ERG and VEP Testing for Your Retinal Health
Understanding ERG and VEP
An electroretinogram, or ERG, records the electrical signals produced by your retina when it is exposed to light. The retina is the thin layer of light-sensitive tissue at the back of the eye. When light hits the retina, specialized cells generate tiny electrical responses. The ERG captures these responses and displays them as a waveform on a screen.
The ERG waveform has two main parts. The first part, called the a-wave, comes from the photoreceptors (the rods and cones that detect light). The second part, called the b-wave, comes from the bipolar cells in the middle layer of the retina. Together, these waves tell a retina specialist how well different layers of your retina are functioning.
A visual evoked potential, or VEP, measures the electrical activity of your entire visual pathway. This pathway runs from the retina through the optic nerve to the visual cortex at the back of your brain. The VEP records how quickly and strongly electrical signals travel along this route.
Because the VEP evaluates the whole visual system, it can detect problems at many points along the pathway. It is especially useful for conditions affecting the optic nerve and brain. One advantage of VEP testing is that it works independently of a patient's consciousness or attention level, making it valuable for testing young children and patients who cannot communicate clearly.
ERG and VEP provide complementary information. The ERG focuses on the retina itself, while the VEP evaluates the signal as it travels from the retina to the brain. If the ERG is normal but the VEP is abnormal, the problem likely lies in the optic nerve or brain rather than the retina. If the ERG is abnormal, the VEP may also be affected because the retina generates the signal that travels through the visual pathway.
A retina specialist may order one or both tests depending on your symptoms and suspected condition. Using them together provides a more complete picture of your visual system health.
Who Benefits From These Tests
The American Academy of Ophthalmology (AAO, 2022) recommends full-field ERG to evaluate inherited retinal degenerative diseases. These include retinitis pigmentosa, congenital stationary night blindness, cone-rod degenerations, and inherited macular dystrophies. The ERG can detect widespread retinal dysfunction even before noticeable vision loss occurs, helping to confirm a diagnosis early.
Some medications can be toxic to the retina over time. Antimalarial drugs such as chloroquine and hydroxychloroquine can produce ring scotomas (ring-shaped blind spots) and a characteristic pattern called bullseye maculopathy. Up to 7 percent of patients on antimalarial medications may show retinal changes detectable with advanced electrophysiological screening methods including multifocal ERG (BCBS Medical Policy Reference, 2023). The AAO recommends multifocal ERG as one objective test option for screening patients on these medications.
ERG and VEP testing can be performed on patients who are unable to read an eye chart or describe their symptoms. Research has shown that 75 percent of pediatric referrals younger than three years old and 87 percent of neurologically handicapped referrals were testable for VEP acuity in a clinical setting (Odom et al., 1983). Newer handheld devices like the RETeval have made ERG even more accessible, with children as young as six months able to remain calm during testing without sedation.
VEP testing is particularly valuable for patients with suspected optic nerve or brain disorders. Conditions such as multiple sclerosis can slow signals along the optic nerve due to damage to the myelin coating. Optic nerve tumors, pituitary tumors, and hydrocephalus (excess fluid pressure in the brain) can also affect VEP results. A retina specialist or neurologist may use VEP to help locate where in the visual pathway a problem exists.
Types of ERG and VEP Tests
Full-field ERG, also called flash ERG, measures the electrical response of the entire retina at once. It is performed under both dark-adapted (scotopic) and light-adapted (photopic) conditions. In dark-adapted testing, a weak flash measures rod function while a strong flash measures a mix of rod and cone activity. In light-adapted testing, the response comes primarily from cone cells.
Full-field ERG is best suited for detecting widespread retinal diseases. Because it averages the response across the entire retina, it may not detect small areas of damage such as those affecting only the macula (the central part of the retina responsible for detailed vision).
Multifocal ERG tests many small areas of the retina individually, creating a detailed map of retinal function. This test is especially useful for detecting localized damage that full-field ERG might miss. The AAO lists multifocal ERG as an objective screening option for hydroxychloroquine toxicity, alongside fundus autofluorescence and spectral-domain optical coherence tomography (SD-OCT).
