Lower Extremity SSEP Obligate Waveform
Cortical Response generated by the Somatosensory Cortex
Supplied by Anterior Cerebral Artery (ACA)
What is the N9 and which artery supplies the area?
Upper Extremity SSEP Obligate Waveform
Peripheral Response generated by Brachial Plexus, Near Field Response
Supplied by Subclavian Artery
What is the N13 and which artery supplies the area?
Upper Extremity SSEP Obligate Waveform
Stationary, post-synaptic potential generated by Upper Cervical Spinal Cord
Recorded from dorsal neck, Near Field Potential
Supplied by Posterior Spinal Arteries
What is the N18 and which artery supplies the area?
Upper Extremity SSEP Obligate Waveform
Subcortical Response, Far Field response generated by Brainstem/Thalamus
Supplied by Basilar Artery
What is the N20 and which artery supplies the area?
Upper Extremity SSEP Obligate Waveform
Cortical response, Near Field Potential generated by the Somatosensory Cortex
Supplied by Middle Cerebral Artery (MCA)
What does “subcortical” mean?
SSEP response generated by the brainstem/thalamus
think “below cortical”
N18 Upper Extremity SSEP
N34 Lower Extremity SSEP
N20 response recorded from CPc-Fpz
Is it a near field or far field potential?
Near field
N34
Near field or far field?
Far field brainstem response
Dermatomal SSEPs follow what pathway?
Dorsal column / Medial Lemniscus pathway
Name the generator for the N18 waveform
Upper Brainstem/Thalamus
Stimulation of what nerve(s) provides the most effective somatosensory monitoring of upper limb
positioning during prone spine surgery?
Ulnar & Median Nerve SSEP combined
What is the anatomical generator of the P20 response recorded directly over the cortex during phase
reversal mapping?
somatosensory cortex
Most appropriate nerve to stimulate for recording robust UE SSEPs during cerebrovascular
procedures?
Median nerve
Which nucleus is the termination of the first order of the SSEP pathway for the upper extremities?
Nucleus Cuneatus
The termination of the second order of the SSEP pathway is in the
Thalamus
Cortical ischemia caused by the Middle cerebral artery (MCA) would cause what SSEP data changes?
Decrease (attenuation) of a unilateral cortical SSEP responses along with stable subcortical waveforms
(median nerve stimulation)
If the surgeons clamps the left Internal Carotid Artery (ICA), which waveform is most likely to attenuate if the patient does not
have adequate collateral perfusion?
Right upper extremity (RUE) SSEP
Injury to the SSEP pathway causes what symptoms?
Numbness, Loss of Vibration Sense, Loss of Conscious Proprioception
Occlusion of which vessel is most likely to correlate with a decrease in left lower extremity cortical
SSEPs?
Right Anterior Cerebral Artery (ACA)
Somatosensory evoked potentials would most likely be analyzed in the clinical diagnosis of which
condition?
- Myasthenia Gravis
- Multiple Sclerosis
- Alzheimer’s Disease
- Hydrocephalus
Multiple Sclerosis
Why? MS is a demyelinating disease. Demyelination causes a decrease (slowing) in the conduction velocity which would result in an INCREASE in the LATENCY of SSEP responses
Generator for N9 Response
Peripheral Nerve
Is the P37 a positive or negative response?
Positive
Generator for N13
Spinal cord
SSEP recordings of P37 and N45 waveforms are likely the result of stimulation to what nerve?
Posterior Tibial Nerve (PTN)
SSEP recordings of N20 and P23 waveforms are likely the result of stimulation to what nerve?
Median or Ulnar Nerves
During a posterior thoracic fusion, the spinal cord is injured and causes swelling. What SSEP response will remain stable despite injury to the cord?
The pop fossa (peripheral) response will remain unchanged following a spinal cord injury
During a posterior thoracic fusion, the spinal cord is injured and causes swelling. What SSEP responses do you expect to observe changes from?
Decrease or loss of subcortical and cortical waveforms.
Alert criteria for SSEP
50% decrease in amplitude
10% increase in latency
65% nitrous oxide will cause the greatest amplitude decrease to what SSEP response?
- N20
- N13
- N9
N20
Ulnar nerve SSEPs can be used to detect ischemia (loss of perfusion) to what structures? (Name 3)
- Limb ischemia
- Spinal Cord ischemia
- Cortical ischemia
What would explain why cortical SSEP amplitudes are often smaller or less robust compared to subcortical SSEP amplitudes?
The thalamocortical projection adds an additional synapse before the signal reaches the
cortex, making the cortical signal more susceptible to anesthesia effects
Following left ulnar nerve stimulation, which active electrode will produce an upward deflection at
approximately 20 milliseconds, when applied to the negative input of the amplifier?
When stimulating a peripheral nerve to elicit an afferent response, the anode should be placed ___ to
the cathode
Anode should be placed distal to the cathode
Erb’s point is valuable in monitoring. Give three reasons.
- verify adequate stimulus
- helps in detection of limb ischemia
- aid in detection of brachial plexus injury
You are obtaining baseline data from a patient with diabetic peripheral neuropathy. Your PTN SSEPs are unreliable. What efforts should you take to try obtaining reliable data?
- Increase pulse width
- Decrease the rep rate below 3Hz
- Stimulate at the pop fossa location
The Surgeon requests relaxant be given during your procedure. What effect will paralytics have on your SSEPs?
May improve artifact, otherwise no effect
During a posterior thoracolumbar fusion, you observe a total loss of bilateral lower extremity SSEP waveforms. tcMEP CMAPs are robust and show no changes. Could a spinal cord injury be present?
Yes, with the posterior approach these SSEP changes may represent spinal cord injury, despite the unchanged tcMEP data.
Think about the approach of surgery and what structures are at risk. In this case, a posterior approach puts the dorsal column at most risk. Dorsal column is where the SSEP pathway is located. It is possible to have a partial spinal cord injury, leaving the anterior portion of the cord intact and fully functional.
The first synapse in the dorsal column/medial lemniscus pathway takes place where?
Medulla
A sensory response recorded at 9ms after ulnar nerve stimulation represents
A peripheral nerve action potential
Increasing number of averages (trials) can help reduce what in the SSEP?
Electrical artifact/noise
- In which type of evoked potential is the scalp position of recording electrodes most critical?
- Electrocochleogram
- BAEPs
- Cognitive evoked potentials
- Lower extremity SSEPs
Lower extremity SSEPs
Think of the homunculus and where lower extremities are located (midline). They have less cortical representation - thus its very important electrode placement is placed correctly.
