1. V I N J U V I S W A N A T H K
VISUAL EVOKED POTENTIAL
2. Visual Evoked Potentials testing measures the
signals from our visual pathway
Small gold cups called electrodes are pasted to your
head to allow us to record those signals
It is just like in a regular eye exam
7. FLASH VEP
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
8. PATTERN REVERSAL VEP
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
9. PATTERN ONSET/OFFSET VEP
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
10.
11. EQUIPMENT REQUIRED
Visual stimulus producing device
Scalp electrodes
Amplifier
Computer and read out systems
12. 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
13. Maximum Value for P100
P100 is 110 milliseconds (ms) in patients younger
than 60 years (it rises to 120 ms thereafter in females
and 125 ms in males. )
(Even though published norms are available in the
medical literature, each individual laboratory should
have its own norms to control for lab-to-lab
variability in technique. )
Interocular P100 latency difference is upto 5 – 6 ms.
> 10ms is gross abnprmality.
14.
15. Analysis Identify the waves
(NPNcomplex)
Determine the absolute peak latencies.
Determine the amplitude of the waves.
Determine the interocular latency difference.
16. Factors influencing VEP
The size of the checks
Pupillary size
Gender (women have slightly shorter P100 latencies )
Age: below 1 yr of age P100 may be 160ms, & above 60
yrs. also it get delayed.
Sedation and anesthesia abolish the VEP.
Visual acuity deterioration up to 20/200 does not alter
the response significantly .
Drugs.(eg. carbamazepine and sodium valproate prolong
P100 latency)
17. What is a normal VEP value?
A normal VEP response to a pattern-reversal
stimulus is a positive peak that occurs at a mean
latency of 100 ms. There are three separate phases in
the VEP waveform: an initial negative deflection
(N70), a prominent positive deflection (P100), and a
later negative deflection (N155)
