Overview of Full-Field Electroretinogram
The full-field electroretinogram (ffERG) is a test used by eye doctors that helps them understand how well the retina, the part of your eye that senses light, is working. This painless test uses a small flash of light to measure the electrical signals in your retina. In other words, it’s like a test that checks how well your retina is responding to light.
The ffERG test can provide important information that helps doctors figure out what might be causing problems with your vision. This is especially helpful when it is not clear what is causing the loss of vision from a standard eye checkup. By looking at different parts of the ffERG results, doctors can identify signs of various eye diseases.
Anatomy and Physiology of Full-Field Electroretinogram
The retina, the light-sensitive layer in the back of your eye, is structured into 10 layers, each containing different types of cells and connections vital for seeing. It has two main parts: the inner retina and the outer retina. The inner retina is made up of ganglion cells and amacrine cells, while the outer retina houses rod and cone photoreceptors.
Photoreceptors are responsible for turning light into an electrical signal, a process we call phototransduction. This signal, containing the visual information we receive, is passed on to another set of cells known as bipolar cells located in the middle of the retina. Rod photoreceptors connect only with certain bipolar cells that increase in activity, while cone photoreceptors can connect with these cells as well as the ones that decrease in activity.
Finally, these electrical signals are sent to the brain via the optic nerve, which is connected to the ganglion cells in the retina. The retina houses more rods than cones, in a ratio of about 3 to 1. However, cones hold more importance in the macula, the particular area of the retina responsible for fine visual details, with the highest concentration found in the central macula or fovea.
Even though there are fewer cones in the macula, the larger surface area of the retina outside of this area makes up a greater total number of cones. These photoreceptors also differ in their sensitivity to light, signal threshold, peak wavelength sensitivity, and the speed of recovery. The most sensitive wavelength for rods and cones is approximately 510 and 560 nm. Besides, rods may be more sensitive than cones, but cones recover quicker.
Why do People Need Full-Field Electroretinogram
The ffERG, or full-field electroretinogram, is a special test that goes beyond the typical eye exam conducted by an eye doctor. This electrical test helps evaluate the health of the retina (the part of the eye that senses light) and the optic nerve (the nerve that sends visual information from the eye to the brain).
This test may be recommended in several situations:
- To diagnose and monitor diseases affecting the retina or the optic nerve.
- To check for any damage to the retina caused by exposure to harmful drugs.
- To assess inflammation within the eye.
- To help evaluate the impact of foreign objects lodged in the eye.
- To check for blockages in the blood vessels of the retina and any resulting damage to the eye tissue due to lack of oxygen (ischemic damage).
- To uncover if someone is pretending to have vision problems for personal or psychological reasons (known as malingering or hysteria).
When a Person Should Avoid Full-Field Electroretinogram
There are no particular reasons why someone should not have a test called a full-field electroretinogram (ffERG). This test measures the electrical responses of various cells in the retina, the back part of your eye and it’s completely safe. Even individuals with seizure disorders can have this test as the frequency of the test does not generally trigger epileptic seizures.
However, for individuals who experience seizures triggered by certain light patterns or flashing lights (photosensitive seizures), doctors will be extra careful. This is particularly necessary if their seizures usually occur at around 30 Hz frequency or if the triggering frequency is not known.
Equipment used for Full-Field Electroretinogram
When a doctor needs to measure how well your eyes are functioning, they might use special tools like electrodes, a device that picks up electrical activity in your body. Another tool is a Ganzfield stimulator; this device helps evenly distribute light or sound to your eyes or ears. They also use a digital recording system, a device that electronically keeps track of your eye’s responses during the test.
Who is needed to perform Full-Field Electroretinogram?
Specially trained technicians conduct a test called ffERG in large medical centers that have a special testing area called an electrophysiology laboratory. This test measures the electrical activity in your eye. Doctors who specialize in eye conditions that affect the retina (the layer at the back of your eyeball that senses light) and the nerves connected to your eyes (neuro-ophthalmologists) are usually in charge of understanding the results from these tests. They are responsible for explaining these results to you.
