Electro oculography & Electro Retinography.pptx

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- 23 rd 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- 23 rd edition. The electroretinogram and electrooculogram: clinical applications by Donnell J. Creel.