Electroretinography (ERG): Principles, Procedure, and Clinical Applications
Electroretinography (ERG) is an important diagnostic test in ophthalmology and vision science used to assess the electrical responses of the retina—the light-sensitive tissue lining the back of the eye—to a brief flash of light or other visual stimuli. By directly measuring the summed electrical activity of retinal cells, ERG provides valuable information about retinal function that cannot be obtained through purely structural imaging techniques such as fundus photography or optical coherence tomography (OCT). ERG is indispensable for diagnosing hereditary retinal dystrophies, evaluating unexplained vision loss, and monitoring retinal toxicity, among other uses.
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Table of Contents
Principles and Physiology
The retina consists of several layers of neurons, including photoreceptors (rods and cones), bipolar cells, and ganglion cells, supported by the retinal pigment epithelium (RPE) and Müller cells. When light hits the retina, photoreceptors undergo a complex cascade of biochemical and electrical events, leading to a change in membrane potential. These electrical changes propagate through bipolar and other retinal cells, generating tiny electrical currents.
ERG records these extracellular potentials using electrodes placed on the cornea or nearby skin. The resulting waveforms reflect the summed electrical responses of different retinal layers. The main ERG components include:
✅ The a-wave: a negative deflection originating mainly from photoreceptors.
✅ The b-wave: a larger positive wave generated by bipolar and Müller cells. Additional components, such as oscillatory potentials, arise from inner retinal cells, including amacrine cells.
Types of Electroretinography (ERG)
ERG can be broadly classified into several subtypes, each tailored to specific clinical questions:
1. Full-field (Ganzfeld) ERG: Evaluates the overall function of the retina by stimulating the entire retina uniformly with a bright flash of light. It is particularly useful for detecting generalized retinal diseases, like retinitis pigmentosa or cone–rod dystrophies.
2. Pattern ERG (PERG): Uses a reversing checkerboard or grating stimulus instead of a diffuse flash. PERG is sensitive to dysfunction of ganglion cells and the macula, and can aid in diagnosing early glaucoma or macular disorders.
3. Multifocal ERG (mfERG): Simultaneously records responses from many discrete retinal locations, creating a topographic map of retinal function. mfERG is valuable for detecting localized retinal dysfunction, such as macular dystrophies or paracentral scotomas.
4. Focal ERG: Targets a specific retinal area, often the macula, to assess localized disease.
Each of these types has standardized protocols developed by the International Society for Clinical Electrophysiology of Vision (ISCEV) to ensure reproducibility and comparability between clinics.
Procedure and Technique
An ERG test is typically performed in an outpatient setting and takes about 30–60 minutes. Preparation often includes:
1. Preparation: The patient's eyes are typically dilated with eye drops, and numbing drops are applied to the surface of the eye to ensure comfort.
2. Electrode Placement: Small electrodes are placed on the cornea (the clear front surface of the eye), often in the form of a thin fiber or a special contact lens. Additional electrodes may be placed on the skin near the eye and on the forehead.
3. Light Stimulation: The patient looks into a dome-shaped device (called a Ganzfeld dome) or at a screen that presents various light stimuli. These stimuli can include:
4. Flashes of light: Different intensities and colors of light are used to selectively stimulate rods (responsible for night vision and peripheral vision) and cones (responsible for color vision and central vision).
5. Patterned stimuli: Checkerboard patterns that reverse contrast can be used to assess specific retinal cell functions.
6. Recording: The electrodes record the tiny electrical signals generated by the retina in response to the light. These signals are amplified and displayed as a waveform on a computer, known as an electroretinogram.
During the test, patients look at a light stimulus, and multiple recordings are averaged to improve the signal-to-noise ratio. Both dark-adapted (scotopic) and light-adapted (photopic) conditions are tested to separately evaluate rod and cone pathways.(alert-passed)
Interpretation of Results
An ophthalmologist or electrophysiologist interprets the waveform, looking at:
1. Amplitude (height): The strength of the electrical response. Reduced amplitude can indicate a loss of retinal cells or impaired function.
2. Implicit time (latency): The time it takes for the electrical signal to be generated and reach its peak. Delayed implicit time can suggest slower retinal processing.
3. Waveform morphology: The overall shape and characteristics of the electrical waves (e.g., "a-wave" representing photoreceptor activity, "b-wave" representing activity of bipolar and Müller cells).
Purpose and Clinical Applications
The ERG is a crucial tool for diagnosing and monitoring a wide range of inherited and acquired retinal disorders.
A. Inherited Retinal Dystrophies
1. Retinitis Pigmentosa (RP): A group of genetic disorders causing progressive vision loss due to rod and cone degeneration. ERG can detect early changes and monitor progression.
2. Leber Congenital Amaurosis (LCA): A severe, early-onset retinal dystrophy causing profound vision loss from birth.
3. Cone Dystrophies: Conditions primarily affecting cone photoreceptors, leading to central vision loss and color vision problems.
4. Congenital Stationary Night Blindness (CSNB): A condition where individuals have difficulty seeing in low light, but their vision does not worsen over time.
5. Choroideremia: A progressive genetic disorder leading to blindness due to degeneration of the choroid, retinal pigment epithelium, and photoreceptors.
B. Acquired Retinal Disorders
1. Retinal Detachment: To assess the viability of the retina before or after surgery.
2. Retinal Vascular Diseases: Such as central retinal artery occlusion or central retinal vein occlusion, to evaluate retinal function.
3. Diabetic Retinopathy: Though not typically the primary diagnostic tool, ERG can sometimes provide additional functional information.
4. Drug Toxicity: To monitor the retina for damage from certain medications (e.g., antimalarial drugs like hydroxychloroquine, which can cause retinal toxicity).
5. Trauma: To assess retinal function after eye injuries.
6. Autoimmune Retinopathies: Such as cancer-associated retinopathy (CAR) or melanoma-associated retinopathy (MAR).
C. Unexplained Vision Loss
When other tests don't reveal the cause of vision problems, ERG can help determine if the retina is involved.
D. Assessing Retinal Function in Patients with Opaque Media
For example, in cases of dense cataracts or vitreous hemorrhage, where it's difficult to visualize the retina, ERG can still provide objective information about its function.
E. Research and Clinical Trials
Serving as an objective biomarker of retinal function to evaluate new therapies.
In pediatric patients, ERG is particularly useful since structural examination may be limited by cooperation, and children may not accurately describe visual symptoms.
Limitations and Considerations
While ERG provides objective data on retinal function, it does have limitations:
➧ It reflects overall retinal function and may not detect very small, localized lesions.
➧ Results can be affected by factors such as media opacities (e.g., cataracts) and poor electrode placement.
➧ Interpretation requires specialized expertise and familiarity with normal age-related variations.
In summary, an ERG test is a powerful electrophysiological tool in ophthalmology that provides objective, quantifiable data about the electrical activity of the retina, aiding in the diagnosis, monitoring, and management of a wide array of retinal diseases.
