The document discusses visual evoked potentials (VEPs), which involve recording electrical signals from the visual cortex in response to visual stimulation. It notes that VEPs have smaller amplitudes than EEGs but can objectively assess macular function and the functional state of the visual system. It describes how steady state VEPs use rapid stimulation to produce sinusoidal waveforms while transient VEPs use discrete deflections with low rates of stimulation. It also discusses the use of flash VEPs, pattern VEPs, and other techniques and provides details on stimulation methods, components of VEP testing equipment, factors that influence VEPs, and abnormal findings that may be observed in various conditions.

2. • Record of gross electrical signal generated at
visual (occipital) cortex in response to visual
stimulation
• The amplitude of VEP-3 to 25 microvolt is
considerably smaller than that of EEG, which
can be as large as 100 microvolt

3. • Averaged and amplified record of action
potentials in visual cortex
• VEP is the only objective technique available
to assess clinically the functional state of the
visual system beyond the retinal ganglion
cells.

4. VEP
abnormality
Visual pathway
abnormality
Non specific

5. • Since foveal projection is magnified at cortex ,
it is an objective indication of macular
function

6. • At rapid rates of stimulation the waveform
becomes sinusoidal – STEADY STATE VEP
• Not used routinely due to inferior information
on latency
• Low rates of stimulation – discrete deflections
• Known as TRANSIENT VEP
• This is commonly employed

8. • FLASH
• Occipital cortex is
relatively insensitive to
flash
• PATTERN
• Cortex is sensitive to
edges of contrast

9. FLASH VEP
PATTERN REVERSAL
VEP
PATTERN
ONSET/OFFSET VEP

11. • Response to diffusely flashing light stimulus
that subtends a visual field of 20 degrees
• Cruder response than pattern VEP
• Merely indicates that light has been perceived
by cortex
• Indications – media haze, infants,poor patient
cooperation

12. • Response to a patterned stimulus -
checkerboard or square and sine wave
gratings
• Frequency of gratings is decribed 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

13. • A pattern is abruptly exchanged with an
equilluminant diffuse background
• More intersubject variability than pattern
reversal VEP
• Useful in detection of pts with malingering ,
pts with nystagmus

14. • 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
• Significantly affected by eccentric fixation,
excessive blinking of eyes and partial closure
of eyes

15. • Visual stimulus
producing device
• Scalp electrodes
• Amplifier
• Computer and read out
systems

17. • Undilated pupil
• Monocular recording
• Refractive correction
• Relaxed position
• 1m distance from monitor

18. AMPLITUDE
LATENCY

22. • P 100 LATENCY ( m sec ) = 102  5
• R-L difference ( msec) = 1.3  2.0
• Amplitude (μV) =10  4.2
• Duration = 63  8.7

23. • If acuity of the patient is in question , the
amplitude is more important
• If detection of a lesion in visual pathway is in
question , latency is more important
• Latency is more reliable than amplitude
(Variabilty – 5% as compared to 25%)

24. • Presence of reduced amplitude is non specific,
gains importance only on serial testing
• B/L symmetry is seen both with flash and
pattern VEP , thus an asymmetrical response
is more indicative of an abnormality

25. • SIZE OF STIMULUS – Decrease in size of
stimulus increases amplitude of VEP
• Position of electrodes on scalp
• AGE- amplitude decreases with age
• ATTENTION OF PATIENT – If subject looks to
side of stimulus , there is rapid fall in size of
response

26. DELAYED LATENCY
• Demyelinating optic
neuritis
• Neurotransmitter
disorders
• Glaucoma
• Uncorrected refractive
error
• Ageing
REDUCED AMPLITUDE
• Optic atrophy
• Toxic
• Compressive
• Uncorrected refractive
error
• Ambylopia

27. • OPTIC NEURITIS / MULTIPLE SCLEROSIS
Increased latency of P100
Even when no defect in visual acuity , colour
vision or field of vision.
About 96% of pts with MS have delayed
latency

28. • COMPRESSIVE OPTIC NERVE LESIONS –
decreased amplitude without much change in
latency
• 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

29. • DEGENERATIVE DISEASES that affect the
spinal cord, cerebellar pathways, or both, such
as Friedreich's ataxia, Huntington's disease,
neurosyphilis, and AIDS, also can affect the
optic nerves and cause visual defects,
including a delay of the pattern VEP

30. • The earliest VEP abnormalities in LEBER'S
optic neuropathy appear to be increases in
P100 latency or changes in the waveform
morphology (i.e., the development a double
positive peak).
• As the condition progresses, the VEP
amplitude decreases to a point where
responses become immeasurable.

31. • Patients with THYROID OPHTHALMOPATHY
may have a prolonged latency of the pattern
VEP before a clinically apparent optic
neuropathy.55
• SUBACUTE COMBINED DEGENERATION
secondary to vitamin B12 deficiency causes
demyelination and prolongs the pattern VEP
latency,56,57 even with an unremarkable
neuroophthalmologic examination.58

32. • AION
Low amplitude but normal latency
TO ASSESS MISPROJECTION OF OPTIC NERVE
FIBRES IN ALBINISM
Nerve fibers that originate in the temporal retina
are misrouted at the optic chiasm. This
misrouting results in an anomalous temporal
nerve fiber decussation and an abnormal
projection to the occipital cortex.-definite VEP
asymmetry

33. • TO ASSESS VISUAL POTENTIAL IN PATIENTS WITH
OPAQUE MEDIA
• Flash VEPs may be useful for detecting
maculopathy or optic neuropathy in patients with
dense media opacities.
• An amplitude reduction of more than 50% or a
latency delay of more than 15 ms is highly
suggestive of dysfunction in the central visual
field.
• important in patients with opacities who are at
high risk for neuronal dysfunction, such as
patients with diabetes, ocular hypertension, or
ocular trauma.

34. • TO ASSESS VISUAL ACUITY IN NON VERBAL
CHILDREN , MENTALLY CHALLENGED AND
APHASIC PATIENTS
• Useful in assessing the integrity of macula and
visual pathway
• Pattern VEP gives a rough estimate of visual
acuity objectively

35. • EVALUATION OF OPTIC NERVE FUNCTION IN
PATIENTS WITH HEAD INJURY
• Pupillary reflexes are often inaccessible because
of periocular edema and/or motoric pupillary
involvement, and patients are often comatose or
sedated. Under these circumstances, the flash
VEP can provide valuable information regarding
optic nerve integrity.
AMBYLOPIA – decrease in amplitude with
relative sparing of latency

36. • GLAUCOMA
• Decreases in pattern VEP amplitude and
prolonged VEP latencies are found in many
patients with glaucoma and some glaucoma
suspects
• Steady-state VEPs appear to be more
sensitive for detecting glaucomatous damage
thantransient responses

37. • MALINGERING AND HYSTERICAL BLINDNESS
• R/O malingering by confirming the fact that
visual pathway is intact even in patients
claiming no PL
• Hysterical response – shows large variations
from moment to moment , eg first half may
produce an absent VEP and second half may
produce normal VEP

38. • VISUAL FIELD DEFECTS
• Asymmetry of amplitudes of VEP recorded
over each hemisphere implies a hemianopic
visual pattern
• Decreased amplitude of VEP recorded over
contralateral hemisphere when each eye is
stimulated separately indicates bitemporal
field defect

39. • Generated simultanously from 60 regions of
central 20 to 25 degrees of visual field
• Local defects are easily missed in conventional
VEPS
• Can detect local demylination - f/u of cases of
optic neuritis
• To confirm unreliable visual fields

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