Vep

DR BHAVIN J PATEL
SR NEUROLOGY
GMC KOTA
Visual Evoked
Potentials

Evoked Potential
 Electrical potentials that occur in the cortex after stimulation of a sense organ
which can be recorded by surface electrodes is known as Evoked Potential.
 eg. SEP, BAER and VEP

VEP
VEPs are electrophysiologic responses recoeded from scalp in response to
stimulation by either patterned or unpatterned visual stimuli.
Stimulation at a relatively low rate (up to 4/s) will produce “transient” VEPs
Stimulation at higher rates (10/s or higher) persist for the duration of the
stimulation and are referred to as “steady-state” VEPs.
Responses evoked by patterned stimuli are “pattern” VEPs
Responses evoked by unpatterned stimuli are “flash” VEPs

Choice of Stimulus
Patterned visual stimuli elicit responses that have far less intra- and
interindividual variability
Greater sensitivity and accuracy
Checkerboard pattern reversal is the most widely
Unpatterned stimuli are generally reserved for patients who are
unable to fixate or to attend to the stimulus

Physiologic basis
The generator site for VEPs is believed to be the peristriate
and striate occipital cortex

Pretest Evaluation
Test should be explained
Ability to fixate important throughout
Avoid Hair Spray or Oil
Cycloplegics generally should not be used
Subjects with refractive errors should be tested with appropriate
corrective lenses

Electrode Placement
Standard Disc EEG electrodes used
Skin is prepared by abrading and degreasing.
Active/Recording Electrode Placed at Oz in midline 4cm above
Inion
Reference Electrode FPz 12 cm above Nasion.
Ground Electrode placed at vertex Cz

Electrode Placement
Montages – International federation of Clinical Neurophysiology (IFCN) recommends 2 channels
minimum
Channel1 – Oz – Fpz
Channel 2 – Oz – Linked ear
 Four Channel montage
Channel 1 : Oz –Fpz
Channel 2- Pz- Fpz
Channel 3 – L5-Fpz
Channel 4 –R5 -Fpz

Stimulus field types
Pattern that extends equally to both sides of the fixation point is referred to as a
full-field stimulus
A pattern presented to one side of the fixation point in one-half – Half field
stimulus
Pattern presented to a small sector of the visual field is designated a partial-field
stimulus
half-field or partial-field stimuli are used, the fixation point should be displaced
to the nonstimulated visual field by a small amount, to prevent stimulation of
both retinal hemifields

Pattern Reversal Visual Evoked
Potential Testing
Negative and positive polarities are designated N and P, respectively.
Peak latencies are expressed in milliseconds
Peaks N75, P100, and N145 are recorded over the occiput
Wave Nl00 is recorded from the midfrontal region
N145 is highly variable and is not used for standard test interpretation
Type of pattern.- Checkerboard ,Bar and sinusoidal grating stimuli

Waveforms
(The NPN complex)
The initial negative peak (N1 or N75)
A large positive peak (P1 or P100)
Negative peak (N2 or N145)
N75
P100
N145

Flash Visual Evoked Potential Testing
Limited to: (1) subjects with severe refractive errors or opacity of ocular media
(2) Subjects who are too young or too uncooperative
Results should demonstrate reproducible peak positive responses to flash
stimulation
Unpatterned visual stimuli commonly consist of brief flashes of light with no
discernible pattern or contour
(LED) board can be viewed from a distance or LED goggles can be placed
directly over the eyes. Goggles have the advantage of producing a very large
field of stimulation that minimizes the effect of changes in direction of gaze

Factors Affecting VEP
The size of the checks
Pupillary size
Gender (women have slightly shorter P100 latencies),
Age
- Children have large amplitude and latency prolonged.
- After 50 yr of age latency prolonged by 2.5 ms/decade.
Sedation and anesthesia abolish the VEP.
Visual acuity deterioration up to 20/200 does not alter the response significantly .
Drugs.

Clinically Significant Abnormality
Changes in latency, amplitude, topography, and waveform
P100 latency prolongation is the most reliable indicator
Waveform abnormalities are generally subjective in nature and difficult to
quantify
Amplitude affected by technical Factors wide individual variation – Hence
interoccular amplitude ratio used
P100 is 110 milliseconds (ms) in patients younger than 60 years .

Clinical Applications of VEP
VEPs are most useful for testing optic nerve function and less useful for
assessing postchiasmatic disorders
Non Specific for etiology
Partial-field studies may be useful for retrochiasmatic lesions; however, they are
not performed routinely
VEP may be abnormal ( low amplitude ) in Refractive error severe ,Retinal
diseases

Optic neuritis-MS – P100 latencies prolonged with or without amplitude loss
NMO – unrecordable P100 waveform with reduced amplitude more likely
Ischemic optic neuropathy – Attenuation of amplitude earlier than latency
Vit B12 deficency – Bilateral asymmetric prolonged p100 latencies
HIV infection:- prolonged latency in initial stage f/b decrease amplitude.

Toxic neuropathy:- decrease amplitude in toxic while prolong latency secondary
to drugs.
Papilledema only – VEP not affected
Hereditary disease:- reduction in amplitude without prolongation of latency
Degenerative disease:- prolongation of latency in parkinsonian patient
Compressive neuropathy:- decrease amplitude with minimal prolongation of
latency

VEP in cortical blindness
Some reports suggest that VEP may show a varied result Or normal VEP
Other reports suggest prognostic importance of VEP with absent VEP response
foretelling poor prognosis
INCONSISTENT PATTERN

Test Protocol for Full-Field Stimulation
Full-field PVEP testing is most sensitive in detecting lesions of the visual system anterior to the
optic chiasm
should be performed monocularly,
black-and-white checkerboard pattern,
at a reversal rate of 4/s or less.
The subject should be placed no closer than 70 cm to the stimulus screen.
Small checks (12—16‟) and small fields (2-4˚) selectively stimulate central vision. These
responses are particularly sensitive to defocusing and decreased visual acuity
Recommended recording time window (ie, the sweep length) is 250 msec; 50-200 responses are
to be averaged. A minimum of 2 trials should be given,

Vep

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