Somatosensory Evoked Potentials/SEP
The Somatosensory Evoked Potential (SEP) is mediated by the Dorsal Column Medial Lemniscal Pathway and can be elicited from a number of locations. for upper extremity, the ulnar nerve which enters into the spine and a more caudal level and more sensitive to positional issues of brachial plexus to prevent palsy and can be verified from lower parts of the cervical spinal cord. the median nerve gives a more robust signal but enters the spinal cord at a higher level and isn't best to catch positional problems. the radial nerve, i have never stimulated here but perhaps can try someday. Can also half step and stimulate at the olecranon fossa, where the nerve is exposed and at the armpit, directly stimulating the brachial plexus. Lets talk a bit about the brachial plexus and what it looks like entangling all of the nerves which then traverse up the brain. lower limb can be elicited from the posterior tibial nerve and popliteal fossa. Goal is to preserve functional integrity
two needles, anode and cathode are placed in a specific location.
these are small signals, and the nervous system is inherently noisy, especially with activation of muscles that can compromise recordings (muscle paralysis assists in cleaner SEPs) averaging is required, typically stimlate approximately 25mA and average over 50 trials, with a stimulus interval of 3.11, etcetera. stimulate a peripheral nerve caudal to the site of surgery in between the site of surgery and the cortex with the lesion in the middle to monitor pathway. Usually used for four extremities so can tell the pathways function in total and recording at the scalp using the 10-20 system.
The SEP is sensitive to spinal compression and ischemia? allows monitoring of posterior column function during surgery, there is a separate blood supply, sensitive to cortical and thalamic ischemia, is observed during carotid endarterectomies if there isn't enough blood profusion to the cortex. monitors cerebral profusion.
Median Nerve - C5-T1** nerve root innervations, Ulnar Nerve C8-T1** this is typically below most cervical surgeries, can compound with posterior tibial nerve which is at the floor of the nervous system, the 2m pathway from the big toe to the medulla.
The SEP can also be used to map the location of the central sulcus by looking for a polarity reversal with a cortical strip electrode placed at eloquent cortex. very useful in positiong of unstable spine.
sensitive to anesthesia, can lower amplitude and increase latency, make it difficult to give information. inhalational gases are fat soluable and guess what the dorsal roots and nervous system is composed of, fats!
upper limb ssep, lower limb ssep, erb's point
somatosensory evoked potential response showing p9, p11, p14, n18, n20 and their related structures
place posterior tibial nerve in between ankle and achilles and then near medial malleolus??? approximately 2-3 cm apart. peroneal cathode medial and superior to the fibial head, anode medial to the fibular head
median nerve between the tendons of the palmaris longus and flexor carpi radialis muscle 2cm proximal to the wrist crease
ulnar stimulation cathod medial to the flexor digitorum superficialis tendon, 2cm proimal o the wrist crease anode
talking about neural genearations, P37, N47, P60
N34, P31 Thalamus (upper brainstem)
N9
cortical dipoe orientation results in an electric dipole directed toward the side ipsilateral to stimulation. anterior is positive, posterior is negative. dipole deflected upward toward Cz also have dipole toward ipsilateral side
N20, P23, lateral and contralateral somatosensory
N18, thalamus and upper brainstem
P14 caudal medial lemniscus
N13 recorded from upper cervical spine
N9 brachial plexus
near field vs far field stimulation
warning criteria is 50% amplitude decrease or greater than 10% latency increase
there are non surgical factors like anethesia from gases, boluses, buildup blood pressure, temperature, ischemia
remain vigilant! have regular inervals of stimulation and recording, especially at key points in surgery! set to manual at every five minutes, during ctitical moments set to automatic and get it in as many trials as possible.
can use erb's point as a measurement tool for proper stimulation if amplitude doesn't keep increasing. look for the potential and look for the stimulation of the wrist moving, can correlate if your sides are direct from emg feedback.