Pattern ERG uses a checkerboard pattern rather than a flash of light to stimulate the retina. It primarily measures the function of retinal ganglion cells (the cells that send signals from the retina to the brain through the optic nerve). Pattern ERG has shown promise in the early diagnosis and monitoring of glaucoma. While the AAO (2015-2016) does not recommend ERG as a standard diagnostic tool for glaucoma, it acknowledges electrophysiologic testing as an additional option for diagnostic testing.
The photopic negative response, or PhNR, is a specific part of the ERG waveform recorded under light-adapted conditions. It appears as a slow negative wave after the b-wave. The PhNR is primarily driven by retinal ganglion cells, making it a useful indicator of ganglion cell health. This measurement provides insight into conditions like glaucoma and optic nerve diseases that affect these critical cells.
Standard VEP uses flash stimulation and records the brain's response to light. Pattern VEP uses a reversing checkerboard pattern and is more sensitive to problems affecting central vision. Pattern VEP and pattern ERG together offer quantitative data for conditions like glaucoma, with potential advantages including earlier diagnosis compared with OCT, less frequent follow-ups needed, and the ability to monitor how well treatment is working.
What Happens During Testing
Your retina specialist will provide specific instructions before your appointment. For some types of ERG testing, you may need to sit in a dark room for about 20 to 30 minutes beforehand. This dark adaptation allows your rod cells to reach their full sensitivity. You may be asked to avoid caffeine or certain medications before the test. Your pupils may or may not need to be dilated depending on the type of test being performed.
Wear comfortable clothing and be prepared for a visit that may last one to two hours, depending on which tests are ordered.
For a standard full-field ERG, numbing eye drops are placed on your eyes. A small electrode, often in the form of a thin fiber or a contact lens, is placed on the surface of each eye. Additional small electrodes are placed on the skin near the eyes. You will be asked to look at a series of light flashes while the electrodes record the retinal response.
The test itself is not painful. Some patients feel mild discomfort from the electrode on the eye or from the bright flashes. The newer handheld RETeval device uses skin electrodes placed below the eye, which many patients find more comfortable than traditional contact lens electrodes.
For a VEP test, small electrodes are placed on the scalp over the back of your head, near the visual cortex. You will sit in front of a screen displaying either a flashing light or a reversing checkerboard pattern. The electrodes measure the electrical signals generated in your brain in response to the visual stimulation. You do not need to do anything other than keep your eyes open and focused on the screen.
VEP testing is painless. The electrodes are attached to the skin with a small amount of paste, similar to those used in other types of brain wave recordings. The entire process typically takes 30 to 60 minutes.
There is no recovery time needed after ERG or VEP testing. If numbing drops were used for ERG, you should avoid rubbing your eyes until the drops wear off, usually within 30 minutes. If your pupils were dilated, you may experience light sensitivity and blurred vision for several hours. It is a good idea to bring sunglasses and arrange for someone to drive you home if dilation is needed.
Your retina specialist will review the results and discuss findings with you, often at a follow-up appointment or sometimes the same day.
What Results Can Show
ERG results are displayed as waveforms with specific shapes and sizes. The a-wave and b-wave each have a normal range for their height (amplitude) and timing (peak time). A reduced amplitude may indicate that fewer retinal cells are responding to light. A delayed peak time may suggest that the cells are responding more slowly than normal. Your retina specialist compares your waveforms to established normal values for your age.
ERG testing can help diagnose or monitor several retinal conditions. These include:
- Retinitis pigmentosa and other inherited retinal degenerations
- Congenital stationary night blindness
- Cone-rod dystrophies
- Drug toxicity from medications like chloroquine and hydroxychloroquine
- Systemic metabolic disorders that reduce retinal function, particularly affecting the b-wave in dim light conditions
- Unexplained vision loss when other tests are inconclusive
VEP testing can help identify problems along the visual pathway from the eye to the brain. Conditions that may affect VEP results include:
- Multiple sclerosis, which causes slowed signal conduction due to damage to the myelin coating of nerves
- Optic neuritis (inflammation of the optic nerve)
- Optic nerve tumors, including gliomas in neurofibromatosis
- Pituitary tumors compressing the optic pathways
- Hydrocephalus causing pressure on visual pathways
- Retinal diseases affecting central vision, such as macular disease or central serous retinopathy
Several factors may influence ERG and VEP results. Media opacities such as cataracts and vitreous opacities can reduce VEP responses, particularly with pattern stimuli. Increasing opacity causes longer peak times and smaller amplitudes. Physiological factors including age, sex, pupil size, and head circumference can also affect VEP measurements.
