EVOKED POTENTIALS:An overview

DR. M. ANBARASI

DEFINITION

An electrical potential recorded from a human or animal following presentation of a stimulus

{EEG / EKG / EMG – detects spontaneous potentials}

AMPLITUDES OF VARIOUS POTENTIALS

• EP - < 1 – few micro volts

• EEG – tens of micro volts

• EMG – milli volts

• EKG – volts

“ SIGNAL AVERAGING’ ”

Is done to resolve the low amplitude potentials

CLASIFICATION OF EVOKED POTENTIALS

SENSORY EVOKED POTENTIALS

MOTOR EVOKED POTENTIALS

EVENT RELATED POTENTIALS

VISUALEVOKED POTENTIAL

AUDITORYEVOKED POTENTIAL

SOMATOSENSORYEVOKED POTENTIAL

SENSORY EVOKED POTENTIALS

• VISUAL EVOKED POTENTIAL (VEP)

• AUDITORY EVOKED POTENTIAL (AEP)

SHORT LATENCY AEP

{Brain stem auditory evoked potentials} MID-LATENCY AEP LONG LATENCY AEP

• SOMATOSENSORY EVOKED POTENTIAL (SSEP)

INTERNATIONAL 10 – 20 SYSTEM OF ELECTRODE PLACEMENT

ANATOMICAL & PHYSIOLOGICAL BASIS OF VEP

TYPES OF VEP

PATTERN REVERSAL VEP

• Primary visual system is arranged to emphasize the edges and movements so shifting patterns with multiple edges and contrasts are the most appropriate method to assess visual function.

FLASH VEP• Stroboscopic flash units

• Greater variability of response with multiple positive and negative peaks

• Activates additional cortical projection systems including retino-tectal pathways.

• Primarily use when an individual cannot cooperate or for gross determination of visual pathway. Ex in infants / comatose patients

• Flash stimuli is also Used to produce ERG.

PARTIAL FIELD STIMULATION

To evaluate Retro-chiasmatic lesions.

Involves additional electrodes

Other valuable investigation - MRI

TECHNICAL RECOMMENDATIONS

ACTIVE

Midline occiput (MO) – 0z

REFERENCE

Vertex - Cz

GROUND

Forehead – Fpz

RECORDING ELECTRODES

PATIENT & SEATING PREREQUISITES

• Each eye tested separately

• Patient seated at a distance of 0.75 to 1.5 meters

• Eye glasses to be worn

• The eye not tested should be patched

• Gaze at the centre of the monitor

RECORDING CONDITIONS

• Band pass : 1 – 300 Hz

• Analysis time : 250 ms

• Number of epocs : minimum 100

• Electrode impedence : < 5 Ω

STIMULATION PATTERNS

• Black & white checkerboard• Size of the checks : 14 x 16 mins {Size & distance from the monitor should produce a visual angle of 10 – 20 °}• Contrast : 50 -80 %• Mean luminance : central field – 50 cd /m2

background – 20 – 40 cd /m2

VEP RESPONSE

• P100 – PRIMARY POSITIVE PEAK latency of 100 msec (upper limit of normal – 117 – 120 msec)

• P 100 amplitude

• Two negative peaks – N 75 & N 145

• Inter eye latency difference for P 100 should be less than 6 – 7 msec

NORMAL VALUES FOR VEP

PARAMETERSMEAN ± SD

SHAHROKHIEt al. 1978

MISRA ANDKALITA

P 100 : LATENCY 102.3 ± 5.1 96.9 ± 3.6

R – L (ms) 1.3 ± 0.2 1.5 ± 0.5

AMPLITUDE (µV) 10.1 ± 4.2 7.8 ± 1.9

CLINICAL UTILITY

• MULTIPLE SCLEROSIS:

