AUDIO VISUAL EVOKED PATENTIAL VEP, BAEP

1. AUDITORY AND VISUAL
EVOKED POTENTIAL
PRESENTED BY-
Dr. PRIYANKA VERMA

2. The commonly tested evoked potentials are
1. Brainstem auditory evoked potentials (BAEPs)
2. Visual evoked potentials (VEPS)
3. Somatosensory evoked potentials (SEPs)
4. Motor evoked potentials (MEPs)

3. VISUAL EVOKED POTENTIALS (VEP)
 The visual evoked potentials are the potentials recorded
from the scalp over the occipital cortex in response to
visual stimuli such as an alternating checkerboard pattern
on a computer screen.
 They represent the responses of the visual pathway to
photo stimulation.

4. • The visual evoked potential is primarily a function of central
visual function, because such a large region of occipital
cortex is devoted to macular projections.
• Thus, peripheral visual loss might be overlooked by visual
evoked potential testing.
• These are useful to detect the abnormality, but cannot
locate exactly the site of defect.

5.  Visual evoked potentials are most useful for testing
optic nerve function and less useful for assessing
postchiasmatic disorders.
 In patients with postchiasmatic lesions, MRI is a more
useful test.

6. 3. Genesis of VEP
I. The process of visual information in retina involves a generation of
electrical activity at three places:
a. The first activity is generated in photoreceptors (rods and cones) by
the action of light;
b. The second in the bipolar cells; and
c. The third electrical activity is generated in the ganglion cells.
II. The retinal ganglion cells are broadly classified into X and Y cells; the
main differentiating features between the two are:

8. The Waveforms of VEPS
 The VEPS consist of a series of waveforms of opposite polarity.
 The negative waves are denoted by N and positive waves by P ,
which is followed by the approximate latency in ms.
 The commonly seen waveforms are N₇₅ ,P₁₀₀ and N₁₄₅ .
 The peak latency and peak to peak amplitudes of these waves are
measured.
 Generally the peak latency, duration and amplitude of P₁₀₀ are
measured.

9.  The normal values of parameters of P100 are:
 Latency (ms) : 100
 Amplitude (uv): 11
 Duration (ms) : 60
 N₇₅ mainly results from foveal stimulation and originates
in area 17.
 P₁₀₀ originates in area 19.
 N₁₄₅ reflects the activity of area 18.

14. Factors that Influence VEP
1. Age - The amplitude of P₁₀₀ is high in infants and children, and is
almost double the adult value. The adult value is reached in 5-7
years. After 50 years, the amplitude decreases.
2. Sex - The P₁₀₀ latency is longer in men, which may be due to bigger
head size in men. However, the P₁₀₀ amplitude is greater in
women, which may be due to hormonal influence.
3. Drugs - The drugs that cause miosis (pupillary constriction), for
example, pilocarpine, increase the P₁₀₀ latency, which is due to
decreased area of retinal illumination. The mydriatics decrease
P₁₀₀ latency

15. 5. Eye movement - The amplitude of P100 is decreased by eye
movement but the latency remains unaffected.
6. Visual acuity - With decreased visual acuity the amplitude of
P100 is decreased, but the latency remains normal.

16.  Method of Recording VEP
 Principle
 The stimulation of the visual pathway generates activities in the
visual cortex.
 A visual stimulus is presented to the subject for a selected
number of times, and the cerebral responses are amplified,
averaged by a computer and displayed on the oscilloscope screen
or printed out on paper.

17. Requirement for Recording
1. Pre amplifier and amplifier
2. Oscilloscope and recording unit
3. Electrode jelly
4. Photo stimulator
5. 10-20 EEG electrode system
Procedure
• For best results, proper instructions should be given to the subject
and a thorough eye examination should be conducted.

19. Pretest instructions
1. The subject should be told about the procedure of the test to get full
cooperation.
2. The subject should avoid applying hair spray or oil after the last hair wash.
3. If the subject uses optical lenses, these glasses should be worn during the
test.
4. The subject should be instructed not to use any miotic and mydriatics 12
hours before the test.
5. The full ophthalmological examination should be carried out to determine
the visual acuity, the pupillary diameter and the field of vision.
6. If there is any field defect, the electrodes may be placed laterally (in addition
to midline electrodes).
7. This is done because the field defects alter the potential field distribution of
P100.

20. Steps
1. Allow the subject to sit comfortably in a chair in a fully relaxed
state.
2. The skin at the point of the placement of electrodes is cleared
with ether or spirit.
3. Place the recording electrode at Oz, using conducting jelly or
electrode paste.
4. Place the reference electrode at Fpz or 12 cm above the
nasion.
5. The ground electrode is placed at the vertex, i.e. at Cz.