verify that there are no technical changes, may see systemic changes, changes from surgical manipulations and techincal stuff. especially if the patient has cervical myelopathy, can see positional changes, improved signals from decompression can help the surgeon alighn the head for trauma. be really careful for inhalational agents for myelopathic patients and epidural abscesses, tumors, etcetera. distractors, clamping of the carotid artery, etc aneurysms etcetera
two needles, anode and cathode are placed in a specific location.
these are small signals, and the nervous system is inherently noisy, especially with activation of muscles that can compromise recordings (muscle paralysis assists in cleaner SEPs) averaging is required, typically stimlate approximately 25mA and average over 50 trials, with a stimulus interval of 3.11, etcetera. stimulate a peripheral nerve caudal to the site of surgery in between the site of surgery and the cortex with the lesion in the middle to monitor pathway. Usually used for four extremities so can tell the pathways function in total and recording at the scalp using the 10-20 system.
The SEP is sensitive to spinal compression and ischemia? allows monitoring of posterior column function during surgery, there is a separate blood supply, sensitive to cortical and thalamic ischemia, is observed during carotid endarterectomies if there isn't enough blood profusion to the cortex. monitors cerebral profusion.
Median Nerve - C5-T1** nerve root innervations, Ulnar Nerve C8-T1** this is typically below most cervical surgeries, can compound with posterior tibial nerve which is at the floor of the nervous system, the 2m pathway from the big toe to the medulla.
The SEP can also be used to map the location of the central sulcus by looking for a polarity reversal with a cortical strip electrode placed at eloquent cortex. very useful in positiong of unstable spine.
sensitive to anesthesia, can lower amplitude and increase latency, make it difficult to give information. inhalational gases are fat soluable and guess what the dorsal roots and nervous system is composed of, fats!
upper limb ssep, lower limb ssep, erb's point
somatosensory evoked potential response showing p9, p11, p14, n18, n20 and their related structures
place posterior tibial nerve in between ankle and achilles and then near medial malleolus??? approximately 2-3 cm apart. peroneal cathode medial and superior to the fibial head, anode medial to the fibular head
median nerve between the tendons of the palmaris longus and flexor carpi radialis muscle 2cm proximal to the wrist crease
ulnar stimulation cathod medial to the flexor digitorum superficialis tendon, 2cm proimal o the wrist crease anode
talking about neural genearations, P37, N47, P60
N34, P31 Thalamus (upper brainstem)
N9
cortical dipoe orientation results in an electric dipole directed toward the side ipsilateral to stimulation. anterior is positive, posterior is negative. dipole deflected upward toward Cz also have dipole toward ipsilateral side
N20, P23, lateral and contralateral somatosensory
N18, thalamus and upper brainstem
P14 caudal medial lemniscus
N13 recorded from upper cervical spine
N9 brachial plexus
near field vs far field stimulation
warning criteria is 50% amplitude decrease or greater than 10% latency increase
there are non surgical factors like anethesia from gases, boluses, buildup blood pressure, temperature, ischemia
remain vigilant! have regular inervals of stimulation and recording, especially at key points in surgery! set to manual at every five minutes, during ctitical moments set to automatic and get it in as many trials as possible.
can use erb's point as a measurement tool for proper stimulation if amplitude doesn't keep increasing. look for the potential and look for the stimulation of the wrist moving, can correlate if your sides are direct from emg feedback.
