A presentation on Visual evoked potential

1. VISUAL EVOKED POTENTIAL
Dr Saurabh Kushwaha
Resident (Ophthalmology)

2. SCOPE
 Introduction
 VEP stimuli
 Types of VEP
 Equipments required
 Prerequisites
 Recording of VEP
 Properties of VEP waveform
 Interpretation
 Clinical applications

3. VISUAL EVOKED POTENTIAL
 EEG is a record of the electrical activity of the brain,
obtained by placing surface electrodes on the scalp
 VEP is an 'evoked' electrophysiological potential
recorded from scalp in response to visual stimuli
 It assess the integrity of the visual pathways from the
optic nerve to the occipital cortex

4. ANATOMIC BASIS OF VEP
 VEP grossly over-represent
the macular region primarily
because anatomically:
• Macular fibers project to
the occipital lobe cortex,
and those from the
peripheral retina project
deeper within the calcarine
fissure
• Also, over the course of
the visual pathway, the
macular field gets
‘amplified’ as it reaches the
cortex

5. VEP STIMULI
 VEP can be evoked by either a flash of light or a
pattern
 Flash Stimuli
• The flash VEP is elicited by
flashes of light produced by a
xenon arc photo stimulator
• Occipital cortex is relatively
insensitive to flash

6.  Pattern Stimuli
• These are presented in a checker-board pattern
• ‘Pattern onset offset‘ form wherein the pattern is
shown for a brief period and then replaced with a
blank screen (of the same intensity)
• ‘Pattern reversal’ type wherein the black and white
checks reverse their orientation
• Luminance contamination: The evoked potential
should be in response only to the changing pattern
and not due to a change in light intensity (brightness)
Therefore, the average luminance must be kept
constant throughout the test

7. TYPES OF VEP
 Flash VEP
• Less commonly used
• Used in uncooperative
and unconscious patients
 Pattern VEP
• Most commonly used in
clinical practice.
 Chromatic patterned stimuli
• Helpful in detecting color
blindness.

8. TRANSIENT AND STEADY-STATE
VEP
 If the visual stimulus is intermittent, thus allowing the
brain to recover its resting state in between, then the
VEP obtained is called transient VEP.
• Transient VEP is used for all practical purposes.
 If however, the stimulus is projected faster so that the
brain does not regain its resting state, a sinusoidal
waveform called steady-state VEP is obtained.
• Not used routinely due to inferior information on
latency or amplitude components.

10. FLASH VEP
 Flash Response to diffusely flashing light stimulus that
subtends a visual field of 20 degrees
 It is performed in a dimly illuminated room
 Cruder response than pattern VEP
 Merely indicates that light has been perceived by cortex
 Indications - media haze, infants, poor pt cooperation

11. PATTERN REVERSAL VEP
 Response to a patterned stimulus - checkerboard or
square and sine wave gratings
 Frequency of gratings is described in CPD - cycles
per degree
 For check pattern visual angle subtended by a single
check is used
 Preferred technique for most clinical purposes, gives
an estimate of form sense and thus visual acuity

12. PATTERN ONSET/OFFSET VEP
 A pattern is abruptly exchanged with an
equiilluminant diffuse background
 More intersubject variability than pattern reversal VEP
 Useful in detection of pts with poor fixation,
malingering, deliberate defocusing, pts with nystagmus

13. EQUIPMENTS REQUIRED
 Visual stimulus producing device
 Scalp electrodes
 Amplifier
 Computer and read out systems

14. PREREQUISITES
 There should be no distracting sound or light waves
 Pattern and flash must both be done in all patients as
pattern cannot be detected in pts with media opacities
 Pattern VEP followed by flash VEP
 Procedure is significantly affected by eccentric
fixation, excessive blinking of eyes and partial closure
of eyes

15. RECORDING OF THE VEP
 Recorded monocularly with undilated pupils
 Refractive correction and in a relaxed position at the
calibrated viewing distance (1m distance from monitor)
 The pupillary size should be noted for each
evaluation
 While monocular stimulation is standard; in children
or other special groups, binocular stimulation may be
used to assess visual pathway conduction from either
eye
 While performing flash VEP, a mechanical patch
should be applied over the unstimulated eye

16. PROPERTIES OF THE VEP
 Amplitude :
• It is the height of the wave (vertical) measured in
microvolts from the preceding trough
• Absolute amplitude is difficult to compare because
of the large variation between normal persons and
variations in sensitivities of the recording equipment
• Relative amplitude (difference between the two
eyes) is more sensitive when looking for unilateral or
asymmetric disease
• In general, absolute amplitude of P100 less than
05 microvolts is abnormal

18.  Latency :
• Measured in milliseconds, it is the delay between
the stimulus presentation and the peak of the wave
in question.
• Latency shows much less variation between
subjects
• However, it is also affected by a number of
factors, including pupil size, refractive error, age and
stimulus factors (pattern size, luminance and
contrast)
• Latency of P100 wave should not exceed 110 ms
in patients under the age of 60 years.