21. 8. The electrodes are connected through the preamplifier to the
cathode ray oscilloscope (CRO).
9. Instruct the subject to fix the gaze at the centre of the screen.
10.The visual stimulus is delivered by photo stimulator; white flashes
should be given at frequencies 1, 3, 6, 10 and 20 flashes/sec.
11.Obtain the response: the peak latency and the peak to-peak
amplitude of these waves are measured and compared with the
normal.

22. Precautions
1. The subject should be instructed properly.
2. The skin of the scalp should be grease-free.
3. Mydriatics and miotics should not be used for minimum of 12 hours before the
test.
4. Visual acuity, pupillary diameter and field of vision must be checked before
starting the test.
5. Additional lateral electrodes should be used if there is any visual field defect.
6. Amplification ranging should be between 20,000 and 1,00,000, for recording
pattern shift VEP.
7. The subject should not sleep during the procedure.

23. DISCUSSION
• For recording VEPs, the eyes are tested one at a time.
• Each eye projects to the occipital cortex through the optic chiasma.
Therefore, unilateral VEP abnormality is obtained by full-field mono-
ocular stimulation, which is likely to be due to prechiasmal lesion.
• If the pattern shift visual evoked potential (PSVEP) is abnormal
bilaterally, it becomes difficult to locate the anatomical site of the
defect.

24. VEP Abnormalities
• The common VEP abnormalities are:
1. Prolongation of latency The commonest cause of prolonged latency of
P100 is demyelination of optic pathways. The amplitude remains
normal.
2. Amplitude reduction - Amplitude reduction of P100 occurs in
ischemic optic neuropathy that causes axonal loss. The latency
remains normal. This also occurs in refractive errors, media opacities
(for example, lens opacity) and retinal diseases.
3. Combined latency and amplitude defects This occurs in optic nerve
compression that causes segmental demyelination and axonal loss.

25. 4. Shape abnormalities Usually two types are commonly observed.
i. Bifid P100 - In bifid P100 two peaks are observed. This is seen
rarely in normal individuals. Its indicates abnormality.
ii. W-shaped VEP When two peaks are separated by 10-50 ms, it
forms W-shaped P100 waveform.

26. Clinical Applications
• The VEP studies provide a sensitive method for documenting the
abnormalities in visual pathways especially anterior to the optic
chiasma.
• The VEP abnormalities are non-specific and are not characteristic of
any specific etiology.
• But VEP studies help in assisting a clinical diagnosis of
demyelinating diseases, ischemic optic neuropathy, nutritional
and toxic optic neuropathies, hereditary and degenerative
diseases, lesions affecting anterior visual pathways and cortical
blindness.

27. •A normal VEP indicate the intactness of visual system.
•Therefore, in the localization of deficit within the visual
pathway, the knowledge of physiological anatomy of
the eye is essential.

28. Brainstem auditory
evoked potentials
(BAEPS)

29. • Brainstem auditory evoked potentials (BAEPS) constitute an
objective hearing test.
• These are the potentials recorded from the ear and the scalp in
response to a brief auditory stimulation.
• The evoked potentials that appear following transduction of the
acoustic stimulus by the ear cells, create an electrical signal that is
carried through the auditory pathway to the brainstem and from
there to the cerebral cortex.

30. • When the signal travels, it generates action potential in all the
fibres.
• These action potentials can be recorded at several points along
the auditory pathway and even from the surface of the body.
• BAEPs assess conduction of the impulse through the auditory
pathway up to the midbrain.

31. Uses of BAEP
Clinically BAEPs are used:
1.To assess hearing in uncooperative patients and very
young children.
2.To detect degree of hearing loss in infants.
3.To assess the functions of the midpart of the brainstem.

32. Auditory Pathway
•The axons of the spiral ganglion, which innervate hair cells
of the ear, form the cochlear nerve.
•The first order of neurons terminates in the cochlear nuclei
in the medulla from where the second order of neurons
arises and ends in the superior olivary nucleus.

33. • The third order of neurons originates from the superior
olivary nucleus and ascends the lateral lemniscus to project
onto the inferior colliculus which is the centre for auditory
reflexes.
• From the inferior colliculi, many fibres project to the
medial geniculate body in the thalamus and from there to
the primary auditory cortex (area 41).

35. Physiological Basis of BAEPS
•BAEPs are recorded within 10 ms after acoustic stimulus is
given.
•A series of potentials are generated corresponding to
sequential activation of different parts of the auditory
pathway, that is, peripheral, pontomedullary, pontine and
midbrain portions of the pathway.

37. Waves of BAEP
• Five or more distinct waveforms are recorded within 10 ms of the
auditory stimulus.
• These waveforms are named wave I, II, III, IV and V (Fig. 44.1).
• If the recording continues, a few more positive and negative waves
are recorded.
Wave I -Originates from the peripheral portion of the eighth
cranial nerve adjacent to the cochlea.
Wave II:- Originates from the cochlear nucleus.
Wave III:- Originates from the superior olivary nucleus.
Wave IV: Originates from the lateral lemniscus.
Wave V: Originates from the inferior colliculi.