Technical factors matter as well. Room lighting, stimulus type, electrode placement, and patient cooperation all influence the quality of recordings. A retina specialist's office takes careful steps to control these variables so that results are accurate and reliable.
Emerging Uses of ERG and VEP
Researchers have found that the retina shares a common embryonic origin with the brain, making it an accessible window into central nervous system function. Electroretinography has shown promise in detecting differences in retinal electrical activity associated with psychiatric disorders including schizophrenia, bipolar disorder, and ADHD. ADHD affects approximately 7.2 percent of children worldwide (Thomas et al., 2015). While this research is still developing, it may eventually lead to new ways of screening for and understanding these conditions.
Traditional ERG equipment required specialized darkened rooms and contact lens electrodes. Newer handheld devices like the RETeval have made testing more accessible, portable, and comfortable. Studies have shown that patients prefer the handheld device over conventional methods and that very young children can be tested without sedation. These advances are expanding where and how ERG testing can be performed, potentially making it available in more clinical settings.
When to See a Retina Specialist
A retina specialist may recommend ERG or VEP testing if you experience difficulty seeing in low light, progressive loss of peripheral or central vision, unexplained changes in color perception, or a family history of inherited retinal disease. These tests are also used when standard eye exams and imaging do not fully explain your symptoms.
Certain vision changes require immediate medical evaluation. See a retina specialist or go to the emergency room right away if you experience a sudden increase in floaters, flashes of light, a curtain or shadow moving across your vision, or sudden vision loss in one eye. While these symptoms may not always lead to ERG or VEP testing, they could indicate serious retinal conditions such as retinal detachment that need urgent care.
If you are taking medications known to affect the retina, such as hydroxychloroquine, your retina specialist may schedule regular ERG testing as part of your monitoring plan. Patients with known inherited retinal diseases may also undergo periodic ERG testing to track disease progression and guide treatment decisions. Regular follow-up helps ensure that any changes in retinal function are detected as early as possible.
Questions and Answers
ERG testing may cause mild discomfort from the electrode placed on the eye surface and from bright light flashes, but numbing drops are used to minimize any sensation. Newer handheld devices use skin electrodes below the eye, which most patients find quite comfortable. VEP testing involves only small electrodes placed on the scalp with paste. Neither test involves needles, incisions, or significant discomfort.
The recordings themselves are available immediately after testing. However, your retina specialist needs time to carefully analyze the waveform patterns, compare them to normal values, and interpret them alongside your other test results and clinical history. Some specialists discuss preliminary findings the same day, while others may schedule a follow-up visit to review results in detail.
Yes. Both ERG and VEP are safe and noninvasive for children. Research has demonstrated that the majority of young children, including those under three years old and those with neurological conditions, can be successfully tested (Odom et al., 1983). Handheld ERG devices have further improved the ability to test infants as young as six months without the need for sedation. These tests are particularly valuable for children who cannot yet describe their vision or cooperate with standard eye charts.
That depends on your condition. Patients being monitored for medication toxicity, such as those taking hydroxychloroquine, may need periodic testing at intervals recommended by their retina specialist. Patients with progressive inherited retinal diseases may undergo repeat testing to track changes over time. For some conditions, a single test may provide enough information for diagnosis. Your retina specialist will advise you on the appropriate testing schedule for your situation.
No. ERG and VEP provide unique functional information about how your retinal cells and visual pathways respond to light, but they do not replace structural imaging tests like OCT or fluorescein angiography. They also do not replace comprehensive eye exams, visual field testing, or fundus photography. A retina specialist uses ERG and VEP alongside other tests to build a complete understanding of your eye health and guide the best treatment plan.