Successfully reported this slideshow.
Your SlideShare is downloading. ×

Evoked potential - An overview

Ad

EVOKED POTENTIALS:
    An overview




  DR. M. ANBARASI

Ad

DEFINITION

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



  {EEG / EKG...

Ad

AMPLITUDES OF VARIOUS POTENTIALS

 •   EP - < 1 – few micro volts
 •   EEG – tens of micro volts
 •   EMG – milli volts
 •...

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Ad

Loading in …3
×

Check these out next

1 of 60 Ad
1 of 60 Ad
Advertisement

More Related Content

Advertisement
Advertisement

Evoked potential - An overview

  1. 1. EVOKED POTENTIALS: An overview DR. M. ANBARASI
  2. 2. DEFINITION An electrical potential recorded from a human or animal following presentation of a stimulus {EEG / EKG / EMG – detects spontaneous potentials}
  3. 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. 4. CLASIFICATION OF EVOKED POTENTIALS SENSORY MOTOR EVENT EVOKED EVOKED RELATED POTENTIALS POTENTIALS POTENTIALS VISUAL AUDITORY SOMATOSENSORY EVOKED EVOKED EVOKED POTENTIAL POTENTIAL POTENTIAL
  5. 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. 6. INTERNATIONAL 10 – 20 SYSTEM OF ELECTRODE PLACEMENT
  7. 7. ANATOMICAL & PHYSIOLOGICAL BASIS OF VEP
  8. 8. TYPES OF VEP PATTERN REVERSAL VEP
  9. 9. • 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.
  10. 10. 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.
  11. 11. PARTIAL FIELD STIMULATION To evaluate Retro-chiasmatic lesions. Involves additional electrodes Other valuable investigation - MRI
  12. 12. TECHNICAL RECOMMENDATIONS RECORDING ELECTRODES ACTIVE Midline occiput (MO) – 0z REFERENCE Vertex - Cz GROUND Forehead – Fpz
  13. 13. 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
  14. 14. RECORDING CONDITIONS • Band pass : 1 – 300 Hz • Analysis time : 250 ms • Number of epocs : minimum 100 • Electrode impedence : < 5 Ω
  15. 15. 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
  16. 16. VEP RESPONSE
  17. 17. • 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
  18. 18. NORMAL VALUES FOR VEP MEAN SD PARAMETERS SHAHROKHI MISRA AND Et al. 1978 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
  19. 19. CLINICAL UTILITY • MULTIPLE SCLEROSIS: VEP abnormality – prolongation of P 100
  20. 20. • 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
  21. 21. • CATARACTS & GLAUCOMA : Decrease in P100 amplitude • Visual aquity: Direct correlation with VEP • Monitoring visual pathway integrity during surgeries
  22. 22. 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
  23. 23. Classification of auditory responses : 1. Electrocochleogram (ECoG) 2. Brainstem Auditory Evoked Potential 3. Mid latency Auditory Evoked Potential 4. Long latency Auditory Evoked Potential
  24. 24. AUDITORY CORTEX LLR AUDITORY CORTEX MGB MGB MLR IC IC BERA SUPERIOR SUPERIOR OLIVE OLIVE CN CN COCHLEA COCHLEA AP & CM
  25. 25. 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.
  26. 26. NORMAL ECoG
  27. 27. 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.
  28. 28. IV V III II I VII VI MGB CN SON LL IC AUD. RAD GENERATORS OF BERA
  29. 29. METHODOLOGY OF BERA ELECTRODE PLACEMENT: ACTIVE – A1 / A2 - Ear lobe REFERENCE – Cz – Vertex GROUND – Fpz - forehead
  30. 30. 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
  31. 31. BERA PARAMETERS • Absolute waveform latencies • Interpeak latencies ( I – III, I – V & III – V ) • Amplitude ratio of wave V / I
  32. 32. NORMAL BERA
  33. 33. WAVE Chippa et al. Misra & Kalita LATENCY (ms) I 1.7 0.15 1.67 0.17 II 2.8 0.17 2.78 0.21 III 3.9 0.19 3.65 0.22 IV 5.1 0.24 5.72 0.3 V 5.7 0.25 5.72 0.3 I – III IPL 2.1 0.15 1.99 0.25 III – V IPL 1.9 0.18 2.08 0.3 I – V IPL 4.0 0.23 4.04 0.225
  34. 34. CLINICAL UTILITY MULTIPLE SCLEROSIS: VEP + BERA changes ( 32 – 72 % ) BERA abnormality : IPL & WAVE v/I amplitude
  35. 35. ACOUSTIC NEUROMA: BAEP abnormality > 90% wave I – III IPL
  36. 36. • 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
  37. 37. 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
  38. 38. 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
  39. 39. Normal MLR
  40. 40. 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’
  41. 41. • 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
  42. 42. Anatomical & Physiological basis of SSEP SENSE ORGANS – PACINIAN AND GOLGI COMPLEXES IN JOINTS, MUSCLES AND TENDONS TYPE A FIBRES DORSAL ROOT GANGLIA GRACILE AND CUNEATE Nu. IN MEDULLA MEDIAL LEMNISCUS Nu POSTEROLATERALIS OF THALAMUS THALAMOPARIETAL RADIATYIONS SENSORY CORTEX
  43. 43. 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
  44. 44. 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
  45. 45. MEDIAN NERVE SSEP • Erb‟s point :N9 – brachial plexus • Cervical spine : N13 – dorsal column nuclei • Scalp : N20 – P23 – thalamocortical radiations & primary sensory cortex
  46. 46. MEDIAN NERVE SSEP
  47. 47. LOWER LIMB SSEP SITES: • Lumbar spine – L3 • Thoracic spine – T12 • Primary sensory cortex - Cz
  48. 48. 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
  49. 49. TIBIAL NERVE SSEP
  50. 50. 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
  51. 51. Abnormal sural nerve SSEP in Right lumbar radiculopathy
  52. 52. • 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
  53. 53. 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.
  54. 54. CLINICAL UTILITY • To diagnose disorders that affect central & peripheral motor pathway • Examples: multiple sclerosis, Parkinsons, CVA, Myelopathy of cervial & lumbar plexus. • Intra-operative monitoring.
  55. 55. 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.
  56. 56. • Waveform is called ‘P 300’ with a positive peak. • Prolongation of P 300 : Dementia Neurodegenerative disorders Schizophrenia Autism

×