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1. DR BHAVIN J PATEL
SR NEUROLOGY
GMC KOTA
Visual Evoked
Potentials
2. Evoked Potential
ī 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.
ī eg. SEP, BAER and VEP
3. VEP
īVEPs are electrophysiologic responses recoeded from scalp in response to
stimulation by either patterned or unpatterned visual stimuli.
īStimulation at a relatively low rate (up to 4/s) will produce âtransientâ VEPs
īStimulation at higher rates (10/s or higher) persist for the duration of the
stimulation and are referred to as âsteady-stateâ VEPs.
īResponses evoked by patterned stimuli are âpatternâ VEPs
īResponses evoked by unpatterned stimuli are âflashâ VEPs
4. Choice of Stimulus
īPatterned visual stimuli elicit responses that have far less intra- and
interindividual variability
īGreater sensitivity and accuracy
īCheckerboard pattern reversal is the most widely
īUnpatterned stimuli are generally reserved for patients who are
unable to fixate or to attend to the stimulus
6. Pretest Evaluation
īTest should be explained
īAbility to fixate important throughout
īAvoid Hair Spray or Oil
īCycloplegics generally should not be used
īSubjects with refractive errors should be tested with appropriate
corrective lenses
7. Electrode Placement
īStandard Disc EEG electrodes used
īSkin is prepared by abrading and degreasing.
īActive/Recording Electrode Placed at Oz in midline 4cm above
Inion
īReference Electrode FPz 12 cm above Nasion.
īGround Electrode placed at vertex Cz
8. Electrode Placement
Montages â International federation of Clinical Neurophysiology (IFCN) recommends 2 channels
minimum
ī§Channel1 â Oz â Fpz
ī§Channel 2 â Oz â Linked ear
ī Four Channel montage
ī§Channel 1 : Oz âFpz
ī§Channel 2- Pz- Fpz
ī§Channel 3 â L5-Fpz
ī§Channel 4 âR5 -Fpz
9.
10. Stimulus field types
īPattern that extends equally to both sides of the fixation point is referred to as a
full-field stimulus
īA pattern presented to one side of the fixation point in one-half â Half field
stimulus
īPattern presented to a small sector of the visual field is designated a partial-field
stimulus
īhalf-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
11.
12. Pattern Reversal Visual Evoked
Potential Testing
īNegative and positive polarities are designated N and P, respectively.
īPeak latencies are expressed in milliseconds
īPeaks N75, P100, and N145 are recorded over the occiput
īWave Nl00 is recorded from the midfrontal region
īN145 is highly variable and is not used for standard test interpretation
īType of pattern.- Checkerboard ,Bar and sinusoidal grating stimuli
13. Waveforms
(The NPN complex)
ī§The initial negative peak (N1 or N75)
ī§A large positive peak (P1 or P100)
ī§Negative peak (N2 or N145)
N75
P100
N145
14.
15. Flash Visual Evoked Potential Testing
īLimited to: (1) subjects with severe refractive errors or opacity of ocular media
(2) Subjects who are too young or too uncooperative
īResults should demonstrate reproducible peak positive responses to flash
stimulation
īUnpatterned visual stimuli commonly consist of brief flashes of light with no
discernible pattern or contour
ī(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
16. Factors Affecting VEP
īThe size of the checks
īPupillary size
īGender (women have slightly shorter P100 latencies),
īAge
- Children have large amplitude and latency prolonged.
- After 50 yr of age latency prolonged by 2.5 ms/decade.
īSedation and anesthesia abolish the VEP.
īVisual acuity deterioration up to 20/200 does not alter the response significantly .
īDrugs.
17. Clinically Significant Abnormality
īChanges in latency, amplitude, topography, and waveform
īP100 latency prolongation is the most reliable indicator
īWaveform abnormalities are generally subjective in nature and difficult to
quantify
īAmplitude affected by technical Factors wide individual variation â Hence
interoccular amplitude ratio used
īP100 is 110 milliseconds (ms) in patients younger than 60 years .
18. Clinical Applications of VEP
īVEPs are most useful for testing optic nerve function and less useful for
assessing postchiasmatic disorders
īNon Specific for etiology
īPartial-field studies may be useful for retrochiasmatic lesions; however, they are
not performed routinely
īVEP may be abnormal ( low amplitude ) in Refractive error severe ,Retinal
diseases
19. Clinical Applications of VEP
īOptic neuritis-MS â P100 latencies prolonged with or without amplitude loss
īNMO â unrecordable P100 waveform with reduced amplitude more likely
īIschemic optic neuropathy â Attenuation of amplitude earlier than latency
īVit B12 deficency â Bilateral asymmetric prolonged p100 latencies
īHIV infection:- prolonged latency in initial stage f/b decrease amplitude.
20. Clinical Applications of VEP
īToxic neuropathy:- decrease amplitude in toxic while prolong latency secondary
to drugs.
īPapilledema only â VEP not affected
īHereditary disease:- reduction in amplitude without prolongation of latency
īDegenerative disease:- prolongation of latency in parkinsonian patient
īCompressive neuropathy:- decrease amplitude with minimal prolongation of
latency
21. VEP in cortical blindness
īSome reports suggest that VEP may show a varied result Or normal VEP
īOther reports suggest prognostic importance of VEP with absent VEP response
foretelling poor prognosis
īINCONSISTENT PATTERN
23. 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 (12â16â) and small fields (2-4Ë) 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,
Editor's Notes
P100 amplitude decreased and latency prolonged when pupils constricted .