verify that there are no technical changes, may see systemic changes, changes from surgical manipulations and techincal stuff. especially if the patient has cervical myelopathy, can see positional changes, improved signals from decompression can help the surgeon alighn the head for trauma. be really careful for inhalational agents for myelopathic patients and epidural abscesses, tumors, etcetera. distractors, clamping of the carotid artery, etc aneurysms etcetera
Transmission; generated by a nerve or tract as an action potential is propagated past the recording site, latency is dependent on the distance from site of stimulation to site of recording
bipolar vs referential recordings
bipolar recording electrodes are relatively closely spaced specific far field responses common mode rejected, near field responses recorded referential recording electrodes are spaced relatively far apart sensitive both far-field and near field responses are recorded, less common mode rejection
note that p31 and n34 are absent in the non-cephalic recording, widespread scalp potential
present in the not cephalic to cephalic recordings referential recording
absent in the cephalic to cephalic recordings bipolar recording
anesthetic effects
effects of inhalational agents on ssep responses; cause a dose-dependent decrease in cortical amplitudes and increase cortical latencies
@ 1/2 MAC as increase gases there is a dose dependent latency and amplitude reduction
the effects of nitrous oxide on cortical amplitude amplification with etomidate administration; shows decreasing amplitude with time
Effects of Midazolam injection on SSEP cortical waveforms
warning criteria; greater than 50% amplitude decrease, greater than 10% latency increase
non surgical factors effecting Evoked Potentials; anesthetic changes - cortical waveforms will change in response to increases in the gases boluses and changes in infusion rates or concentrations, BP changes, temperature changes, limb ischemia
When to expect changes - always, you should always remain vigilant when monitoring SSEP responses as changes could occur at any time for numerous reasons, there are 3 types of changes that we may expect to see, systemic, surgical manipulations, technical
periods during procedure to expect changes; intubation especially if the patient has cervical myelopathy, positioning, decompression, placement of distractors, placement of instrumentation, clamping of carotid artery
*LH* round 2
Effects of midazolam (versed) injection on SSEP cortical waveforms; decrease amplitude over time
Change criteria; greater than 50% amplitude decrease; greater than 10% latency increase
Non-surgical factors effecting Evoked potentials; anesthetic changes; cortical waveforms will change in response to increases in the gases, boluses and changes in infusion rates or concentrations, blood pressure changes, temperature changes, limb ischemia
When to expect changes, always you should always remail vigilant when monitoring SSEP responses as changes could occur at any time for numerous reasons. There are 3 types of changes that we may expect to see; systemic; surgical manipulations, technical
Periods during procedure to expect changes; intubation, especially if the patient has cervical myelopathy, positiioning, decompression, placement of distractors, placement of instrumentation, clamping of carotid artery
Surgical Procedures where SSEPs are indicated; cervical and thoracic decompressions and fusions, carotid endarterectomis, brainstem procedures, cerebral and aortic aneurysms, lumbar fusions
Effects of midazolam (versed) injection on SSEP cortical waveforms; decrease amplitude over time
Change criteria; greater than 50% amplitude decrease; greater than 10% latency increase
Non-surgical factors effecting Evoked potentials; anesthetic changes; cortical waveforms will change in response to increases in the gases, boluses and changes in infusion rates or concentrations, blood pressure changes, temperature changes, limb ischemia
When to expect changes, always you should always remail vigilant when monitoring SSEP responses as changes could occur at any time for numerous reasons. There are 3 types of changes that we may expect to see; systemic; surgical manipulations, technical
Periods during procedure to expect changes; intubation, especially if the patient has cervical myelopathy, positiioning, decompression, placement of distractors, placement of instrumentation, clamping of carotid artery
Surgical Procedures where SSEPs are indicated; cervical and thoracic decompressions and fusions, carotid endarterectomis, brainstem procedures, cerebral and aortic aneurysms, lumbar fusions
SEP Runoff
dorsal column / medial lemniscus / thalamus / primary sensory cortex
