sseps

SSEPs

electrical activity in response to touch; give info on proprioception and vibration sensations

What do SSEPs monitor?

- individual extremities for positioning effects

-dorsal column of spinal cord

- Brainstem and cerebral cortex

SSEPpathway

- ascending/afferent

- peripheral stimulation; cortical recording

    - UN or MN and PTN or peroneal N or popliteal fossa

Where are the synapses of the DCML Pathway?

1. nucleus cuneatus/gracillis in MEDULLA OBLONGATA

2. Thalamus

3. Primary somatosensory cortex

Where does decussation/crossover happen?

- Medial lemniscus pathway in thalamus

- Pyramids in thalamus

Where does decussation/crossover happen?

- Medial lemniscus pathway in medulla oblongata

- Pyramids in the medulla-spinal cord junction in medulla oblongata

Fasiculus Cuneatus

• upper extremities

• Enters at T6 and up

• Lateral distr. in dorsal column

Fasciculus Gracilis

• lower extremities

• Medial distr. in dorsal column

What is a neurons resting potential?

-70 my

What substance does neuron contain more and less of?

More potassium inside, more sodium outside

Relative Refractory Period

Period after absolute refractory period where AP reaches hyperpolarization and begins to return back to its resting potential

• doesn’t include period where its back to resting potential

SSEP Stimulation electrodes called..

Anode (+) - where hyperpolarization occurs

Cathode (-) - where depolarization occurs

Cathodal Stimulation

Anions flow from cathode into tissue and back to anode

Absolute Refractory Period

Period from start of AP to return to below threshold

Comes before refractory period

Starts AP therefore NO other AP can be started during this period

How should you place electrodes for cathodal stimulation

Anode > cathode > recording electrode

• cathode is proximal

• Anode is distal

“The black (-) cat goes up the tree”

Anodal Blocking

Anode is more proximal than cathode, causing the blockage of ascending sensory signals from traveling up

SSEP stimulation units

Milliamperes (mA)

What distance should anode and cathode be? What happens if they’re too far or too close?

• 2-3 cm

• Too close = small amplitudes bc current penetration very deep

• Too far = increased in resistance therefore increase in stimulus artifact

Ohm’s Law

• voltage (v) = current (i) x resistance (R)

  • Voltage = electrical force needed to drive electrical current

  • Resistance = opposition to current flow

    • Increased distance = increased resistance to current

  • Current remains constant - constant current stimulation

Factors that increase resistance

• increased distance between anode and cathode

• Flaky/oily skin (skin quality)

• Contact impedance (electrode contact)

Factors that increase voltage

• increased resistance

• Increased current

Polarity Types

1. Normal - cathode is stimulating electrode and is proximal

   1. For SSEPS

2. Reverse - anode and cathode switched at stimulating unit

   1. Now anode stimulates

   2. For TOF

3. Biphasic - switches anode and cathode over and over

   1. Neutralizes voltage at cost of lower amplitude

   2. Decreases stim artifact

Stimulating Ulnar N

• Anode (+): 2-3 cm above wrist

• cathode (-): medial to palmaris longus; 2-3 cm above anode

• Alt site: medial epicondyle of humerus

Stimulating Medial N

• Cathode: 2-3 cm proximal to anode, between palmaris longus and flexor carpi radials (FCR)

• Anode: 2-3 cm from wrist

• Alt site: cubical fossa (inside elbow)

Posterior tibial nerve (PTN) (L4-S3)

• Plantar flexion - abductor Hallicus muscle

• Cathode: between medial malleollus and Achilles tendon

• Anode: 2-3 cm distal to cathode

Peroneal N (L4-S2)

• plantar eversion - peroneus longus and peroneus Brevis

• Cathode - below fibulae head

• Anode - 3 cm distal to cathode

Saphenous N is alt stim site for above the knee amputees

Changes in SSEPs

• Stimulate below surgery

  • Record above stimulation site, around surgical site, and above surgical site

• Monitor upper and lower SSEPs on ALL cases

  • Cervical case: upper and lower SSEPs run through cervical region

  • Thoracic/Lumbar case: lower SSEPs run through surgery pathways; upper SSEPs give you info on positional issues of arms and shoulders

  • Global changes: changes in all SSEPs (upper and lower) indicate high level of anesthesia or significant decrease in BP

Near field

Generator is close to recording electrodes

• Cortical responses: recording electrodes (CPz, CP3, CP4) placed over/near generator (somatosensory cortex)

• Peripheral responses: Erbs point (upper SSEP) and pop fossa (lower SSEP) placed over/near generators (brachial plexus-erbs and sciatic nerve-pop fossa)

Far field

Generator far from recording electrodes

• recording electrodes (FPz) places far from generator (Brainstem - caudal medial lemniscus and thalamus)

Stationary Potential

• not seen Intraoperatively @ MPH

• From gray matter

• Amplitude changes, latency constant

Propagating potential

• from white matter

• Latency changes, amplitude constant

Where do peripheral responses come from? What waveform latency do they produce?

