1. Evoked potentials are electrical potentials recorded from the brain following presentation of a stimulus. There are several types including visual, auditory, and somatosensory evoked potentials. 2. Visual evoked potentials assess the visual pathway by recording electrical activity in the brain in response to visual stimuli. Patterns or flashes of light are used to stimulate the retina. 3. Auditory evoked potentials evaluate the auditory pathway by recording brain activity following auditory clicks or tones. This can help detect lesions in the auditory nerve or brainstem.

1. Evoked potential
Presentar : Ashik Dhakal
Moderator : Ms. Ziona

2. Content
• Introduction
• Types
• Anatomical and physiological response
• Variables influencing EP
• Clinical implication of EP

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

4. Classification

5. Sensory evoked potential
• Visual evoked potential (VEP)
• Auditory evoked potential (AEP)
• Somatosensory evoked potential (SSEP)

6. Electrode placement

7. Measurements for electrode placement
• Nasion - Anion
• pre auricular
• Circumference
• Parasaggital
• Horse shoe

8. Visual evoked potential

9. Anatomical and physiological basis of VEP

11. Basic VEP abnormalities
• Latency prolongation
• Amplitude reduction
• Combined abnormalities

12. Types of VEP
1. Pattern reversal VEP
• Primary visual system is arranged to emphasise the edges and
movements.
• So, shifting patterns with multiple edges and contrasts are the most
appropriate method to assess visual function.

13. 2. Flash VEP
Stroboscopic flash units
• Primarily use when an individual cannot cooperate or for gross determination
of visual pathways. eg, in infants/comatose patient.

14. Procedure
• Electrodes : active (Oz), reference (Cz), ground (Fpz)

15. • Patient position :
• Each eyes separately
• Patient seated at the distance of 0.75 to 1.5 meters.
• Eyes glasses to be worn
• Eye not tested should be patched
• Gaze at the centre of the monitor.

16. • Stimulation patterns
• Black and white checkerboard
• Size of the checks : 14*16
• Contrast : 50-80 %

17. • Video

18. VEP response
• The P100 latency of PSVEP(pattern shift) increases with decrease of
luminance.
• Reduce contrast between black and white squares — increases latency
and decreases amplitude of P100.
• Constricted pupil — increases average latency of P100.

19. Normal values

23. Variables influencing VEP
Age-
• influencing latency of P100 at the rate of 2.5ms/decade after 5th decade.
• Age related changes in retina and rostral part of visual system.
Gender-
• P100 latency- longer in adult male compared to female.
• Less than 19 does not vary with gender.
• :. Larger head size and lower core body temperature in male.
• Amplitude- mean P100 amplitude > in female rather male

24. Eye dominance-
• P100 wave obtained by stimulating the dominant eye is shorter and
amplitude is greater compared to non dominant eye.
• :. Attributed to neuroanatomic symmetries of human cortex.
Eye movement-
• Eye movement- reduces the amplitude of P100 : latency not affected.
• nystagmus— normal visual pathway— normal P100 latency.

25. Visual acuity—
• Pronounced diminution of visual acuity— P100 latency remains normal
• Visual acuity as low as 20/120 - P100 latency normal.
• Further reduction of visual acuity— amplitude decreases.
Drugs—
• Drugs producing pupillary constriction (Pilocarpine) can increase P100 latency
• :. Attributed to reduced area of retinal illumination.
• Unmotivated patient- may alter - P100 latency or amplitude by closing the eye,
gazing of the screen, converging in front of target (nose).

26. Clinical utility
• Cataract and glaucoma : decrease in P100 amplitude .
• Optic neuritis
• Monitoring visual pathways integrity during surgeries.

27. Multiple sclerosis:
• Focal demyelination in visual pathway- delays in conduction.
• P100 latency is prolonged with or without attenuation of amplitude.
• Prolonged by 10-30ms — in definite MS, rarely 100ms.

28. 7 days post methyl prednisolone therapy
In Multiple Sclerosis

29. Limitations of VEP
• Disturbances anywhere in the visual system can produce abnormal VEP,
localising value of VEP is limited.

30. BAEP
• AEP/ BAEP are the potentials recorded from the ear and vertex — in response
to — brief auditory stimulation, to assess — the conduction through the auditory
pathway.
• Clinically helpful in
• Assessment of hearing in uncooperative patients and very young children.
• Severity of hearing deficits in infants.
• Function of the middle portion of the brainstem.

31. Anatomy and Physiological Basis

32. Procedure
• Subject lying supine with a pillow under his head.
• Room should be quit.
• Clean the scalp and apply electrodes.
• Check for impedence.
• Apply ear phone (red for the right ear and blue for the left ear)
• Select the ear in the stimulator and apply masking to the opposite ear.

33. Electrode placement

34. Identification of waves
• Identify wave V which is the most persitent wave. It comes as IV-V
complex, and wave V comes to the baseline.
• Go in reverse order, wave IV, III, II, I.
• Observe their latency., eg, latency of wave I will be less than 2msec.

35. Waveform
Waveform Generators
I VIII nerve
II Cochlear nucleus
III
Superior Olivary
nucleus
IV Lateral Lemniscus
V Inferior colliculi

36. BAEP testing

37. Basic Abnormalities
• Absence of waveform
• Absence of inter peak latency
• Right to left symmetry

38. Interpretation
• Wave I : small amplitude, delayed or absent may indicate cochlear lesion.
• Wave V : small amplitude, delayed or absent may indicate upper brainstem
lesion
• I-III IPL : Prolongation may indicate lower brainstem lesion.
• III-V IPL : prolongation may indicate upper brainstem lesion.
• I-V IPL : prolongation may indicate whole brainstem lesion. Shortening of
wave the interval with normal latency of wave V indicate cochlear
involvement.

