The electroretinogram (ERG) measures the electrical activity of the retina in response to light stimulation. It provides objective assessment of overall retinal function. The ERG response has different components - a-wave from photoreceptors, b-wave from bipolar and Muller cells. ERG is used to diagnose retinal diseases like retinitis pigmentosa and macular degeneration. It helps evaluate progression of retinal degenerations and assists in determining retinal involvement in visual complaints. Factors like pupil size, age, light/dark adaptation influence the ERG response which needs clinical correlation for interpretation.

1. ELECTRORETINOGRAM (ERG)
By,
G.Shirisha,
13IMMO20

2. ERG
• ERG is the corneal measure of an action potential
produced by the retina when it is stimulated by light of
adequate intensity.
• It is the composite of electrical activity from the
photoreceptors, Muller cells & RPE.

3. HISTORY
• 1865 : First known recording of an ERG (amphibian retina)
Swedish physiologist Alarik Frithiof
• 1877 : Holmgren, James Dewar of Scotland (humans)
• 1908 : Einthoven and Jolly separated the ERG response into
three components: a-wave, b-wave and c-wave
• 1941 : American psychologist Lorin Riggs introduced the
contact-lens electrode (clinical use)
• 1967 : Ragnar Granit Nobel Prize for Physiology and Medicine
(demonstrated the physiology of the receptor potential of each
component of the ERG)
• 1989 : ISCEV standards(International Society for Clinical
Electrophysiology of Vision)
• 1992 : Erich Sutter mfERG

4. PROCEDURE
According to ISCEV 2015 guidelines:
• Maximally dilate the pupils
• Before Dark adapted protocols- 20-30 min
of dark adaptation
• Before light adapted protocols- 10 min of
light adaptation
• Present low strength flashes before stronger
flashes- so that the partial light adaptation
due to bright light does not occur

5. • Insert corneal contact electrodes (when these are used) under
dim red light after dark adaptation period. Avoid strong red
light. Allow 5 min of extra dark adaptation after insertion of
contact lens electrode.
• Allow at least 30 min recovery time in ordinary room
illumination after use of strong light for retinal imaging
(fundus photography, fluorescein angiography and others).
• Request the patient to fix and not move eyes. Ocular
movements can change the positions of electrodes, can cause
blockage of light by eyelids or electrode and may induce
electrical artifacts.

6. ELECTRODES
• GROUND ELECTRODE – FOREHEAD
• REFERENCE ELECTRODE – OUTER
CANTHUS
• ACTIVE ELECTRODE -Cornea (contact
lens electrode) in flash ERG
Conjunctival sac – used in pattern ERG

8. TYPES OF ERG
• FULL FIELD ERG
• FOCAL ERG
• MULTIFOCAL ERG
• PATTERN ERG

9. Full-Field ERG
• The full-field ERG, also referred to as the standard or
flash ERG
• Retinal potential elicited by a brief flash of light,
recommended to be about 5 ms in duration, that evenly
illuminates the entire retina (Marmor and Zrenner, 1998).

10. FOCAL ERG
• Used for detecting small focal lesions or pathologies
which are missed by standard full field ERG.
• A small stimulus of 4⁰ size is projected on area of retina
to be tested.
• Due to light scattering & poor signal to noise ratio, this
technique is mostly used in research setting than in
clinical setting.

11. MULTIFOCAL ERG
• The stimuli consists densely arranged black
or white hexagonal elements displayed on
CRT monitor.
• These hexagonal elements change from
light to dark independently & this change
results into recording of mfERG.

12. PATTERN ERG
• It mainly represents inner retinal activity (especially
ganglion cell activity)
• Useful in differentiating optic nerve disorders from
macular disorders.
• Unlike flash ERG, pattern ERG is a very small response.
• Recorded with full correction of refractive errors as
visualization of stimulus for extended time is essential
for recording.

13. PERG WAVEFORMS
• P1 or P50 : Initial corneal positive response.
• N1 or N95 : Immediate cornea negative response.
• This 50 & 95 represents the time in milliseconds from the
onset of stimulus to peak of positive or negative response.

14. Indications & Clinical Uses of ERG
• Evaluation of visual function in infants & children.
• To determine presence or absence of retinal function.
• To evaluate progression of retinal degeneration.
• To confirm diagnosis of a particular disease (dystrophies).
• For early detection of toxic retinopathies.
• Assisting in diagnosing the retinal conditions in which
clinical findings don't match with visual complaints
(unexplained visual loss).

