The document discusses electro-oculography (EOG) and electroretinography (ERG), including their definitions, test procedures, and interpretations of results. EOG measures the electrical potential differences related to eye movements and retinal pigment epithelium function, while ERG evaluates the electrical responses of retinal cells to light stimuli. Both methods offer valuable non-invasive insights into retinal health, although they have specific limitations.

1. Electro-oculography
&
Electroretinography
-By Dr. N. Revanth

2. Specific Learning Objectives:
• Definitions of Electro-oculography (EOG) & Electroretinogram
(ERG).
• Different types of EOG & ERG.
• Test procedure.
• Normal patterns & interpretations.
• Advantages & limitations.

3. Electro-oculography:
Definition:
• The electrooculogram (EOG) measures the cornea-positive
standing potential relative to the back of the eye by attaching skin
electrodes outside the eye near the lateral and medial canthus.

4. Introduction:
• EOG measures the potential difference between the cornea and
the retina ( mainly retina pigment epithelium) during eye
movements.
• Cornea and retina maintain a constant electrical potential
difference creating a constant “ocular dipole”.
• When the eyes move, this potential difference changes.
• The EOG is the method by which these minor electrical potential
differences are captured by placing electrodes on the surface.

5. History:
• Elwin Marg first described & named EOG.
• Geoffrey Arden is the first to use and compare the amplitudes of
electric potentials under light and dark-adapted states, hence the
name “ Arden Ratio”.
• EOG became one of the first techniques to provide non-invasive
insight into the functional status of the RPE.

6. Test procedure:
• The patient should be in stable indoor lighting for at least 30 mins
before the test.
• Should not be exposed to any strong illumination during the rest
period.
• The patient should be dilated.
• He/she should be told to remain still except for eye movement
during the test procedure.

7. • Surface electrodes are
placed at the outer
canthi of the eye &
medial canthi of the eye
and reference electrode
is placed on the
forehead or near the
ear.

8. • The patient alternates the gaze between two fixed points and the
voltage change during the eye movements is recorded.
• The difference in potential is called standing potential.
• It indirectly measures the transepithelial potential of retinal
pigment epithelium(RPE).

9. Arden Ratio:
• Dark trough/DT: During 15 mins of dark adaptation the standing
potential usually reaches a minimum level, called dark trough.
• Light peak/LP: During 15 mins of light adaptation the standing
potential usually reaches its highest value at 7-12 min called light
peak.
• The light peak: dark trough(usually preferred) ratio is referred to
as the Arden ratio.

10. • Normal wave forms in EOG during light & dark adaptations
Image source: Donnell J. Creel’s ERG & EOG Clinical applications

11. Interpretation:
• Light peak-to-dark trough ratio of > 1.8 or 180% is normal and less
than 1.5 or 150% is abnormal, in people below 60 years.
• In people aged above 60 yrs, >1.7 is considered normal.

12. Advantages and limitations:
• simple., non-invasive and provides information about retinal
pigment epithelium.
• Limitations include, it cannot provide information about retinal
functional beyond the RPE.

13. Electroretinography
Definition:
• In electroretinography (ERG), changes in the resting potential of
the eye when induced by the stimulation of the eye with a light
stimulus are measured.

14. • The flash of light elicits a biphasic waveform recordable at cornea
when placed an electrode.

15. History:
• Ragnar Granit is the first to identify that ERG waveform reflected
different components of retinal activity, particularly rods and
cones.

16. Introduction:
• ERG measures the electrical responses of various retinal cells,
particularly rods and cones to light stimuli.
• Photoreceptor cells generate electrical signals in response to light
stimuli.

17. Test procedure:
• Pupils are dilated first, station may be set to dark or light
adaptation depending on the test that would like to be performed.
• Contact lens electrode is placed on cornea and reference
electrodes are placed on the skin.
• Flashes or patterns of light are presented ti evoke retinal
responses.

18. Electrode placements:

19. • Waveform:
Image source: Parsons diseases of the Eye- 23rd
edition.

20. Types of ERG:
• 1). Pattern ERG
• 2). Multifocal ERG
• 3). Full field ERG.

21. • Rods are tested in dark adaptation with a blue light stimulus-
Scotopic ERG
• Cones are tested in bright light or flickering light stimulus-
Photopic ERG..

22. Advantages and limitations:
• Provides detailed functional assessment of retinal health.
• Also can identify a wide range of retinal disorders.
• Limitations include, it requires specialized equipment & can be
uncomfortable to patients due to the use of corneal electrodes.

23. Summary:
•EOG: Provides insight into RPE function and eye movement control.
•ERG: Offers a detailed evaluation of retinal cell function, aiding in
diagnosing various retinal conditions.

24. References:
• Parsons Diseases of the Eye- 23rd
edition.
• The electroretinogram and electrooculogram: clinical applications
by Donnell J. Creel.