ERG is a diagnostic test used to measure the electrical responses of the retina to light stimuli. This test helps assess the function of the retina, which is crucial for diagnosing various retinal diseases and conditions that affect vision.
What is ERG (Electroretinography)?
An electroretinogram (ERG) is a test that measures the electrical activity of the retina in response to light. The retina contains cells called photoreceptors (rods and cones) that are responsible for detecting light and initiating electrical signals. These signals are sent to the brain and interpreted as visual information. The ERG records these electrical signals to assess how well the retina is functioning.
The test is primarily used to evaluate the health of the retina, including conditions that affect the rod and cone cells (which are responsible for vision in low light and detailed, color vision, respectively).
Types of ERG:
- Standard ERG (Full-field ERG)
- This is the most common form, where the patient is shown a series of light flashes. The resulting electrical activity from the retina is measured and recorded.
- It’s used to evaluate the general health of the retina, including the function of rods and cones.
- Multifocal ERG (mfERG)
- This test measures electrical responses from different parts of the retina at once, allowing the doctor to evaluate specific areas of the retina.
- It’s more detailed than a standard ERG and is often used to diagnose macular diseases or conditions affecting the central retina, such as macular degeneration or diabetic macular edema.
- Pattern ERG (PERG)
- This form of ERG is often used to assess the function of the optic nerve and is particularly useful in evaluating glaucoma.
- It involves presenting a patterned stimulus (like alternating black-and-white stripes) and measuring the electrical responses from the retina.
- Flash ERG
- Flash ERG is typically used to evaluate the rod and cone function with a series of light flashes.
- It helps in diagnosing retinal diseases that affect both rods and cones, such as retinitis pigmentosa.
How is an ERG Test Performed?
The test is relatively simple, but it does require some preparation:
- Preparation: The patient will be asked to sit comfortably in a dark room. The eyes may be dilated using eye drops.
- Electrode Placement: Small electrodes are placed around the eyes, including one on or near the cornea, a ground electrode on the forehead, and possibly others near the eyes.
- Light Stimulation: The patient is exposed to a series of light stimuli, such as flashes or patterns.
- Data Collection: The electrodes measure the electrical signals from the retina, which are recorded as waveforms.
- Analysis: The waveforms are analyzed for abnormalities that indicate retinal dysfunction.
Conditions Diagnosed or Monitored Using ERG:
- Retinitis Pigmentosa (RP):
- Genetic disorders causing degeneration of the retina, especially rod cells.
- ERG often shows a diminished or absent response, confirming diagnosis.
- Macular Degeneration:
- ERG can detect macular abnormalities in both dry and wet AMD.
- Diabetic Retinopathy:
- ERG assesses retina function in diabetic patients to detect early damage.
- Cone and Rod Degenerations:
- Helps differentiate between diseases affecting rods or cones, such as cone dystrophy.
- Leber’s Congenital Amaurosis (LCA):
- Genetic disorder causing severe early vision loss. ERG is essential for diagnosis.
- Retinal Detachment:
- ERG can detect functional abnormalities suggesting detachment before visible changes appear.
- Glaucoma:
- Pattern ERG helps monitor optic nerve health in glaucoma patients.
- Toxic Retinopathy:
- ERG assesses retinal effects of toxic drugs like chloroquine or tamoxifen.
Advantages of ERG Testing:
- Objective measurement: Directly assesses retinal response to light, not dependent on patient feedback.
- Early detection: Identifies functional abnormalities before structural changes occur.
- Non-invasive: The procedure is quick and painless.
- Monitoring: Useful for tracking disease progression or treatment effectiveness in conditions like RP or AMD.
