Evoked potential - An overview

1. EVOKED POTENTIALS:
An overview
DR. M. ANBARASI

2. DEFINITION
An electrical potential recorded from a
human or animal following presentation of
a stimulus
{EEG / EKG / EMG –detects spontaneous
potentials}

3. 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

4. CLASIFICATION OF EVOKED POTENTIALS
SENSORY
EVOKED
POTENTIALS
MOTOR
EVOKED
POTENTIALS
EVENT
RELATED
POTENTIALS
VISUAL
EVOKED
POTENTIAL
AUDITORY
EVOKED
POTENTIAL
SOMATOSENSORY
EVOKED
POTENTIAL

5. 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)

6. INTERNATIONAL 10 –20 SYSTEM OF
ELECTRODE PLACEMENT

8. ANATOMICAL & PHYSIOLOGICAL
BASIS OF VEP

9. TYPES OF VEP
PATTERN REVERSAL VEP

10. •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.

11. 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.

12. PARTIAL FIELD STIMULATION
To evaluate Retro-chiasmatic lesions.
Involves additional electrodes
Other valuable investigation -MRI

13. TECHNICAL RECOMMENDATIONS
ACTIVE
Midline occiput (MO) –0z
REFERENCE
Vertex -Cz
GROUND
Forehead –Fpz
RECORDING ELECTRODES

14. 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

15. RECORDING CONDITIONS
•Band pass : 1 –300 Hz
•Analysis time : 250 ms
•Number of epocs : minimum 100
•Electrode impedence : < 5 Ω

16. 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 /m
2
background –20 –40 cd /m
2

17. VEP RESPONSE

18. •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 differencefor P 100 should
be less than 6 –7 msec

19. NORMAL VALUES FOR VEP
PARAMETERS
MEAN SD
SHAHROKHI
Et al. 1978
MISRA AND
KALITA
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

20. CLINICAL UTILITY
•MULTIPLE SCLEROSIS:
VEP abnormality –prolongation of P 100

21. •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

22. •CATARACTS & GLAUCOMA :
Decrease in P100 amplitude
•Visual aquity:
Direct correlation with VEP
•Monitoring visual pathway integrity during
surgeries

23. 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

25. Classification of auditory responses :
1.Electrocochleogram (ECoG)
2.Brainstem Auditory Evoked Potential
3.Mid latency Auditory Evoked Potential
4.Long latency Auditory Evoked Potential

26. COCHLEA
COCHLEA
CN CN
SUPERIOR
OLIVE
SUPERIOR
OLIVE
IC
IC
MGB
MGB
AUDITORY CORTEX AUDITORY CORTEX
AP & CM
BERA
MLR
LLR

27. 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.

28. NORMAL ECoG

29. 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.

30. CN SON LL IC
I
V
IV
III
II
VII
VI
MGB
AUD. RAD
GENERATORS OF BERA

31. METHODOLOGY OF BERA
ELECTRODE PLACEMENT:
ACTIVE –A1 / A2 -Ear lobe
REFERENCE –Cz –Vertex
GROUND –Fpz -forehead

32. 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

33. BERA PARAMETERS
•Absolute waveform latencies
•Interpeak latencies ( I –III, I –V & III –V )
•Amplitude ratio of wave V / I

34. NORMAL BERA

35. 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

36. CLINICAL UTILITY
MULTIPLE SCLEROSIS:
VEP + BERA changes ( 32 –72 % )
BERA abnormality : IPL &
WAVE v/I amplitude

37. ACOUSTIC NEUROMA:
BAEP abnormality > 90%
wave I –III IPL

38. •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 sensitivityin patients unable to undergo
audiometry . Ex. Infants

39. 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

40. 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

41. Normal MLR

42. 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’

44. •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

45. 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

46. 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

47. 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

48. MEDIAN NERVE SSEP
•Erb‟s point :N9 –brachial plexus
•Cervical spine : N13 –dorsal column nuclei
•Scalp : N20 –P23 –thalamocortical
radiations & primary sensory cortex

49. MEDIAN NERVE SSEP

50. LOWER LIMB SSEP
SITES:
•Lumbar spine –L3
•Thoracic spine –T12
•Primary sensory cortex -Cz

51. 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

52. TIBIAL NERVE SSEP

53. 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

54. Abnormal sural nerve SSEP in Right lumbar
radiculopathy

55. •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

56. 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.

57. CLINICAL UTILITY
•To diagnose disorders that affect central &
peripheral motor pathway
•Examples: multiple sclerosis, Parkinsons,
CVA, Myelopathy of cervial & lumbar
plexus.
•Intra-operative monitoring.

58. 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.

59. •Waveform is called ‘P 300’with a positive
peak.
•Prolongation of P 300 :
Dementia
Neurodegenerative disorders
Schizophrenia
Autism