39. Factors that Affect BAEP
• 1. Age -The latency of BAEP is affected by age, Older adults have
slightly longer I to IV inter peak latency compared to younger
individuals.
• 2. Sex - Women have shorter latency and higher amplitude of BAEPS.
• 3. Height - The height of the subject has no direct correlation with
latency or amplitude of BAEPS.

40. • 4. Temperature Increased body temperature decreases the latency and
decreased temperature increases the latency of BAEP.
• 5. Drugs - Barbiturates and alcohol prolong the latency of wave V. These
drugs affect latency by decreasing the body temperature instead of
directly acting on the auditory pathway.
• 6. Hearing loss - Hearing deficit affects BAEPS. Therefore, hearing tests,
especially to detect conductive deafness and examination of the ear to
diagnose ear block by cerumen, should be done prior to recording BAEPS.

41. METHODS
Methods of Recording BAEPS
Principle
A brief auditory stimulation generates action potentials in the
auditory pathway. These potentials are recorded from the ear and
vertex as BAEPs.
Requirements
1. Recording electrodes
2. Amplifier and average
3. Electrode paste
4. Earphone

43. Procedure
1. Place the recording electrode on both the ear lobes or mastoid process .
2. Place the reference electrode on a point slightly in front of the vertex .
3. Place the ground electrode on a point in front of the reference electrode.
4. Connect the recording electrodes to the amplifier.
5. Use amplifications of 2,00,000-5,00,000.
6. Set the low filter at 100 Hz and high filter at 3000Hz.
7. Give a brief click stimulus of 0.1 ms duration.
8. Observe the recording of potentials.
9. Repeat 2-3 times and see that recordings are superimposed to check the
reproducibility.
Note: The BAEP repetition should be superimposed almost exactly.

45. Precautions
1. The subject should be properly instructed and motivated to
provide full cooperation.
2. The subject should be fully relaxed, otherwise hypnotics can
be used to achieve maximum relaxation.
3. The room should be quiet and comfortable.
4. The skin of the scalp and mastoid should be grease free.

46. Measurement of BAEP Waveforms
The following parameters are measured for analysing the
waveforms of BAEPS:
1. Absolute latency and amplitude
2. Interpeak latencies
3. Amplitude ratio of V/I
4. Inter-ear-interpeak difference

47. Absolute Latency and Amplitude
• The absolute amplitude is measured as the height (expressed
in uv) from the peak of the wave to the trough of that wave.
• The absolute latency is measured as the distance (expressed in
ms) from the beginning of the first wave to the peak of that
wave.
• Interpeak Latencies
• The interpeak latencies (IPLs) commonly measured are I-V, I-III
and III-V. This is measured as the distance between the peak of
both the waves (expressed in ms).

48. I-V Interpeak Latency
1. The normal value is 4.5 ms.
2. It represents conduction from the proximal part of the eighth
nerve through pons to the midbrain.
3. It is slightly less in females and more in elderly men.
4. It is prolonged in: Demyelination. Degenerative diseases
,Hypoxic brain damage

49. I-III Interpeak Latency
1. The normal value is about 2.5 ms.
2. It measures conduction from the eighth nerve across the
subarachnoid space into the core of lower pons.
3. It is prolonged in: Inflammation or tumor of the eighth nerve
Diseases at the pontomedullary junction.

50. III-V Interpeak Latency
1. The normal value is about 2.4 ms.
2. It measures conduction from the lower pons to the midbrain.
3. It is prolonged in prolongation of I-V IPL. The isolated
prolongation III-V IPL is not considered significant.

51. Amplitude Ratio of V / I
• Wave I is generated outside and V is generated inside the CNS.
Therefore, the V/I ratio compares the relationship of the signal
amplitude.
• Normal value The ratio is normally between 50 per cent and
300 per cent.

52. Clinical implication
• If the ratio is less than 50 per cent. this suggests small wave V,
which indicates a central impairment of hearing.
• If the ratio is more than 300 per cent, this suggests small
amplitude of wave I, which indicates peripheral hearing
impairment.s

53. Clinical Application - BAEP
• The changes in brainstem auditory evoked potentials have been
correlated with diseases at different levels of the auditory pathway.
• BAEP is usually helpful in –
1. localizing the lesions in the brainstem.
2. It is useful in diagnosing diseases like cerebellopontine ,angle
tumor, intrinsic brainstem tumor, multiple sclerosis, coma, brain
death and strokes affecting the brainstem (thrombosis of
vertebrobasilar system).
3. It is also useful in pediatrics for assessing auditory function in
children whose hearing cannot be tested behaviorally.

54. THANK YOU