VEP abnormality – prolongation of P 100

• DEMYELINATING DISORDERS

Increase in response latency

• AXONAL LOSS DISORDERS

reduction in response amplitude

• MIGRAINE HEADACHES

more commonly seen soon after the attacks and with flash stimuli

• CATARACTS & GLAUCOMA :

Decrease in P100 amplitude

• Visual aquity:

Direct correlation with VEP

• Monitoring visual pathway integrity during surgeries

LIMITATIONS OF VEP

• Normal cortical response is obtained if entire visual system is intact

• Disturbances anywhere in the visual system can produce abnormal VEP

localizing value of VEP is limited

Classification of auditory responses :

1. Electrocochleogram (ECoG)

2. Brainstem Auditory Evoked Potential

3. Mid latency Auditory Evoked Potential

4. Long latency Auditory Evoked Potential

COCHLEA COCHLEA

CN CN

SUPERIOROLIVE

SUPERIOROLIVE

IC IC

MGB MGB

AUDITORY CORTEX AUDITORY CORTEX

AP & CM

BERA

MLR

LLR

ELECTROCOCHLEAOGRAM (ECOG)

• Electrodes placed transtympanically into middle ear

• Cochlear microphonics (CMs)

• Summation potentials (SPs)

• Action potentials (wave I of BERA)

• Valuable in diagnosing cochleovestibular disorders.

NORMAL ECoG

BRAINSTEM AUDITORY EVOKED POTENTIALS

• BERA / BAEP / SHORT LATENCY AEP

• It is the evoked transient response of the first 10 msec from the onset of stimulation

• Produces waveforms when passing through brainstem.

CN SON LL IC

I

VIV

IIIIIVII

VI

MGBAUD. RAD

GENERATORS OF BERA

METHODOLOGY OF BERA

ELECTRODE PLACEMENT:

ACTIVE – A1 / A2 - Ear lobe

REFERENCE – Cz – Vertex

GROUND – Fpz - forehead

AUDITORY STIMULUS

• Breif electrical pulse “ click”

• Intensity – 65 – 70 dB above

threshold

• Rate – 10 – 50 clicks / sec

• Averaging of 1000 – 2000 stimuli

• The other ear is masked with

‘ white noise’ of 30 – 50 dB

BERA PARAMETERS

• Absolute waveform latencies

• Interpeak latencies ( I – III, I – V & III – V )

• Amplitude ratio of wave V / I

NORMAL BERA

WAVE LATENCY (ms)

Chippa et al. Misra & Kalita

I

II

III

IV

V

I – III IPL

III – V IPL

I – V IPL

1.7 ± 0.15

2.8 ±0.17

3.9 ± 0.19

5.1 ± 0.24

5.7 ± 0.25

2.1 ± 0.15

1.9 ± 0.18

4.0 ± 0.23

1.67 ± 0.17

2.78 ± 0.21

3.65 ± 0.22

5.72 ± 0.3

5.72 ± 0.3

1.99 ± 0.25

2.08 ± 0.3

4.04 ± 0.225

CLINICAL UTILITY

MULTIPLE SCLEROSIS:

VEP + BERA changes ( 32 – 72 % )

BERA abnormality : IPL &

WAVE v/I amplitude

ACOUSTIC NEUROMA:

BAEP abnormality > 90%

wave I – III IPL

• COMATOSE PATIENTS : COMA due to toxic or metabolic cause – no BAEP

abnormality due to structural brainstem lesion – changes in

BAEP

• HEAD INJURY : More severe BAEP abnormality – poorer prognosis

• Monitor auditory pathway during surgery

• Hearing sensitivity in patients unable to undergo audiometry . Ex. Infants

LIMITATIONS OF BERA

• AEPs parallel haering but not test hearing

• It reflects the synchronus neural discharge in the auditory system

• Should be preceded by PURE TONE AUDIOMETRY

MID LATENCY AEP

• Electrical activity in the post stimulus

period of 10 – 50 ms

• ORIGIN:

Thalamocortical tracts, Reticular fromation of BS, Medial geniculate body & Primary auditory cortex

• Both neurogenic & myogenic origin

Normal MLR

LONG LATENCY AEP (LLR)

• Electrical activity in the post stimulus period of 50 to 500 ms

• Five wave peaks – P1, N1, P2, N2 & P3

• P3 – P300 : related to cognitive and perceptive functions of brain.

