This document discusses electrodiagnostic tests including electroretinography (ERG) and visual evoked potentials (VEP). It provides details on: - The components and origins of ERG waves - How ERG represents function of the retina up to the bipolar cell level - Techniques for ERG recording and factors that influence results - Clinical applications of ERG in evaluating retinal diseases It also outlines: - What VEP measures and the types of VEP tests - The waves seen on pattern and flash VEP - Factors that can influence VEP results - Clinical uses of VEP in assessing conditions of the visual pathway and optic nerve.

1. ELECTRO-DIAGNOSTIC
TESTS
(ERG, VER)
Dr Pravda Chaturvedi
II year, MS Ophthalmology
Dr S N Medical College.
cpravda17@yahoo.com

2. ERG
The graphical tracing of the summated
action potential generated in retina in
response to change in retinal illumination is
called ELECTRORETINOGRAM.
First recorded by Frithiof Holmgren (1865)
In humans by Dewar (1877)
Extensive work thereafter by Riggs (1941)
Sutter and Tran developed multifocal ERG in
1992.

3. ERG doesn’t represent horizontal cells, amacrine
cells and ganglion cell function.
It represent RPE, photoreceptors, bipolar cells
and muller cells.

4. ERG: components

5. ERG: components
‘a’ is negative wave. Amplitude is from
baseline to trough & implicit time is from
onset of stimulus to trough of ‘a’ wave
‘b’ is large positive wave. Amplitude is
trough of ‘a’ to peak of ‘b’ & implicit time is
from onset of stimulus to peak of ‘b’
‘c’ is lower amplitude, prolonged +ve wave,
shows metabolic activity of RPE. Clinically
not much significant.

6. ERG: origin
‘a’ origin: photoreceptors
‘b’ origin: Muller’s cells + bipolar cells.
Mainly from Muller’s in response to
increase (ECF) K+
in bipolars
‘c’ origin: RPE
Oscillatory potentials (small wavelets on
ascending limb of ‘b’) from amacrine
cells. Feedback inhibitory potential.

7. Physiologic basis of ERG
 a wave –
I.Light falling – Hyperpolarisation
II.Outer portion of photoreceptor – positive
III. Inner portion - negative
IV. both rods and cones contribute.
V.Appropriate stimulus distingushes the origin.
VI.Blue dim flash light: rod
VII.Bright red flash light: cone

8. Physiologic basis of ERG
b wave-
I. Positive wave. Source muller cells.
II. Light strikes photoreceptor, releases
potassium, membrane potential of muller
cells change.
III. Factors affecting a wave influence b wave
too.
IV. Oscillatory potential are the feedback
inhibition waves. Seen in photopic recordings
only.

9. Physiologic basis of ERG
C wave –
I.RPE – in response to rod signals only.
II. Direct contact of rod cells with RPE.

10. Measurement of components

11. ERG recording
Electrodes:
 active,
 Reference
 ground
Scotopic trial: dark adapted retina for 20
mins. Red flash, blue filter, white light.
Photopic trial: dark lamp, room light,
granezfeld bowl. Red flash, white flash, white
flash for oscillating potential, 40 hz flicker
test.

13. Scotopic vs Photopic
Scotopic:
• After 20 mins of dark adaptation
• Large amplitude and large implicit time.
• Response of 125 million rods + 6.5 million cones.
• Isolated Rod function: stimulate fully dark
adapted eye with a flash of dim blue light.

14. Photopic:
• Under fully light adapted eye with bright flash of
white or red light. This suppresses rod response.
• Lower amplitude and shorter implicit time.
• Both cone and rod responses can be isolated with
orange red stimulus.
• Isolated cone response: flicker light stimulus of
30-40 Hz

15. Factors influencing ERG
1. Stimulus,
2. Recording equipment.
3. Dark adaptation.
4. Age and sex.

16. Factors influencing ERG
1.Stimulus –
• With increase in stimulus, a wave continues to
increase in size and b wave reaches its max.
• Latency shortens.
• Flicker light above 30 Hz stimulates cones only.
• At higher frequencies ERG becomes sinusoidal.

