Pattern ERG (pERG) is a specialized diagnostic eye test that assesses the electrical activity of the retina. This non-invasive procedure measures how the retina responds to specific visual stimuli, providing insights into its overall function. It evaluates the health of certain retinal cells and the pathways responsible for central vision.
The Retina’s Vital Role in Vision
The retina is a thin layer of tissue at the back of the eye, where visual images are initially projected. It contains light-sensing photoreceptors (rods and cones) that convert light into electrical signals. These signals are then processed and transmitted through a network of specialized cells.
The macula, a small central area within the retina, is particularly important for sharp, detailed central vision, necessary for tasks like reading and recognizing faces. Retinal ganglion cells (RGCs) are near the inner surface of the retina and receive visual information from photoreceptors via intermediate neurons. These RGCs have long axons that extend from the eye to the brain, forming the optic nerve. The health and proper functioning of both the macula and retinal ganglion cells are crucial for transmitting clear visual information to the brain.
How Pattern ERG Measures Retinal Function
Pattern ERG works by recording the tiny electrical signals generated by retinal cells in response to specific visual stimuli. The test uses alternating black-and-white checkerboard patterns or gratings that reverse contrast at a specific rate. This patterned stimulation helps isolate the activity of the inner retinal layers, particularly the retinal ganglion cells.
The electrical responses recorded during a pERG test produce a waveform with distinct components, reflecting different aspects of retinal cell activity. These components include a negative wave around 35 milliseconds (N35), a positive wave at approximately 50 milliseconds (P50), and a larger negative wave at about 95 milliseconds (N95). The P50 wave is thought to originate from retinal cells, including the cone cells in the macula, while the N95 wave primarily reflects the function of the retinal ganglion cells in the inner retina. By analyzing the amplitude (strength) and implicit time (speed) of these waves, eye care professionals can assess the health and function of the macula and retinal ganglion cells.
Eye Conditions Identified by Pattern ERG
Pattern ERG is a diagnostic tool that helps identify and monitor several eye conditions by assessing the function of retinal ganglion cells and the macula. It is particularly useful in the diagnosis and management of optic nerve and macular dysfunction. For example, in cases of optic nerve problems, pERG might show a reduction in the N95 amplitude, while the P50 amplitude might remain normal or only slightly reduced in severe cases. This specific pattern helps distinguish optic nerve issues from conditions primarily affecting the macula.
A significant application of pERG is in the early detection and monitoring of glaucoma, a condition that damages the optic nerve. pERG can detect subtle changes in retinal ganglion cell function, sometimes before visible structural damage to the optic nerve or changes in visual fields are apparent. This early detection is important for timely intervention and treatment. Additionally, pERG can be used to monitor certain inherited or acquired retinal disorders, such as macular dystrophies or toxic retinopathies, and has shown abnormalities in conditions like diabetic retinopathy and idiopathic intracranial hypertension.
What to Expect During a Pattern ERG Test
Undergoing a Pattern ERG test is a straightforward and non-invasive experience. Patients do not need pupil dilation for this test, but it is recommended to wear your most recent distance eyeglasses or bring your equivalent prescription for accurate results. Also avoid wearing make-up on the morning of the test and remove any earrings. If you wear contact lenses, you will need to remove them for the duration of the test.
During the procedure, small electrodes are placed on or near the eye to record the electrical responses. These electrodes might be placed on the skin near the eye, or small electrodes may be placed directly on the cornea after topical anesthesia. You will be seated in front of a specialized monitor displaying a pattern of alternating black-and-white squares, such as a checkerboard, and instructed to focus on a target at the center of the screen. The test is well-tolerated, with minimal discomfort. The entire test lasts about an hour.