Electroretinogram and Clinical Applications by Dr. Vikas Khatri

1. ELECTRORETINOGRAM
Dr. Vikas Khatri

2. ERG is the corneal measure of an electrical potential generated by the retina
in response to a change in illumination which is recorded in biphasic waveform
ELECTRORETINOGRAPHY

3. HISTORY
1865 : First known recording of an ERG (amphibian retina)
Swedish Physiologist Alarik Frithiof
1908 : Einthoven and Jolly separated the ERG response into
three components: a-wave, b-wave and c-wave
1967 : Ragnar Granit Nobel Prize for Physiology and
Medicine (demonstrated the physiology of the receptor
potential of each component of the ERG)
Ragnar Granit

4. The electrical basis of ERG recordings
ERG responses are recorded with an Active
extracellular electrode positioned
• Cornea(Humans)
• Vitreous (Experimental Animals)
• At different levels inside the retina
(Experimental Animals)

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Active
Electrode
Negative
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ECF
The electrical basis of ERG recordings
ERG responses are recorded with an Active
extracellular electrode positioned
• Cornea(Humans)
• Vitreous (Experimental Animals)
• At different levels inside the retina
(Experimental Animals)

6. ERGPhotoreceptors, Bipolar Cells, Ganglion Cells and Muller Cells are
arranged in parallel and therefore, their currents are in parallel and
sum up, giving rise to a strong radial extracellular current
Cells arranged horizontally cancel the current of each other and cell
arranged obliquely have small currents.
Therefore, when homogenous light stimulation is directed at the
whole retina only radial extracellular currents are formed.
Current Pathway A - Current flowing through local route remaining
entirely within the retina
Current Pathway B - leaves the retina through the vitreous and
anterior ocular tissue and returns to the retina through sclera>
choroid>pigment epithelium layer

7. Electrical current flows through a resistor, a gradient of
electrical potential is formed
Ohms Law,
V = I x R
V - Potential Difference
I - Current
R - Resistance
Each tissue - Retina, Vitreous, Sclera, Choroid, Pigment
epithelium - behave as an electrical resistor
Change in one of the resistances will cause a change in
the magnitude of the current in the extraocular pathway
and the ERG can change irrespective of retinal function.
Patient undergone vitrectomy surgery and injection of
silicon oil into the vitreous, the resistance of the vitreous
increases by several folds causing the current to be so
reduced that the ERG becomes very small in amplitude.
So if V (Potential Differece by Retina) = Constant
Current inversely proportional to Resistance

8. Types of Electroretinogram
 Full Field ERG
 Focal ERG
 Multifocal ERG
 Pattern ERG

9. FULL FIELD ERG
If 20% or less of the retina is affected with a diseased state the Full-field ERGs are usually normal
Summed activity of all retinal cells, and consists of overlapping positive and negative component
potentials that originate from different stages of retinal processing
Amplifier
Stimulator Electrodes

10. 1. Active electrode
It’s the main electrode.
Can be Placed on :
• Cornea
• Conjunctiva
• Sclera
• Skin
Application of electrodes
Types of Electrodes :

11. 2. Reference electrode
Silver chloride electrode.
Placed on – Outer Canthi or Zygomatic fossa
Serves as the Negative pole as it is placed closer to
the electrically negative posterior pole of the eye.
Refrence
electrode
3. Ground electrode
Placed on – Forehead or Earlobe.

12. Recording Protocols
Dark room with non-reflecting walls
Full pupillary dilatation
30 minutes of dark adaptation
Rod response (Scotopic)
Maximum combined response
Oscillatory potentials
10 minutes of light adaptation
Single flash cone response (Photopic Response)
30 Hz flicker.

13. Stimulus for ERG
Stimulus and background light should be homogeneous and
cover the entire retina.
Strobe lamp and LEDs - mobile and can be easily placed in front
of a person whether sitting or reclining
The Ganzfeld bowl - large white bowl which is used to stimulate
the retina during the recording of the ERG.
It diffuses the light & allows equal stimulation of all parts of
retina.

14. a-Wave
Initial negative wave
Photoreceptors
Resting State ( No light)
Depolarized
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Glutamate
Hyperpolarised
Hyperpolarised
Light
Depolarised
No Inhibition on Bipolar Cells
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Third order Neuron
Na+
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15. b-Wave
Resting State ( No light)
Depolarized
Glutamate
Hyperpolarised
Hyperpolarised
Light
Depolarised
No Inhibition on Bipolar Cells
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Third order Neuron
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Muller Cell
Permeable to K+
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Positive wave from
Bipolar and Muller Cells