stimulate the peripheral nerve, record at multiple levels (brachial plexus, medulla, cortex)
small signals, signal averaging required
sensitive to compression of the spinal cord but not spinal ischemia
monitors posterior posterior column function in spine surgery
sensitive to cortical and thalamic ischemia
monitors cerebral perfusion in vascular surgery
can use to map the location of the central sulcus with surface electrodes
useful in positioning of an unstable cervical spine
most common agents lower amplitude and increase latency (the same effects as compromise of neural pathways)
*LH*
Somatosensory Evoked Potentials
Pathways Waveforms and Generators
Goal of SSEP monitoring is during surgery to preserve functional integrity of the peripheral nerve monitored, spinal cord, brain stem and brain (cortical and subcortical structures) to detect injury early before neural damage becomes permanent
how is this accomplished; by stimulating a peripheral nerve distal (caudal) to the surgical site and by recording evoked potentials from the cortical and subcortical structures proximal to the surgical site with the assumption that the spinal cord or other structure at risk must be functioning properly if recording of caudal sensory stimulation can occur caveat intact SSEP responses do not necessarily indicate intact motor pathways
Nerves generally monitored Upper Extremities Median Nerve C5-T1 nerve root innervations; Ulnar nerve C8-T1 nerve root innervations C7-T1 in some patients
Tibial and peroneal pathways; peripheral nerve enters the spinal cord through the dorsal roots and ascends to the brainstem via the fasciculus gracilis, enter the medulla and synapse at the nucleus gracilis, fibers decussate and continue superiorly through the medial lemniscus and synapse a second time in the thalamus and radiate to the medial somatosensory area
inhalation gases affect nucleus synapse sites
tibial and peroneal stimulation locations; tibial cathode posterior to medial malleolus, anode 2-3 cm distal to cathode; peroneal cathode medial and superior to the fibular head, anode medial and inferior to the fibular head
median and ulnar pathways; peripheral nerve enters the spinal cord through the dorsal roots, ascends to the brainstem via the fasciuclus cuneaus, enters the medulla and synapses at the nucleus cuneatus, fibers decuassate and continue superiorly through the medial lemniscus, synapse a second time in the thalamus, radiate to somatosensory cortex
median and ulnar stimulation locations; median cathode between the tendons of the palmaris longus and flexor carpi radialis muscle 2 cm proximal to the wrist crease anode between the tendons of the palmaris longus and flexor carpi radialis muscle at the wrist crease
ulnar cathode medial to the flexor digitorum superficialis tendon, 2cm proximal to the wrist crease anode medial to the flexor digitorum superficialis tendon at the wrist crease
Tibial peaks and generators; CPi-CpC, Fz-CPz; P37, N47, P60; medial contralateral somatosensory cortex, near field recording
Fz-CS5; N34, P31, Thalamus (upper brainstem) far-field scalp recording; caudal medial lemniscus, far-field scalp recording
IC-T12; LP; Post-synaptic spinal cord response, near-field stationary response
PFp-PFd;N9;CNAP recorded from tibial nerve, near-field transmission response
Cortical dipole orientation results in an electric dipole directed toward the side ipsilateral to stimulation
Tibial Nerve Cortical Dipoles
Cortical dipole orientation results in an electric dipole directed toward the side ipsilateral to stimulation; paradoxical lateralization; dipole deflected upward toward CZ also have dipole deflected toward ipsilateral side
Median Peaks and Generators
Cpi-CPc; N20, P23; Lateral and contralateral somatosensory cortex, near-field
Fz-CS5; N18; Thalamus/Upper Brain-stem, far-field scalp recording. P14; caudal medial lemniscus, far-field scalp recording. N13; recorded from upper cervical spine - nucleus cuneatus recorded from lower cervical spine - post-syaptic spinal recording, near-field stationary recording
EpC-EPi; N9; brachial plexus, near-field transmission recording
cortical dipole orientation results in an electric dipose directed toward the side contralateral to stimuation
Near field vs far field recordings
near field - electrode recording evoked potential is near the source of generation
Far field electrode recording evoked potential is far from the source of generation
stationary vs transmission responses
stationary generated by a specificc anatomical structure at a fixed location latency is fixed; nerve synapse, change in direction of nerve, change is media through which the nerve travels