• Also called ‘cortical evoked potential’

• Evoked potentials of large diameter sensory nerves in the peripheral & central nervous system

• Used to diagnose nerve damage or degeneration in the spinal cord

• Can distinguish central Vs peripheral nerve lesion

Anatomical & Physiological basis of SSEP

SENSE ORGANS – PACINIAN AND GOLGI COMPLEXES IN JOINTS, MUSCLES AND TENDONS

DORSAL ROOT GANGLIA

TYPE A FIBRES

GRACILE AND CUNEATE Nu. IN MEDULLA

Nu POSTEROLATERALIS OF THALAMUS

MEDIAL LEMNISCUS

SENSORY CORTEX

THALAMOPARIETAL RADIATYIONS

METHODOLOGY

• STIMULUS:

Electrical – square wave pulse by surface or needle electrode

• DURATION:

100 – 200 msec at a rate of 3 – 7 / sec• INTENSITY:

for producing observable muscle twitch

or 2.5 – 3 times the threshold for SNS

Unilateral stimulation for localization

Bilateral stimulation for intra-operative monitoring

UPPER EXTREMITY SSEP

SITES:

• ERB’s point

• Cervical spine –C2 or C5

• Contralateral scalp overlying the area of

the primary sensory cortex - C3 or C4

Reference : forehead Fz

Ground : proximal to stimulation site

MEDIAN NERVE SSEP

• Erb’s point :N9 – brachial plexus

• Cervical spine : N13 – dorsal column nuclei

• Scalp : N20 – P23 – thalamocortical radiations & primary sensory cortex

MEDIAN NERVE SSEP

LOWER LIMB SSEP

SITES:

• Lumbar spine – L3

• Thoracic spine – T12

• Primary sensory cortex - Cz

TIBIAL NERVE SSEP RESPONSE

• L3 – negative peak with latency 19 ms (L3 S) – nerve roots of cauda equina

• T12 - negative peak with latency 21 ms (T12 S) – dorsal fibers of spinal cord

• Scalp: positive peak – P37

negative peak – N45

- thalamocortical activity

TIBIAL NERVE SSEP

INTERPRETATION:

• presence or absence of waves

• absolute and interpeak latencies

latencies > 2.5 – 3 SD of mean – abnormal

LESIONS:

normal response distal to lesion

abnormal response proximal to lesion

Abnormal sural nerve SSEP in Right lumbar radiculopathy

• PERIPHERAL NERVE DISEASES:

slowing of conduction velocity – prolong latencies of all peaks.

IPL are useful

• Central conduction time:

Upper extremity – N13 – N20

Lower extremity – L3S – P37

MOTOR EVOKED POTENTIALS

• Used to assess motor functions of deeper structures

• Stimulus may be electrical or magnetic

• Similar to SSEP but stimulus is given centrally recorded peripherally in distant muscles.

CLINICAL UTILITY

• To diagnose disorders that affect central & peripheral motor pathway

• Examples: multiple sclerosis, Parkinsons, CVA, Myelopathy of cervial & lumbar plexus.

• Intra-operative monitoring.

EVENT RELATED POTENTIALS

• Record cortical activity evoked by a stimulus with cognitive significance

• Stimuli : presenting randomly occuring infrequent stimuli interspersed withmore frequently occuring stimuli.

• Patient to attend only to infrequent stimuli.

• Waveform is called ‘P 300’ with a positive peak.

• Prolongation of P 300 :

Dementia

Neurodegenerative disorders

Schizophrenia

Autism