20.  Waveform :
• Age dependent and are standardized for a
population between 20 - 60 years of age.
• In a standard wave pattern, the time from stimulus
onset to the maximum positive or negative excursion
of the VEP is recorded as "peak time“
• The flash VEP pattern comprises a series of positive
and negative deflections, with a peak time varying
between 30 ms to 300 ms, the most robust peaks
being the N2 and the P2 peaks at about 90 ms and
120 ms peak time respectively

21. • Pattern reversal VEP waveforms comprise the N75,
P100 and N135 peaks (Fig. A)
• Standard pattern onset-offset VEPs demonstrate
three main peaks in adults; the C1 positive peak at
about 75 ms, the C2 negative peak at about 125 ms
and C3, another positive peak at about 150 ms (Fig. B)

22. FACTORS INFLUENCING VEP
 Size of stimulus - Decrease in size of stimulus increases
amplitude of VEP
 Position of electrodes on scalp
 Age - amplitude decreases with age
 Gender - P100 latency is longer in adult males and
mean amplitude is greater in females
 Pupil size - Pupillary constriction increase P100 latency
which is attributed to decreased area of retinal
illumination
 Eye movements - reduces the amplitude of P100 but
latency is not affected
 Attention of patient - If subject looks to side of stimulus,
there is rapid fall in size of response

23. INTERPRETATION
 Each eye projects to B/L occipital cortex via optic
chiasma
 Unilateral VEP abnormality - Anterior visual pathway
lesion (pre chiasmal lesion)
 Bilateral VEP abnormality - No localizing value
 Latency prolongation
• P 100 Latency prolongation > 3 SD or interocular
latency difference > 10 m sec is significant
• Prolonged P 100 latency - demyelinating lesions,
retinopathies and glaucoma.

24.  Amplitude reduction
• Amplitude of P 100 shows wide individual variation
• Hence, Inter ocular amplitude ratio is used to
detect abnormalities
• Inter ocular P 100 amplitude ratio > 2 is significant
• Reduced amplitude indicates axonal lesions like
AION
 Combined latency & amplitude abnormalities
• Optic nerve compression produce results in
segmental demyelination and axonal loss
• Hence it produces combined latency and
amplitude abnormalities

25. CLINICAL APPLICATIONS

26. PATTERN VEP
 Optic neuritis and multiple sclerosis (MS):
• Characteristically shows a delayed P100 latency
• Finding persists even after recovery of visual acuity,
hence, it can be useful in confirming a previous attack of
optic neuritis
• However, it may not be able to delineate a fresh attack
• Patients with MS but without a history or clinical
features of optic nerve involvement can show abnormal
VEP responses to pattern stimulation, suggesting
subclinical visual pathway involvement.

27. Left retrobulbar optic neuritis showing a delayed P100 component

28.  Nonarteritic anterior ischemic optic neuropathy
(NAAION):
• Typically shows reduced wave amplitude but
latency is not significantly delayed
• VEP in the clinically uninvolved eye is invariably
normal
 Compressive lesions:
• Shows prolonged latency at an early stage, though
not as much as in optic neuritis
• Much higher incidence of waveform abnormalities
than in patients with demyelinating disease
 Functional visual loss:
• Distinguishes between organic and functional
visual loss
• A normal pattern VEP establishes the presence of
an intact visual pathway

29. Optic nerve Ethambutol toxicity showing slow P100 peak times

30.  Patients with motor disorders:
• Such patients may appear visually impaired
because the eyes cannot track a moving target.
• in children with cerebral palsy, is a preferred
modality, but may be difficult to perform it because of
seizure activity, wandering eye movements or
depressed cortical activity due to anticonvulsants.
 Nystagmus:
• Use of horizontal gratings produces less blur.
• Pattern onset preferred rather than pattern reversal
provide more accurate information.

31. THANK YOU
 Children with neurofibromatosis type 1 are vulnerable to
development of optic nerve glioma
 VEP can be a more sensitive and cost effective test to
follow the progress of nerve pathology than MRI tests alone

32.  Malingering and Hysteria:
• Patients with Hysterical Blindness.
• VEP remains normal with vision as low as 1/60.
• VEP can be enhanced by using large fields, large
checks and binocular vision.
 During Orbital or Neurosurgical Procedures:
• Continuous record of optic nerve function in form
of VEP to prevent inadvertent damage to the nerve
during surgical manipulation

33. FLASH VEP
 Can be performed in shorter time
 No need for active patient participation
 Used in adults and infants with dense media
opacities to test the visual pathway integrity
• A good correlation between VEP prediction and
actual postoperative visual acuity has been seen in
patients with dense cataracts.
 Used as a prognostic tool prior to vitrectomy in
diabetic vitreous hemorrhage
 To evaluate the central nervous system in high-risk
neonates, especially those born prematurely, with
intraventricular hemorrhage, or hydrocephalus.

34. MULTIFOCAL VEP
 mfVEP recorded with the same equipment as for mfERG
 VEPs recorded simultaneously from multiple regions of
the visual field
 Visual field divided into 60 sectors, each having 16
checks (8 black and 8 white)
 Sectors and checks are scaled differently (peripheral
sectors larger) so that they are all of approximately equal
effectiveness for cortical stimulation.
 Pseudo-random sequences and a software algorithm
allow the on-board computer to rapidly extract information
simultaneously from each of the stimulated sectors.
 mfVEP provides a probability plot like automated
perimeters

35.  Used to detect small abnormalities in visual signal
transmission from centric and eccentric field and provides
a topographical display of these deficits
 To rule out non-organic visual loss
 To diagnose and follow-up patients of optic neuritis and
multiple sclerosis
 To confirm unreliable or questionable visual field
examinations

36. MULTICHANNEL VEP
 This technique uses multiple active (parasagittal)
electrodes
 This technique provides localizing value
 Chiasmal lesions show a crossed asymmetry, i.e.
findings of one eye show an asymmetrical distribution that
is reversed when the other eye is stimulated.
 Retrochiasmal lesions show uncrossed asymmetry,
wherein findings of each eye show an asymmetrical
distribution across the hemispheres that is similar when
either eye is stimulated

37. THANK YOU