16. c-Wave
Retinal Pigment Epithelium
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Apical Membrane (Towards Retina)
More Permeable to Potential K+
Basal Membraen (Towards Choroid )
Less Permeable to Potential K+
The standing potential of the eye
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+ + + + +
- - - - - - - - - - - - -
+ + + + + + + + + +
- - - - - - - - - - - - -
+ + + + + + + + + +
++++++
Resting State ( No light)
Depolarized
Light > Hyperpolarised
Increased Extracellular
Positive Charge
Increase in Potential Difference due to light
induced electrical activity in the photoreceptors
ExtracellularSpace
c-wave
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17. • c-wave originates from the pigment epithelium, it depends upon the
integrity of the photoreceptors
• ERG c-wave can be used to assess the functional integrity of the
photoreceptors, the pigment epithelial cells and the interactions between
them

18. Scotopic Threshold Response
The effects of BaCl2 on the STR and extracellular potassium ion
concentration recorded from the proximal retina of the dark-
adapted cat. Barium ions eliminate the STR but have no effect
on the light-induced increase in extracellular potassium
concentration in the proximal
When very dim light stimuli (below the b-wave
threshold) are applied in the dark-adapted
state, a slow corneal negative potential is
recorded
Muller Cell response to K+ ions which affect the
membrane potential of Muller cells
Corneal negative component that is sometimes
followed by a positive component

19. Oscillatory Potentials (OPs)
When a bright light stimulus is used to elicit the ERG
in humans or in animals, low amplitude oscillating
waves can be identified on the rising phase of the b-
wave.
Frequency - 100-150Hz
Can be extracted with a band pass filter with a low-
frequency cutoff of 75 Hz.
Origin - Neuronal Inhibitory Feedback Circuits in
Inner Retina and Amacrine Cells
Isolating the oscillatory potentials from the bright flash ERG
response of the human eye (a) by applying a digital filter (b). An
FFT procedure was applied to the isolated oscillatory potential in
order to obtain the power spectrum (c)

20. Oscillatory potentials are significantly attenuated in various retinal degenerations amongst them are the following:
• Retinitis pigmentosa
• Central serous retinopathy
• Retinoschisis
• Early stage of Diabetic retinopathy
• Hypertensive retinopathy
• CRVO and CRAO
• Takayasu’s (pulseless) disease
Two patients with diabeticretinopathy (DR)
Delayed implicit time (Case 1)
Reduced amplitude (Case 2)

21. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Rod-mediated vision is very sensitive to dim light stimuli in the
dark-adapted state.
In Constant background illumination, Rod Photoreceptor
saturates and does not respond to light increment or decrement
ERG is of considerably larger amplitude (about 4 folds) and is
characterized by slow temporal properties; time to peak of the b-
wave is about 60ms
This ERG response is a mixed rod-cone response (the cone
system is operational too) but mainly reflects the activity in the
rod system since the cone system contribution is considerably
smaller.
Cone-mediated vision is not as sensitive but is characterized by
the ability to adapt to bright lights: processes that allow vision to
adapt to background illumination over a wide range of
intensities.
ERG under these conditions is of small amplitude but of very fast
kinetics

22. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
b-wave Amplitude increases
• Size of the pupil is the major determinant of light intensity to the retina
• 3 fold change in pupil diameter = 9 fold change in light intensity reaching the retina
a-wave precedes b-wave
a-wave and the b-wave increase in
amplitude
Oscillatory potentials seen on the
rising phase of the b-wave
b-wave Amplitude increases

23.  State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Scotopic Condition
- STR (Scotopic Threshold Response)
First wave at -8.2 log units
- b-wave
Appears at -5.8 log units
Saturates at -3.4 log unit
- a-wave
Appears at -1.7 log unit
- OP (Oscillatory Potential)
Light intensity higher than -0.8 log units
0 log unit = is 44.2 candela/meter2/second1
Factors affecting the ERG

24. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Photopic Condition
Short-flash ERGs elicited by increasing stimulus intensities
b-wave maximum amplitude at 3.0 log unit
Photopic Hill Phenomenon – b-wave amplitude decreases
after maximum flash intensity forming inverted U shape
(cone photoreceptor desensitization and Pigment bleach)

25. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Rod curve peaks at the blue-green region of the visible spectrum
(around 500nm)
Spectral sensitivity curve of cone-mediated vision is the sum of all three spectral types of cone;
long-wavelength sensitive (red),
medium-wavelength sensitive (green),
short-wavelength sensitive (blue),
In net, Cone has peak sensitivity in the orange range of the visible spectrum (around 560nm)

26. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Dark-adapted state
Dim Blue Stimuli Bright Red Stimuli
The red light stimulus
produces an ERG response
composed of two parts;
a fast wave (30ms)
a slow wave (100ms)
The blue stimulus elicits a
slow positive ERG of the more
sensitive rod system
With this procedure, cone-mediated function can be isolated from the large amplitude
rod ERG and allows analysis of cone vs rod system in the dark-adapted state
Equality in the slow ERG
component when two light
stimuli were balanced