39. Normal values

40. Patient Related Factors
• Age:
• Older adults have slightly longer I-V (IPL) by 0.1-0.15ms compared
to younger individuals.
• Gender:
• Females have shorter latency and higher amplitude of BAEP.
• The I-V IPL is shorter by 0.1ms in females compared to males.

41. • Temperature:
• On lowering body temperature IPL is prolonged.
• On 1 degree C reduction of body temperature, 0.17ms increase in wave V
latency.
• At 32.5 degree C the BAEP values are distinctively abnormal.
• At 27 degree C waveform disappear.
• Drugs:
• Slight prolongation of wave V latency with barbiturates or alcohol,
• :. Attributed to lowering of body temperature.

42. • Hearing Impairment:
• Can alter BAEP, therefore, before starting the study, external ear
should be examined with an auto-scope for blockage by
cerumen (earwax).
• Audiometry test or hearing should be tested.

43. • Clinical neurophysiological correlation
• Cerebelopontine angle tumor
• Intrinsic brainstem tumor
• MS

44. Somatosensory evoked potential
• Somatosensory Evoked Potential (SSEP) is a noninvasive diagnostic test to assess the
speed of electrical conduction across the spinal cord.
• The technique involves applying electrical stimulus at specific nerves in the arms and
legs and measuring the impulses generated by the stimulus at various points in the
body.
• If the spinal cord is pinched, the electrical signals will travel slower than usual.
• Can distinguish central vs peripheral nerve lesion.
• SSEP may also be used to monitor spinal cord function during surgical procedures,
particularly for cervical or thoracic spine surgery.

45. Anatomical and physiological basis of SSEP

46. Procedure
• The information about sensory symptoms, signs and peripheral nerve injury should be
obtained.
• SSEP can be recorded by stimulating any large nerve. eg. median and posterior tibial
nerves.
• Positioning: Comfortable position, comfortable temperature
• Stimulus : electrical — square wave pulse by surface or needle electrode
• Duration, intensity : 100-200 msec, 15mA.
• In peripheral neuropathy higher current with higher duration is required.
• Unilateral stimulation for localization , Bilateral stimulation for intra-operative
monitoring.

47. Upper extrimity SSEP
• Electrode placement :
• Recording el : 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. Electrode placement for SSEP of median nerve

49. Median nerve SSEP
• Channel 1 Scalp (Cc-Fz): N20 -P23 - thalamocortical radiation and
primary sensory cortex
• Channel 2 (Cc-EPc) - P14
• Channel 3 (C5Sp- EPc) Cervical spine : N13 - dorsal column nuclei
• Channel 4 (Epi-EPc) Erb’s point : N9 - brachial plexus

52. Patient-Related Factors
• Age:
• In young children- N9 and N13, potentials of Median SEP occurs early.
• In elderly- Normal limit for latencies are longer by 5-10%, occurs in
>55years old.
• Gender:
• Females have a shorter central conduction time than males. (reason
unknown)
• Sleep: may change median N20 latency by increasing the amplitude.

53. • Drugs:
• Sedation- for better recording in uncooperative patients.
• 10mg Diazepam orally- reduces muscle artefacts of SEP recording,
improvement in lumbar and neck recordings, increased reproducibility,
increased ease of measurement and greater tolerance of the procedure.
• Temperature:
• Peripheral nerve conduction is slowed by lowering the limb temperature.
• With temperature changes, the peripheral conduction is affected more
than the central.

54. • Clinical application of SSEP
• SSEP have good correlation with impairment of joint position and
vibration sensations but not with pinprick and touch.
• For SSEP abnormalities, significant degree of sensory impairment is
necessary.
• A combination of both latency and amplitude abnormalities are found in
compressive lesions.

55. • Important conditions where SSEP are evaluated-
• Demyelinating disease
• Trauma
• Degenerative diseases like cervical and lumbar spondylosis
• Spinal cord tumor

56. • Surgical Monitoring
• Intraoperative monitoring (IOM): provide a physiological look at :
• BRAIN
• BRAINSTEM
• SPINAL CORD
• CRANIAL NERVES
• And NERVE ROOTS
• During neuro-spinal procedures IOM prevents the possible iatrogenic injury to the
nervous system

57. • Electrode is placed in 10-20 placement system.
• SSEP and MEP both are used in neuro spinal surgeries, as there are two
different pathways, and it is important to keep tract of both.
• SSEP are mainly used in scoliosis surgery, neurosurgical (tumors,
vascular malformations, bony malformations, and trauma of spinal
cord) and
• cardiovascular surgery (if the aortic cross-clamping time exceeds 18min
and 14s it can result in ischemic myelopathy. Thus, spinal cord is
vulnerable to ischemia especially during surgery for aneurysm and
coarctation of aorta)

58. 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.

59. Pathways

60. Electrode placement

61. Effect of activation in MEP

63. Clinical utility
• To diagnose disorders that affect central and peripheral motor pathways.
• examples: multiple sclerosis, Parkinson’s D, CVA, Myelopathy of cervical
and lumbar plexus.
• Intra-operative monitoring.

64. Event related/cognitive evoked potentials
• Record cortical activity evoked by a stimulus with cognitive significance.
• Stimuli : presenting randomly occurring infrequent stimuli interspersed
with more frequent occurring stimuli.

65. Electrode placement

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

68. • Video

69. Variables affecting P300
• Attention
• Task - task harder - latency increases.
• Age :
• latency increases with > age. 1-1.5 ms/year after age of 20.
• Amplitude decreases after the age of 80 years.

70. • Drugs
• Technical parameters
• intraindividual variability

71. Clinical implication
• Prolongation of P 300:
• Aging
• Dementia
• Neurodegenerative disorders
• Schizophrenia
• Autism multiple sclerosis

72. Thank you