Electroretinography (ERG) is a diagnostic test that measures the electrical responses of the retina, the light-sensitive tissue at the back of the eye. It helps eye care professionals assess retinal function by recording its electrical activity in response to light. ERG provides objective information about retinal health, aiding in the diagnosis and monitoring of various eye conditions.
Understanding the Retina’s Electrical Responses
The retina functions by converting light into electrical signals, a process initiated by specialized cells called photoreceptors. These photoreceptors consist of rods and cones; rods are responsible for vision in dim light and peripheral vision, while cones handle bright light and color perception, predominantly located in the macula. When light strikes these photoreceptors, it triggers a change in their membrane potential, converting light energy into electrical signals.
These initial electrical signals are then transmitted to other retinal cells for further processing. Bipolar cells receive input from the photoreceptors and, in turn, relay information to ganglion cells. Horizontal and amacrine cells also contribute to this complex network, modulating the signals before they are sent as action potentials along the optic nerve to the brain for visual interpretation.
What Electroretinography Measures
The electrical signals generated by retinal cell layers in response to light form a characteristic waveform with distinct components: the a-wave and the b-wave. These waves provide insights into the function of different parts of the retina.
The a-wave is the initial negative deflection of the ERG waveform, representing the activity of the photoreceptor cells, both rods and cones. Its amplitude reflects the health of the outer retina, indicating how well these light-sensing cells are converting light into electrical signals. Following the a-wave, the b-wave is a positive deflection that mainly originates from the activity of bipolar cells and Müller cells in the inner retina. The amplitude and implicit time (the time it takes for the wave to reach its peak) of both the a-wave and b-wave are analyzed to determine the functional status of the retinal cells.
Common Types of ERG Tests
Several types of electroretinography tests evaluate specific aspects of retinal function. These tests utilize different visual stimuli and recording techniques to isolate and measure the electrical activity of various retinal cell populations.
Full-field ERG (ffERG) assesses the overall electrical response of the entire retina to a diffuse flash of light. This test is performed using a Ganzfeld dome, which uniformly illuminates the eye, providing a broad overview of retinal function. It is useful for diagnosing widespread retinal diseases.
Pattern ERG (PERG) evaluates the function of the retinal ganglion cells and the macula, the central part of the retina responsible for sharp, detailed vision. This test uses a patterned stimulus, such as an alternating checkerboard, to elicit specific electrical responses. PERG is often used for conditions affecting the optic nerve, as ganglion cells are the cells that form the optic nerve.
Multifocal ERG (mfERG) provides a detailed functional map of localized areas of the retina, especially the macula. Instead of a single, overall response, mfERG uses multiple flickering hexagonal stimuli to record electrical activity from different regions simultaneously. This allows for the detection of subtle, localized retinal dysfunction that might be missed by full-field ERG.
The Electroretinography Procedure
Undergoing an electroretinography test involves several steps. The procedure begins with dilating eye drops to widen the pupils for light stimulation. Numbing eye drops are also applied for comfort, especially if an electrode will be placed directly on the eye.
Once the eyes are prepared, small electrodes are positioned to record the electrical signals. A common method involves placing a thin fiber or contact lens electrode directly onto the cornea. Additional skin electrodes are placed on the forehead or temple to serve as a ground.
The patient then looks into a light-stimulating device, often a Ganzfeld dome, which presents flashes or patterns of light. The electrical responses generated by the retina are transmitted from the electrodes to a monitor for viewing and recording, appearing as wave patterns. The entire test takes about an hour, and while patients may feel a slight scratchy sensation from the electrodes, it is well-tolerated.
Diagnosing Eye Conditions with ERG
Electroretinography plays a role in diagnosing and monitoring a variety of retinal diseases. It helps clinicians understand how well the retina is functioning, often complementing structural imaging tests.
For inherited retinal dystrophies like retinitis pigmentosa, ERG is used to identify the type and progression of photoreceptor degeneration. In these conditions, the dark-adapted ERG (rod response) may be diminished or absent, indicating rod dysfunction, followed by abnormal cone function in later stages. Similarly, for cone-rod dystrophies, ERG helps differentiate the primary involvement of rods versus cones.
ERG, particularly multifocal ERG, can also assess localized macular function in cases of macular degeneration, especially in early or atypical presentations. While structural changes might not be immediately apparent, mfERG can detect subtle functional changes in the macula. In diabetic retinopathy, ERG can indicate retinal ischemia or dysfunction even before overt structural changes are visible, allowing for earlier intervention.
Furthermore, ERG detects early signs of retinal damage caused by drug toxicity, such as with medications like hydroxychloroquine (Plaquenil). The test can identify changes in retinal function before severe, irreversible vision loss occurs. ERG aids in accurate diagnosis, helps guide treatment planning, and offers insights into the potential prognosis for patients with various retinal disorders.