28. Factors affecting the ERG
 State of adaptation
 Light intensity
 Colour of light stimulus
 Frequency of photo stimulation
Separate rod-mediated vision from cone-mediated vision
Maximum frequency of stimulation that can be perceived
as flickering – Critical Fusion Frequency(CFF)
• Highest CFF - Rod vision – 15-20Hz
• Highest CFF - Cone vision – 30-50Hz
Bright light stimuli at a frequency of 30Hz is applied in
order to isolate the cone system from the rod system

29. International Society for Clinical Electrophysiology of Vision (ISCEV)
Standardised the protocols for performing electrophysiological tests (1989)
Ensures uniformity and thus comparability between labs
Dark-adapted 0.01 ERG
A rod-driven response of bipolar cells
Dark-adapted 3.0 ERG
Combined responses arising from photoreceptors
and bipolar cells of both the rod and cone systems;
rod dominated

30. Dark-adapted 3.0 ERG with Band-pass Filter
Oscillatory potentials
Responses primarily from Amacrine cells
Dark- adapted 10 ERG
Combined response with enhanced a-waves
reflecting photoreceptor function

31. Light- adapted 30 Hz flicker ERG
A sensitive cone-pathway-driven response
Light-adapted 3.0 ERG
A cone-driven response of bipolar cells
 Recorded with a stimulus intensity of 3.3 log units
 Background illumination - 40 candela/meter2/second (Sufficient to suppress all rod activity)

32. Measurement of ERG Components
Amplitude
a-Wave amplitude – baseline to trough of a-wave
b-Wave amplitude – trough of a-wave to peak of b-wave
Time Sequences
Latency – Stimulus to onset of wave response
Implicit Time – Stimulus to Maximum wave response

33. Ratio of b-wave to a-wave
ERG analysis that is based only upon amplitude measurements may lead to
erroneous conclusions if the :
• Pupil is not maximally dilated
• Different recordings conditions
• Exchange of information between laboratories
So this Ratio give us information about normal signal transmission between the
photoreceptor and post-synaptic neurons

34. ELECTRORETINOGRAM
CLINICAL APPLICATIONS

35. Normal ERG
Normal ERG can be seen in patients of :
• Localized macular dysfunction
• Optic nerve diseases
• Central nervous system disease such as amblyopia.

36. Subnormal ERG
Amplitudes of all components are reduced approximately to the same degree
• Early stages of rod–cone dystrophy
• PRP for Diabetic retinopathy (ERG components are reduced by 40–45%) b/a ratio remain normal
• Vitreous Haemorrhage (Hazy Media) - ERG + USG can be used to differentiate between Total RD and Dense
Vitreous Membrane
Case 1 - ERG is recordable, even if the amplitude is
small, the thick membrane in the vitreous cavity is not totally
detached retina, but vitreous membrane
Case 2 - When the ERG is unrecordable, the thick membrane
is most likely totally detached retina

37. • Localized damage of the photoreceptors (Partial RD or Sectoral RD) - Amplitude of the full-field ERG is
proportional to the area of functioning retina.
Reduction of the ERG amplitude corresponds proportionally to the extent of RD

38. Negative ERG
Indicates that the amplitude of the b-wave is smaller than that of the a-wave (b/a ratio <1.0)
Normal a-wave with a reduced b-wave localizes the defect to post-synaptic photo transduction processes
Negative ERG can be of useful Prognostic or Diagnostic value in retinal diseases
Endophthalmitis after intraocular lens implantation
Endophthalmitis (<1 Week)
b/a ratio < 1.0
Endophthalmitis (<1 Week)
b/a ratio > 1.0
Late-onset endophthalmitis
b/a ratio > 1.0
Good PrognosisWorst Prognosis
Should Undergo
Vitrectomy Urgently

39. Central Retinal Artery Occlusion
No b-Wave
a-Wave Recordable : Outer Retina - Choroidal perfusion
Ophthalmic artery occlusions usually result in
unrecordable ERGs

40. Central Retinal Vein Occlusion
Ischemic CRVO shows Negative ERG more frequently than Non Ischemic type
b/a ratio can be an important index for evaluating the prognosis of CRVO

41. Proliferative Diabetic Retinopathy
If patient present with VH it is difficult to predict the surgical and visual outcome
after vitrectomy
Most diabetic patients with vitreous hemorrhage have already undergone PRP and
PRP reduces the ERG amplitude without changing the b/a ratio, So it is difficult to
arrive at a prognosis of the outcome after vitrectomy using only the amplitudes
b/a ratio provides more useful information about the visual prognosis after
vitrectomy

