SSEP: Somatosensory Evoked Potentials

SSEPs give us info about:

proprioception and vibration; electrical activity of brain in response to touch

SSEPs are use to monitor:

- patient's extremity for positional effects

- dorsal column

- brainstem & cerebral cortex

left side stimulation sends signals to which part of the brain?

right

3 synapses:

1st order

2nd order

3rd order

1) nucleus cuneatus or nucelus gracillis in medulla

2) thalamus

3) SS cortex

peripheral nerve stimulation sends signals from the __ extremity to the __ and into the __

distal; dorsal root ganglion; dorsal white column

ipsilateral dorsal white column includes:

fasciculus cuneatus & fasciculus gracillis

fasciculus cuneatus

upper extremity sensory signals

T6 and above

LATERAL distribution of dorsal column

fasciculus gracillis

lower extremity signals

L2 & up

MEDIAL distribution of dorsal column

SSEPs use __ stimulation

cathodal = negatively charged anions flow from the cathode back to the anode

cathode

negatively charged; attracts cations

depolarization

PROXIMAL

anode

positively charged; attracts anions

hyperpolarization

DISTAL

anodal blocking

blocking of nerves caused by a hyperpolarization under the anode electrode

SSEP stimulation units are in:

miliAmps (mA)

SSEPs use __ current

constant

why are the cathode & anode placed 2-3cm apart?

too close = SMALL amp/amp cancels out

too far = LARGE stimulus effect

ohm's law

V=IR

voltage

amount of electrical force needed to drive a current between 2 points

resistance

measure of opposition to the flow of electric current

increased distance between anode & cathode =?

increases resistance to current flow

high voltage =?

large stimulation effect

stimulation sites for upper SSEP

ulnar & median

ulnar:

cathode = medial to palmaris longus

anode = dorsum of the wrist

median:

cathode = between tendons of palmaris longus & flexor carpi radialis

anode = dorsum of the wrist

stimulation sites for lower SSEP

PTN & PN

PTN:

cathode = midway between medial mallelous & achilles tendon

anode = distal to cathode

PN:

cathode = below fibular head

anode = distal to cathode

saphenous nerve is typically monitored in __ cases

XLIF/DLIF

unofficial alternate site for lower SSEP

pop fossa

if an insult occurs, the recorded responses BELOW the insult WILL/WILL NOT be affected?

will not

if an insult occurs the recorded responses ABOVE the insult WILL/WILL NOT be affected?

will

surgeon must stimulate __ & record __

below; above

cervical cases monitor __

upper and lower SSEP

thoracic/lumbar cases monitor __

lower SSEP

global changes

see changes in ALL SSEPs

near-field

generator is close to the recording electrodes

types of near-field responses

cortical and peripheral

cortical near-field response

CPZ, CP3, CP4 is placed near SS cortex

peripheral near-field response

Erb's point & pop fossa are placed near brachial plexus & sciatic nerve

far-field

generator is far from recording electrodes

types of far-field responses

subcortical

subcortical response

FPZ is FAR from generator (brainstem --> CML, BT)

stationary potential

generated in grey matter

amp CHANGES = latency SAME

types of stationary potentials

cervical & lumbar b/c they are recorded from spinal cord

propagating potential

generated in white matter

latency CHANGES = amp SAME

types of propagating potentials

erb's point & pop fossa

3 SSEP responses recorded intraoperatively:

1) peripheral

2) subcortical

3) cortical

upper SSEP stimulation peripheral response (nearfield/propagated)

erb's point over brachial plexus

lower SSEP stimulation peripheral response (nearfield/propagated)

pop fossa over sciatic nerve

waveform characteristics include:

1) morphology (shape)

2) latency (time)

3) amplitude (height)

latency is measured in

milliseconds

amplitude is measured in

microvolts

"N" represents:

peaks; negative polarity

"P" represents:

troughs; positive polarity

upper SSEP peak values

N9 (erb's = propagated)

N13 (stationary cervical potential)