• Erbs and Popliteal fossa are peripheral responses

• Their stimulating points are brachial plexus ad tibial/sciatic N

• They produce N9 latency waveforms

• They are near-field and propagated

Where do subcortical responses come from? What waveform do they produce?

• caudal medial lemniscus (CML) and Brainstem-thalamic junction are subcortical responses

• They receive responses from erbs and politeal fossa

  • CML and Brainstem-thalamic junction produce P14-N18 latency waveform

• They are Fairfield

Waveform Characteristics

• Morphology: shape of wave

  • Peaks - negative polarity (N)

  • Trough - positive polarity (P)

• Latency: time

  • Milliseconds (ms)

  • Waveform number is expected latency AFTER stimulation

• Amplitude: height

  • Microvolts (uV)

Waveform for UPPER stimulation

Peripheral: N9

Cervical: N13

Subcortical: P14-N18

Cortical: N20

Analogous peaks

Share same generator

Waveform for LOWER stimulation

Peripheral: N9

Lumbar: N22

Subcortical: P31-N34

Cortical: P37

Latency is higher because the lower limbs are more distal to the SScortex than the upper limbs

Peripheral responses upper electrode position

• erbs point: @ clavicular notch (2cm above middle of clavicle)

• 2 electrodes:

  • Red/+/anode on left erb

  • Black/-/cathode on right erb

• Alt sites:

  • Sub-clavicular

  • Axillary crease

  • Posterior trap muscle

Peripheral responses lower electrode position

• popliteal fossa: behind knee

• 2 electrodes:

  • Cathode 5cm above knee crease

  • Anode 2cm above knee crease

Inactive cephalic electrodes in subcortical responses

Anything that is NOT a cephalic site

Active cephalic electrodes in subcortical responses

Anything in cephalic region

Where do cortical responses come from? What waveform do they produce?

• Primary somatosensory cortex (based on stimulated limb) are cortical responses

• They receive responses fromcaudal medial lemniscus (CML) and Brainstem-thalamic junction

• Primary somatosensory cortex produces N20 latency waveform

• They are nearfield and stationary

Cortical Electrode Locations

• upper extremity SSEPs - CP3 or CP4

  • Since more lateral

• Lower extremity SSEPs -CPz

  • Since more mediaL

Inactive cephalic electrodes

FPz

Non-cephalic electrodes

• Not on scalp

• Called references

• Ex. Mastoid

Channel logic rules

• somatosensory cortical potentials - active electrode must be active cephalic electrode

• Isolated subcortical potential - active electrode must be inactive cephalic electrode

• Channels must be created w intentional of isolating a single desired potential in each respective channel

Common mode rejection

Same signals received by two electrode cancel out

Cortical SSEP blood supply

MCA

ACA

Subcortical SSEP blood supply

Vertebrobasilar artery

SSEP peripheral blood supply

Erbs - subclavian artery

Politeal fossa - popliteal artery

Paradoxical Lateralization

When CPz doesn’t give a good response but ipssilateral side to stimulated side give better response

High frequency filters (HFF)

Diminish signals above set frequency

• low pass filters - allow lower freq to pass; cuts out high freq

Low freq filters (LFF)

Diminish signals low set frequency

• High pass filters - allow higher freq to pass; cuts out low freq

Band pass

Only range of freq allowed to pass

• combined low and high pass filters

Phase lag

Increased latency

Decreased high frequency filters over time

Phase lead

Decreased latency

Increased low frequency filters over time

Notch filter

Aims at cutting out narrow band of freq

Con: ringing effect that filters out surrounding Hz of targeted Hz

Ex. 60Hz notch filter

• 58-62 Hz is filtered dur to proximity to 60 Hz

Used fr sRMG and EEG

Recording parameters

• analysis time

  • Upper 5-10 ms

  • Lower 10-15 ms

• Trials

  • Upper 100-200+

  • Lowers 200+

• LFF

  • 10-30Hz

• HFF

  • Cortical 250-500 Hz

  • Subcortical 500-1500Hz

  • Peripheral 1500 Hz

Stimulation parameters

• intensity: how high you stim

  • Upper 20-50 mA

  • Lowers 35-100 mA

• Pulse width/ duration how long each pulse

  • Upper and lower 200-500 us (0.2-0.5 ms)

• Repetition rate: how often

  • Upper and lower 1-5 Hz (cycles/second)

    • Not whole integer

Baseline range changes (waveform repeatability)

1% latency

15% amplitude

Alert criteria

50% amplitude

10% latency

Ideal SSEP anesthetics 1MAC

Des 6%

Sevo 2%

ISO 1%

Ideal ssep map >60 mmhg