erg & eog

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.