Preparing for Full-Field Electroretinogram
Electrode placement and patient preparation are essential processes in medical procedures involving vision tests. Below is an easy to understand process of how this is done:
Placement of the Recording Electrodes:
Recording electrodes are placed on the eye’s surface, on the skin of the lower eyelid, or nearby the bottom edge of the cornea. The type of electrode used determines the exact placement. To keep the surface of the eye safe, doctors use a non-irritating liquid that conducts electricity or ‘artificial tears’. A local anesthetic helps to reduce any discomfort from the electrodes touching your eye. These electrodes are then linked to the positive terminal in the recording system.
Reference Electrodes:
The reference electrodes are linked to the negative side of the recording system.
Ground Electrodes:
These electrodes are connected to the ground terminal of the recording system, and can be placed on several locations such as the earlobe, the hard bony lump behind your ear (mastoid), or on your forehead.
Preparing the Patient:
Per the guidelines by the International Society for Clinical Electrophysics of Vision (ISCEV) – a group that sets standards for vision testing, the patient needs to be in a special room that’s electrically isolated. This helps to get accurate results for the tests. The patient’s pupils need to be fully dilated, and there are times when they need to adjust to different light conditions before testing. If the test is performed in the dark, the patient needs to sit in the dark for at least 20 minutes before the test. If in the light, they need to be in a well-lit area for at least 10 minutes before starting the test.
During the testing, the strength of the flashed light changes, starting from a low strength and gradually increasing. The electrodes are adjusted under dim red light after the dark adjustment. An extra 5 minutes is allowed for the eyes to adjust after placing the electrodes.
It is necessary for patients to have a 30-minute break in a normally lit room if they have been exposed to strong light from another type of vision test like fundus photography – the photographing of the back of the eye, or fluorescein angiography – a test that uses a special dye and camera to look at the blood flow in the retina.
During testing, it’s important for patients to keep their eyes steady while looking at a specific point inside the stimulator. Patients who can’t see the point are often told to look straight ahead and keep their gaze steady.
How is Full-Field Electroretinogram performed
The full-field electroretinography (ffERG) is a test that measures the electrical response of the retina, or the back part of your eye, to different kinds of light. This technique is specifically used to measure the overall or global reaction of your eye to a whole-field light stimulus. This helps doctors see how well the rod and cone cells in your eye are working. These cells are important because they allow us to see, so by studying them, doctors can diagnose various eye diseases. They do this by changing the color, brightness, and speed of the light, as well as the current state of your retina.
The ffERG test results show a few different types of waves. Here’s what they mean:
a-wave: This is the initial dip in the wave you see in the results. It gives information about the outer part of the retina and how well it functions.
b-wave: This is the part of the wave that increases after the a-wave. It gives information on how the light is being converted into an electrical signal in the retina.
Oscillatory potentials (OPs): These are rhythmic wavelets that are seen on the rising slope of the b-wave and represent the electrical activity of certain inner retinal cells, primarily amacrine cells.
Photopic negative response (PhNR): This part of the wave is a light-adapted, negative deflection that comes after the b-wave. It shows the response of the retinal ganglion cells in response to a brief light flash.
The results from the test are analyzed in various ways including amplitude, implicit time, latency, and the ratio of the b-wave to a-wave. Here’s what these terms mean:
Amplitude: This measures how much electrical response is generated by various retinal cells when exposed to light.
Implicit time: This refers to the time it takes for the electrical response to reach its maximum height. It reflects how quickly an electrical signal is conducted.
Latency: This is the time from the start of exposure to light to the start of the response, as opposed to the peak of the response which is the implicit time.
The b-wave to a-wave ratio: This gives doctors an idea of how the inner part of the retina functions compared to the outer part.
