Multifocal Electroretinograms

How was Sure. Donald Creel. Hello, I'm Don Creel, director of Electrophysiology at the Moran Eye Center in Salt Lake City.

Today we're gonna talk about multifocal electroretinogram. Eric Sutter adapted the mathematical sequences called binary M sequences so that one can isolate from a single electrical signal and electroretinogram approximation from each square millimeter of the retina. These electroretinogram looked like a flash electroretinogram but are in fact a mathematical average of an approximation of a B wave.

Let's go take a look at the recording equipment. When selecting a room for electrophysiological recording, choose a room with a little electrical interference. Do not choose a room adjacent to a source of high voltage interference, such as an elevator, high amperage, autoclave, or centrifuges.

A room with a sink is convenient for washing your hands and cleaning electrodes. An internet connection is good to have so you can share data and the manufacturer of your system can download software updates and access your computer to help solve problems. Be compulsive about the organization of your room.

Keep your dilating drops balance salt solution in same location so they are easily accessible. Be compulsive about your recording procedures. Use the same room lighting.

Place every patient In the same location. The Moran Eye Center system is Eric Sutter's Vera System manufactured by EDI. The Vera system uses a micro display to display the stimulus pattern.

Most multifocal ERG systems display their stimuli on a video display with the subject viewing at a distance of a half meter or less. Most stimulus displays use a pattern similar to this. The fixation image varies between manufacturers.

The VERA system allows one to choose any size X circle or Maltese cross. These shapes can be adjusted to any font and pen size from as small as several millimeters to the full width of the stimulus field. Using the Maltese cross is useful in patients with macular scotomas.

You can adjust the size and width of the tails of the Maltese cross so the patient can estimate the center and hold that fixation point. Regardless of the system that's used, the results are usually displayed on a video monitor and would appear such as this. I have an automatic backup system that backs up whatever data has been recorded every few minutes.

I recommend this in case there's a problem with the computer. Again, I recommend being compulsive about your layout of what you use for your dilating drops, your artificial tears and solutions you might use to clean your electrodes. I use a dilute solution, a 10%dilute solution of clothing bleach I dilute from the original down to 10%and keep it in a smaller bottle, such as a one liter bottle with distilled water available.

For the rinsing. Clearly mark a container to be your cleaning, which we mark with our red dot. We clean the contact lenses in that and then rinse it and tap water and distilled water.

Follow the recommendations of the manufacturer of your contacts. If you use reusable contact Lenses. There are a Number of kinds of eye electrodes.

I'm gonna show you some of them here. This is the ERG jet, which looks like this placed on the eye. This is the ardent gold foil, which is a piece of cassette tape with gold foil on one side when placed in the eye, hanging usually from the lower lid.

The gold foil touches the sclera and records the eye electricity. You can use a small silver wire. These are called DTL.

These are disposable. They come with these self adhesive locations that go near the inner and outer can, and then the silver wire touches the sclera and the lower part of the eye. I use the clear bipolar buran type speculum contact that's manufactured by Hanson Laboratories in Iowa City, Iowa.

If you're not using a bipolar contact lens, you need to select another location for the negative reference in ophthalmology. The forehead, the mastoid or the ear lobe were commonly used for the ground. You can use any location such as the wrist.

I happen to use the ear lobe. I clean well first with an alcohol. Then I clean with a commercial cleaner that's sold as an EEG preparation paste.

The better your connections, the better your data. I'm gonna go through the main procedure of hooking up a patient. For a multifocal electroretinogram, I numb the eye with several drops of a topical anesthetic.

We use propane. I usually give two or three drops to make sure the eye's good and numb. You want to avoid moving the contact across the cornea, so I ask the patient to look down, place the contact high up on the sclera, and then close the contact straight down so it doesn't abbra the cornea.

In my testing situation, I then use head cups to secure the head, to make the head as stable as possible, asking the patient if they're comfortable, and then line up the micro display. I guide the patient's hand to a focus knob on the VERA system. You can then see the eye displayed.

You can see the pattern falling on the pupil. One of the features of the VERA system is the external video of the eye, allowing you to coach subjects to maintain fixation if they're gaze FALs. You can also switch to the fundus view, enabling the projection of a pattern onto the retina in animal models or anesthetize patients.