dorsal column / medial lemniscus / thalamus / primary sensory cortex
stimulate the peripheral nerve, record at multiple levels (brachial plexus, medulla, cortex)
small signals, signal averaging required
sensitive to compression of the spinal cord but not spinal ischemia
monitors posterior posterior column function in spine surgery
sensitive to cortical and thalamic ischemia
monitors cerebral perfusion in vascular surgery
can use to map the location of the central sulcus with surface electrodes
useful in positioning of an unstable cervical spine
most common agents lower amplitude and increase latency (the same effects as compromise of neural pathways)
*LH*
Somatosensory Evoked Potentials
Pathways Waveforms and Generators
Goal of SSEP monitoring is during surgery to preserve functional integrity of the peripheral nerve monitored, spinal cord, brain stem and brain (cortical and subcortical structures) to detect injury early before neural damage becomes permanent
how is this accomplished; by stimulating a peripheral nerve distal (caudal) to the surgical site and by recording evoked potentials from the cortical and subcortical structures proximal to the surgical site with the assumption that the spinal cord or other structure at risk must be functioning properly if recording of caudal sensory stimulation can occur caveat intact SSEP responses do not necessarily indicate intact motor pathways
Nerves generally monitored Upper Extremities Median Nerve C5-T1 nerve root innervations; Ulnar nerve C8-T1 nerve root innervations C7-T1 in some patients
Tibial and peroneal pathways; peripheral nerve enters the spinal cord through the dorsal roots and ascends to the brainstem via the fasciculus gracilis, enter the medulla and synapse at the nucleus gracilis, fibers decussate and continue superiorly through the medial lemniscus and synapse a second time in the thalamus and radiate to the medial somatosensory area
inhalation gases affect nucleus synapse sites
tibial and peroneal stimulation locations; tibial cathode posterior to medial malleolus, anode 2-3 cm distal to cathode; peroneal cathode medial and superior to the fibular head, anode medial and inferior to the fibular head
median and ulnar pathways; peripheral nerve enters the spinal cord through the dorsal roots, ascends to the brainstem via the fasciuclus cuneaus, enters the medulla and synapses at the nucleus cuneatus, fibers decuassate and continue superiorly through the medial lemniscus, synapse a second time in the thalamus, radiate to somatosensory cortex
median and ulnar stimulation locations; median cathode between the tendons of the palmaris longus and flexor carpi radialis muscle 2 cm proximal to the wrist crease anode between the tendons of the palmaris longus and flexor carpi radialis muscle at the wrist crease
ulnar cathode medial to the flexor digitorum superficialis tendon, 2cm proximal to the wrist crease anode medial to the flexor digitorum superficialis tendon at the wrist crease
Tibial peaks and generators; CPi-CpC, Fz-CPz; P37, N47, P60; medial contralateral somatosensory cortex, near field recording
Fz-CS5; N34, P31, Thalamus (upper brainstem) far-field scalp recording; caudal medial lemniscus, far-field scalp recording
IC-T12; LP; Post-synaptic spinal cord response, near-field stationary response
PFp-PFd;N9;CNAP recorded from tibial nerve, near-field transmission response
Cortical dipole orientation results in an electric dipole directed toward the side ipsilateral to stimulation
Tibial Nerve Cortical Dipoles
Cortical dipole orientation results in an electric dipole directed toward the side ipsilateral to stimulation; paradoxical lateralization; dipole deflected upward toward CZ also have dipole deflected toward ipsilateral side
Median Peaks and Generators
Cpi-CPc; N20, P23; Lateral and contralateral somatosensory cortex, near-field
Fz-CS5; N18; Thalamus/Upper Brain-stem, far-field scalp recording. P14; caudal medial lemniscus, far-field scalp recording. N13; recorded from upper cervical spine - nucleus cuneatus recorded from lower cervical spine - post-syaptic spinal recording, near-field stationary recording
EpC-EPi; N9; brachial plexus, near-field transmission recording
cortical dipole orientation results in an electric dipose directed toward the side contralateral to stimuation
Near field vs far field recordings
near field - electrode recording evoked potential is near the source of generation
Far field electrode recording evoked potential is far from the source of generation
stationary vs transmission responses
stationary generated by a specificc anatomical structure at a fixed location latency is fixed; nerve synapse, change in direction of nerve, change is media through which the nerve travels