Lastly, there are six standard protocols used in this test to isolate the rod and cone visual systems. These are named based on whether the eye is adapted to the dark or light and the strength of the stimulus flash.
An additional protocol known as PhNR can also be used to study the function of ganglion cells and the inner retina. In this test, a red LED flash is used on a steady, blue LED background. Blue stimuli on a yellow background can also be used to minimize the effect of different colors on the measurement of ganglion cell activity.
Possible Complications of Full-Field Electroretinogram
The ffERG test is a safe way to measure your eye’s response to light. It’s a simple test that won’t hurt, but some people might feel a little uncomfortable during the procedure. In very uncommon instances, it might cause a small scratch on the surface of the eye, depending on the type of tool used.
There are certain factors that can affect the results of this test:
* If the testing conditions (like brightness, flash intensity, the test room, how long you’ve been in light or darkness, and the size of your pupil) aren’t exactly right
* If the equipment used for the test (like the electrodes that make contact with your skin or eye) isn’t perfectly positioned
* If you move your eyes or blink during the test
* If your vision isn’t properly corrected
* Changes in your eyes that occur as you age
* Cloudiness in the eyes
* Changes in your eyes that happen naturally over the course of the day
* Sedation might make your response less noticeable
* Some differences might arise based on the type of device used for the test
It’s important to know that these factors do not make the test dangerous or unhelpful, but they can affect the accuracy of the results.
What Else Should I Know About Full-Field Electroretinogram?
The full-field electroretinogram (ffERG) measures the electrical activity of your retina, the light-sensitive layer of tissue at the back of your eye. It can be useful in diagnosing various retinal diseases. This is because changes in the electrical activity of the retina often happen early on and can be spotted before you can see anything wrong on an image of the retina.
Cone dystrophy, a genetic disorder that mostly affects the cone cells of the eyes, often shows up on the ffERG as a normal response in darkness but a prolonged response time. In light, the response can be reduced or even missing altogether. Achromatopsia is one example of cone dystrophy.
On the other hand, cone-rod dystrophy is a progressive condition that first affects the cone cells, and then the rod cells. In the early stages of this disease, the ffERG might show reduced or absent responses to light, while in later stages, responses in darkness may also diminish or disappear.
Rod dystrophy mainly affects the rod cells of the eye. The ffERG of someone with rod dystrophy will show reduced or absent responses in the dark. Certain types of congenital stationary night blindness are examples of rod dystrophy.
Vitamin A deficiency can also affect the results of an ffERG, because vitamin A is part of a substance, rhodopsin, that rod cells need to work properly. If your body doesn’t have enough vitamin A, your eyes may struggle to see in the dark, and your ffERG will show a reduced response in darkness.
Rod-cone dystrophy starts with rod cell problems and later affects the cone cells as well. In the early stages, the responses to both darkness and a bright light mix of rod and cone cells are reduced or absent on the ffERG. As the disease goes on and cone-cell function deteriorates, the responses to light also show changes. Retinitis pigmentosa is the most common type of inherited rod-cone dystrophy.
Certain retinal vascular diseases, such as a blockage in the blood vessels of the retina, can also affect the results of the ffERG. For instance, when the central retinal artery or vein is blocked, this leads to tissue damage and a much reduced response in darkness, as well as slower responses to flickering light.
Diseases of the inner retina, like optic neuropathies (nerve damage) due to conditions like glaucoma and Leber’s Hereditary Optic Neuropathy, also show changes on the ffERG, with normal waves but a decrease in the photopic negative response (PhNR), a measure of retinal ganglion cell health.
A normal ffERG can help distinguish between real and non-organic, or psychological, vision loss.
But the ffERG is not perfect; it measures overall retinal function, which means it can miss smaller issues. Focal retinal diseases, like age-related macular degeneration and Stargardt’s disease, which affect only one area of the retina, can sometimes show up as normal on an ffERG. So, if the doctor suspects these diseases, they might recommend additional, more specific testing.