15. Components
• a-wave: initial corneal-negative deflection, derived from
the cones and rods of the outer photoreceptor layers
• b-wave: corneal-positive deflection; derived from the
inner retina, predominantly Muller and ON-bipolar cells
• c-wave: derived from the retinal pigment epithelium and
photoreceptors

16. • d-wave: off bipolar cells.
• Dark adapted Oscillatory potentials: Responses primarily
from the amacrine cells/inner retina.
• Latency of response refers to the onset of the stimulus to the
beginning of the a-wave.
• Implicit time or peak time is a measure of the time interval
from onset of the stimulus to the peak of the b-wave

18. Factors affecting the ERG
• Physiological : Pupil, Age, Sex, Ref. Error, Diurnal Variation,
Dark adaptation, anesthesia
• Instrumental : amplification, gain, stimulus, electrodes
• Artifacts : Blinking, tearing, eye movements, air bubbles
under electrode.

20. ISOLATED ROD RESPONSE
• Produced by dark adapting patient for
20-30 min. & then stimulating retina
with dim light flash(2.5 log
units/24db) which is below cone
threshold and the time interval is
2seconds.
• The resultant waveform has
‘prominent b (positive) wave ‘& no
detectable ‘a (negative) wave’.

21. MAXIMAL COMBINED RESPONSE
• It is a larger waveform generated by using bright flash
(10 seconds of interval)in dark adapted state(30 mins)
which maximally stimulates both rods & cones.
• It results in prominent ‘a (negative) wave & ‘b (positive)
wave’with ‘oscillatory potentials’ which are
superimposed on ‘b wave’.

23. SINGLE FLASH CONE RESPONSES
• It is obtained by maintaining the
patient in light adapted state(10
mins) & stimulating the retina with
bright white flash(10 sec interval).
• The rods are suppressed by light
adaptation & donot contribute to the
waveform.

24. 30 Hz Flicker Response
• With patient in light adapted state(10 mins), a flickering
stimulus at 30 Hz(30 stimuli/sec) can also be used to filter
rod response & measure cone response

26. Interpretation of ERG
• ERG is abnormal only if more than 30% to 40% of retina
is affected
• A clinical correlation is necessary
• Media opacities, non-dilating pupils & nystagmus can
cause an abnormal ERG
• ERG reaches its adult value after the age of 2yrs
• ERG size is slightly larger in women than men

27. Negative ERG
• Characterized by large a-wave.
Causes:
 X-linked retinoschisis
 Congenital stationary night blindness
 Quinine, vigabatrine toxicity, siderosis
 Melanoma associated retinopathy

28. INTERPRETATION
Non recording ERG Leber congenital Amaurosis,
Retinitis pigmentosa,
Total RD
Retinal Aplasia
Abnormal or non recordable photopic
ERG
(often mild rod ERG abnormalities)
Cone degenerations
Achromatopsia
X-linked blue cone monochromatism
X-linked cone dystrophy
Non recordable Rod ERG
(abnormal dark adapted bright flash
ERG)
Normal to near normal
Congenital stationary blindness
Early RP
Barely or Non recordable Scotopic ERG
(abnormal photopic B-Wave ERG)
Rod Cone Degenerations
Night blindness

29. Normal MFERG
2D Map
3D Map

30. RETINITIS PIGMENTOSA

31. CONE DYSTROPHY

32. Chloroquine retinopathy

33. Macular Degeneration Stargardt’s Disease

34. Limitations of ERG
• Since the ERG measures only the mass response of the
retina, isolated lesions like a hole hemorrhage, a small
patch of chorioretinitis or localized area of retinal
detachment can not be detected by amplitude changes.
• Disorders involving ganglion cells (e.g. Tay sachs’
disease), optic nerve or striate cortex do not produce any
ERG abnormality

35. REFERENCES
• eyewiki.aao.org/Electroretinogram
• webvision.med.utah.edu/book/electrophysiology/the-
electroretinogram-clinical-applications
• electroretinographic-modifications-induced-by-agomelatine-a-novel-
aven-peer-reviewed-fulltext-article-NDT
• wonderbaby.org/articles/whats-erg
• slideshare.net/shenoyvignesh/electrophysiology-in-ophthalmology
• slideshare.net/susumarashdeh/erg-21541872
• intechopen.com/books/electroretinograms/electroretinogram-in-
hereditary-retinal-disorders