42. b/a ratio>1.0 and the OPs are clearly recordable -
b/a ratio >1.0 but the OPs are absent -
b/a ratio <1.0 with absent Ops -
Hiraiwa T, Horio N, Terasaki H, et al. Preoperative electroretinogram and postoperative visual outcome in patients with diabetic vitreous hemorrhage. Jpn J Ophthalmol 2003;47:307–11.
The postoperative visual acuity for group C was significantly worse than for group A or group B
This observation is important when we discuss the visual prognosis with patients before surgery

43. ERG recordings in a normal patient and one with retinitis
pigmentosa with residual cone physiology
Retinitis Pigmentosa
ERG is usually normal or only slightly subnormal in these
infectious diseases.
Pigment in the retina is prominent in many infectious
diseases. Early stages of Syphilis and Rubella can mimic
the fundus appearance of RP

44. Full-field ERGs are best for quantifying cone dystrophy
Scotopic ERGs fairly normal in appearance but with
slow implicit times.
Cone Dystrophy
30 Hz flicker ERG
Photopic white ERG
Dependent on
Functioning of Cones

45. Stargardt’s disease
Full-field ERGs in these disorders are normal except
in very late stages where full-field ERGs may
become slightly subnormal.
Macular multifocal ERGs are dramatically abnormal.

46. Congenital Stationary Night Blindness
CSNB show essentially normal fundi and most patients with CSNB have moderately low visual acuity,
So, ERG can be used to differentiate them from other diseases with normal fundi, low visual acuity, and
normal ERG.
Normal a-wave
b/a < 1.0
Normal a-wave
b/a > 1.0
Small a-wave
b/a < 1.0
Without
Photopic Hill Phenomenon
Small a-wave
b/a < 1.0
With
Photopic Hill Phenomenon
• Psychological eye
problems
• Amblyopia
• Optic nerve disease
• CNS disease
• Congenital
Stationary Night
Blindness
• Retinitis Pigmentosa • Oguchis disease

48. Foreign bodies and Trauma
Estimate the extent of retinal dysfunction
Small piece of stainless steel or plastic
outside the macula may have a minor affect
on the retina.
Copper or Iron would likely have
deleterious affects within a few weeks
In general if b-wave amplitudes are
reduced 50% or greater compared to the
fellow eye, it is unlikely that the retinal
physiology will recover unless the foreign
body is removed.
Fundus photo of a patient with a hole in the retina caused
by a metallic foreign body

50. Multifocal electroretinography is better way to assess the
Drug Toxicity and should be Screened every 5-6 months
to see Functional Retinal Changes
Chloroquine Retinopathy

51. Extinct ERG
Advanced stage of rod–cone dystrophy
• Retinitis pigmentosa
• Gyrate atrophy
• Choroideremia
• Total Retinal Detachment
 Even when the macular area is preserved, ERG may become undetectable.

52. FOCAL ERG
Evaluate Macular Function
Change in macular OPs are the most sensitive indicator in variable macular diseases.

53. Selective reduction of macular OP
amplitude is observed in :
• Macular Edema
• Epimacular membrane
• Central Serous Chorioretinopathy
Pseudophakic Cystoid Macular Edema

54. Occult macular dystrophy (OMD)
Focal ERG or Multifocal ERG - key for
diagnosis
Normal fundus, FFA, Full-field ERGs,
but abnormal focal macular ERG, multifocal
ERG, OCT show mild Photoreceptor
abnormalities

55. MULTIFOCAL ERG
Record many focal retinal responses simultaneously in a
brief time period
Array of hexagons 64 or 103 hexagons over 30–40° of
central visual field
Recorded under photopic condition (To asses Foveal
response)
Every frame change (13.3ms) of the monitor, each
hexagon can be either white or black based on a
pseudorandom probability sequence (called an m-
sequence)

57. Photoreceptor
layer
Disruption
Normal Occult Macular Dystrophy

58. Full-field ERG showing Normal Response
Occult Macular Dystrophy

59. Age Related Macular
Degeneration (AMD)

60. Patient with Early Age Related Macular Degeneration (AMD)

61. Multifocal ERG recordings in a patient with
Stargart’s disease

62. PATTERN ERG
Measure of macular function and generalized
bipolar cell function.
Checkerboard stimulus composed of white and
black squares.
Reduction of PERG amplitude reflect the reduced
activity of dysfunctional RGCs.
Inner retinal activity under light-adaptation.

63. The Normal Pattern Electroretinogram :
• N35- a small negative component with a peak time occurring around 35ms
• P50- a prominent positive wave emerging around 50ms
• N95- a wide negative wave around 95ms

64. Macular diseases:
• The P50 component was shown to be altered in all patients with
retinal and macular diseases.
Optic nerve disease:
• N95 component was abnormal in 81% of patients with diseases of
the optic nerve. The P50 component remain normal.

65. Thank You