Electro-Oculogram (EOG): A Comprehensive Guide
The Electro-Oculogram (EOG) is a diagnostic test used to measure the resting potential between the cornea and the retina of the eye. It provides valuable insights into the function of the retinal pigment epithelium (RPE) and is particularly useful in diagnosing various retinal disorders.
Principle of EOG
EOG relies on the principle that the eye behaves like a dipole, with the cornea being positively charged and the retina negatively charged. Electrodes are placed near the medial and lateral canthi to detect voltage changes as the patient moves their eyes. These voltage changes are recorded in both fully dark-adapted and fully light-adapted conditions.
Procedure
- Electrode Placement: Electrodes are positioned near the orbital margins close to the medial and lateral canthi.
- Ground Electrode: A forehead electrode serves as a ground reference.
- Adaptation Periods: The patient undergoes two phases of adaptation:
- Dark Adaptation: The patient remains in the dark for approximately 15 minutes.
- Light Adaptation: The patient is exposed to light for the next 10–15 minutes.
- Eye Movement Monitoring: The patient follows fixation lights positioned at different angles while the electrodes record the voltage changes.
- Graph Generation: The EOG generates a graph representing the electrical responses in light and dark conditions.
Arden Ratio
The Arden Ratio is a critical parameter in EOG interpretation. It is calculated using the formula:
Normal Values
- Normal: Arden Ratio > 185
- Subnormal: 165–185
- Abnormal: < 165
Clinical Significance
EOG is primarily used to assess the health of the retinal pigment epithelium (RPE) and diagnose conditions such as:
- Best’s Vitelliform Macular Dystrophy – Characterized by a significantly reduced Arden Ratio.
- Butterfly Macular Dystrophy – A rare hereditary disorder affecting the macula.
- Macular Degeneration – Includes age-related macular degeneration (AMD) where EOG may be useful in assessing RPE function.
- Retinitis Pigmentosa – A degenerative retinal disease where EOG can help determine disease severity.
Advantages of EOG
- Non-invasive and painless procedure.
- Useful in diagnosing hereditary and degenerative retinal diseases.
- Provides insight into the functional status of the RPE.
Limitations of EOG
- Less specific compared to Electroretinography (ERG).
- Requires patient cooperation, making it less suitable for young children or uncooperative individuals.
Conclusion
The Electro-Oculogram (EOG) is a valuable tool in ophthalmology, particularly for diagnosing retinal pigment epithelium-related disorders. By understanding the Arden Ratio and EOG interpretation, clinicians can effectively assess and diagnose retinal conditions, leading to better patient management and outcomes.