P14 = far field subcortical --> CML

N18 = far field subcortical --> brainstem/thalamus

N20 = cortical (SS cortex)

lower SSEP peak values

N9 = pop fossa propagated

LP = stationary lumbar potential

P31 = far-field subcortical --> CML

N34 = far-field subcortical --> brainstem/thalamus

P37 = cortical (SS cortex)

do upper or lower extremity responses have high latencies? why?

lower b/c of distance it takes for the stimulus to travel

analogous peaks

share same generator

subcortical upper & lower analogous peaks

P14 & P31 = CML

N18 & N34 = brainstem/thalamic

spine potential upper & lower analogous peaks

N13 & N22 (AKA LP) = spinal cord

alternate sites for erb's point

sub-clavicular

axillary crease

posterior trapezius muscle

subcortical responses are recorded from the __ electrode

FPZ

inactive cephalic electrode

FPZ b/c there is nothing DIRECTLY underneath it though the generator (brainstem) is farfield

reference electrodes when obtaining a subcortical response

mastoid (M1/M2) or the C5S

cortical SSEPs are recorded from the __

SS cortex

recording electrodes on the SS cortex

CPZ

CP3

CP4

upper extremity cortical SSEPs are recorded from

CP3 (right) or CP4 (left) --> lateral portion of brain

lower extremity cortical SSEPs are recorded from

CPZ (left and right) --> midline of brain

ground electrode

place between stimulating & first recording electrode

1 ground PER amplifier ! !

active cortical electrodes

CPZ

CP3

CP4

can FPZ be used as an active electrode

yes but for ONLY obtaining farfield cortical response

where are SSEP recording electrodes placed

scalp

montage

made up of several recording electrodes

cephalic electrodes

FPZ

CPZ

CP3

CP4

non-cephalic electrodes

M1 (or C5S)

subcortical signals are received by ALL __ electrodes

cephalic

common mode rejection

same signals received by 2 electrodes

1) takes out subcortical response

2) takes out noise

cortical responses are recorded by referencing a __ electrode to an __ __ electrode

cephalic; inactive cephalic

cephalic-cephalic channel

cortical responses ONLY

inactive cephalic-noncephalic channel

subcortcial responses ONLY (EX: FPz-M1 or M1-FPz)

what does switching between the active and reference electrode do to the waveform

inverts it

active cephalic-noncephalic

cortical and subcortical response

examples of noncephalic electrodes

erb's, pop fossa

the dorsal column pathway is perfused by the __

posterior spinal arteries

blood supply for cortical responses

upper = MCA

lower = ACA

blood supply for subcortical responses

vertebrobasilar

blood supply for peripheral responses

upper = subclavian

lower = popliteal

auto-regulation is effective if its __

BELOW 60 mmHg

upper SSEPs cortical reponse primary & secondary channels

primary: CPc - CPi

secondary: CPz-FPz

lower SSEPs cortical reponse primary & secondary channels

primary: CPz-FPz

secondary: CPi-CPc

paradoxical lateralization

obtaining a better response from the ipsilateral side in lower SSEPs

upper SSEP obligate peaks

P14

N18

lower SSEP obligate peaks

P31

N34

recording montages for upper SSEP

1) cortical: CPc-CPi

2) cortical: CPz-FPz

3) subcortical: M1-FPz

recording montages lower SSEP

1) cortical: CPi-CPc

2) cortical: CPz-FPz

3) subcortical: M1-FPz

4) peripheral: PFd-PFp

filters

reduce noise & tell the computer what frequency range to look for signals

high frequency filters (HFF)

attenuate signals ABOVE set frequency

low frequency filters (LFF)

attenuate signals BELOW set frequency

band pass

pass a band of frequencies while attenuating ALL frequencies outside the band

combination of LOW frequency & HIGH frequency filters

high pass filters

filter out low frequencies & allow HIGH frequencies to pass

low pass filters

filter out high frequencies & allow LOW frequencies to pass

when are you supposed to change filters?

before setting baselines

changing filters during a surgery can cause:

phase lag or phase lead

phase lag

increase latency (comes later); changing high frequency filters

phase lead

decrease latency (comes in earlier); changing low frequency filters