I wanna reemphasize the procedures used to connect the patient and putting the contact lens in clean very well. Whatever locations you're going to use either for your negative reference or for your ground, the better the connection, the better the data. Again, numb the eye with several drops of a topical anesthetic such, such as procaine.

The patients only need to be dilated. With my datic drops. One only needs about a seven millimeter pupil for the stimulus image to fall on the central 40 degrees or so of the retina.

Again, in the electrodes I use, I have them look down well and then place the contact straight in so it doesn't ab braid the cornea at all. To secure its placement on the eye, I line up the center of the pupil with the center of the contact lens and put a small piece of tape on the cheek to hold its position and then place the patient in head cups to help secure the head and ask them if they're comfortable in its position. Then I line up the micro display, always patch the other eye.

Some people have such a dominant eye that even though you are asking them to look at the pattern with the one eye, they won't look directly at it if you leave the dominant eye open, and then I ask the patient to focus so that they have a sharp image. Ask the patient just to attend to their fixation point, and this is what it looks like during a recording. You can see the pattern moving on the cornea.

This is what they're seeing inside of the micro display, and as most stimuli are displayed on video monitors of other multifocal electroretinogram systems, it's important that the patient maintains fixation. If you don't maintain fixation, you get multiple points indicating the patient is focusing at different locations during the recording instead of just one central peak. One of the many applications of the multifocal electroretinogram is to track the progression of macular degeneration.

This patient shows the beginnings of macular degeneration in one eye. You can see the reduced ERGs in the central portion of this eye. The color display would look like this.

Comparing that to normal individuals below another application of the multifocal electroretinogram is to detect and follow the progression of patients that are taking medications that can be toxic to the eye, such as chloroquine or Plaquenil. These individuals, if the medication is toxic, can produce ring scotomas usually between five and 10 degrees from the fovea. This is the display of an individual with significant retinal toxicity from Plaquenil.

You can see the scotomas areas around the foveal area between five and approximately 10 to 15 degrees, and the ring scotoma that this produces. This is a very severe expression of toxicity from Plaquenil. This is the visual field of an individual who had been struck in the eye showing the visual field loss.

This shows the multifocal electroretinogram, which are attenuated in the similar area as the visual field loss, and this is the color representation of the field loss. Yet central sparing with scotomas area beginning approximately five degrees from the fovea and extending to about nine o'clock in his visual field. Traditionally with multifocal electroretinogram, people mainly looked at what you might call the B wave amplitude.

It's important to also look at the implicit times. Diseases vary whether the implicit times are affected. In addition to amplitude attenuation, this is a patient with birdshot chorio retinopathy.

These individuals can have any stage of retinal degeneration, but in the early stages, characteristically they just have slowing of the implicit times, especially of their cone physiology. This displays slowing of implicit times beyond two or three standard deviations. You should look at implicit times in all patients.

This figure shows the multifocal electroretinogram of a patient with big blind spots syndrome. You can see the depressed area around the normal blind spot greatly enlarged. This shows the color transformation of the amplitude in the area of the big blind spot syndrome.

This shows the implicit times of a big blind spot syndrome. Patient transformed into a color scale. The white areas show slowing of five standard deviations.

The pink area is slowing of four standard deviations in implicit time, and the red area is three standard deviations. In implicit times, as I mentioned earlier, it's important that you start looking regularly at the implicit times of patients and not just the amplitudes of their multifocal electroretinogram. This is a patient with Stargardt.

You can see the attenuated multifocal electroretinogram in the central portion of the retina. This figure shows the color transformation of the amplitudes with No central peak visible. In summary, I encourage those recording multifocal electroretinogram to broaden their horizons and apply new protocols and analyses.

The multifocal electroretinogram is not just the one analysis used by most, which represents the mathematical approximation of BWA amplitudes. Look at other analyses, especially implicit times, apply different protocols appropriate for each patient. Ask if there are other protocols specifically designed for different patient populations.

Multifocal electroretinogram comprise a system of many possible protocols and analyses.