17. Factors influencing ERG
2. Recording equipment
Faulty connection and inappropriately
positioned lead can give wrong waveforms.
3. Dark adaptation
ERG increases in size and b wave becomes slower
4. Age and sex
ERG reaches its adult value by 2 years, it
declines with age and women have slightly
larger ERG.

18. ERG Waveforms
1. Normal Waveforms Rod response / scotopic
blue / dim white
usually smoother, dome shaped. Initial –ve ‘a’ wave is not seen & is
hidden by ‘b’.
Longer implicit time. Only rods contribute

19. 2. Max combined response /
scotopic white flash /
mesopic response
deep ‘a’ wave with tall ‘b’.
Longer
implicit, larger
amplitude &
cones contribute

20. 3. Oscillatory potentials
5

21. 4. Single flash cone response / photopic
white flash
small ‘a’ & ‘b’ waves. Waveforms are
more peaked with shorter implicit
& smaller amplitude. Cone function

22. 5. 30 hz flicker
multiple peaked waveforms. Cone
function

23. mfERG
Records central 40-50 degrees of retina under
light adapted conditions.
It records 61 or 103 hexagons.
Responses are displayed individually and
presented as topography.
Most useful in assessment of macular functions

24. Clinical Applications
Useful for functional abnormalities of retina up
to bipolar cell.
ERG is normal in disorders of ganglion cell layer
and higher visual pathway.
Measures diffuse response of retina.

25. Clinical Applications
1. Useful in assesment of retinal function
in case of media opacities.
2. Retinitis pigmentosa and other
inherited retinal dystrophies
3. Diabetic retinopathy and vascular
occlusions.
4. Retinal detachment.
5. Toxic state of RPE, cancer associated
retinopathy, Vit A deficiency

26. Visual
Evoked
Potential (Response)
VEP / VER

27. Visual evoked potential
Gross electrical signal generated at visual
cortex in response to visual stimuli
 Impulses carried to visual cortex via visual
pathway
 Recorded by EEG
 It is the only objective technique to assess
clinical and functional state of visual pathway
beyond retinal ganglion cells.

28. Types of VEP
1) Pattern VEP
checker-board patterns on TV monitor
2) Flash VEP
diffuse flash light for uncooperative
subjects, simpler test, can be used
in opaque or hazy media, closed lids

29. VEP
Pattern VEP then Flash VEP then Bright
Flash VEP.
Patient provided spectacle correction
Un-dilated pupils.
 Seated 1 meter from monitor, one eye
checked at a time.
Electrodes in midline at forehead, vertex
& occipital lobes
2-3 different checker sizes are shown
Recording is done

30. Technique
No distracting light or sound.
Equipments: stimulus producing device,
electrodes, amplifier, reading and display system.
Flash VEP: Ganzfeld hemisphere bowl, flash
repeated 75 times.
Bright flash: 50 stimuli, if awake aptient has to
look at stimulator 5cm away.

32. VEP
Pattern VEP
initial +ve (P1) -ve(N1)second+ve (P2) wave
Positive wave – 70 100 ms
Negative wave – 100 – 130 ms
Positive wave - 150 –200 ms
Amplitude is 10-25 microns and reaches its adult value
by 6 months.
Abnormal <10microns, Absent <3microns
Flash VEP:
Complex, M shaped multiphasic curve, two +ve, two –ve
waves. Several secondary waves.

35. Factors Influencing
1. Stimulus
2. Position of electrodes
3. Age and sex: child has larger readings, reaches
peak at 5-8 yrs, gradual decline seen in adults.
4. Attention of patient to the stimulus

36. Clinical applications
Reduced amplitude
• Poor co operation
• Refractive errors
• Amblyopia
• Purposeful defocussing
• Optic nerve hypoplasia, compression, ant ischaemic optic
neuropathy
b/l symmetery
Change in latency
Gross abnormality in flash VER

37. VEP Indications
1. optic nerve disease
2. Visual acuity
3. Malingering, hysterical blindness
4. Visual potential in opaque ocular media
5. Unexplained visual loss
6. Amblyopia
7. Glaucoma