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Visual Electrophysiology Unit

Commonly asked questions for patients and families

Your child has been referred to the Visual Electrophysiology Unit at HSC for one of the following tests:

  • An electroretinogram (ERG)
  • A focal ERG
  • A visual evoked potential (VEP)
  • An electro-oculogram (EOG)
  • Volunteering

Here are some commonly asked questions about these tests and what they each involve.

What is an ERG?
An ERG (electroretinogram) is an eye test, which is used to measure the electrical activity in the retina. The retina is the light sensitive lining inside the eye which makes a picture of what we see, rather like the film in a camera. The ERG is used to test how well the retina is working when a disease of the retina is known or suspected. Sometimes an ERG is performed on the relatives of people who have a retinal problem.

What will happen when I come for an ERG test?
Usually your vision is tested first to see how well you can see in each eye. This is done in the same ways your eye doctor uses. Your colour vision may be tested by asking you to pick out coloured shapes or colour chips or by reading coloured numbers or symbols.

After the vision and colour tests, eye drops are put in both your eyes to dilate the pupils (make the black part in the centre of your eye bigger). After a few minutes you may notice that your near vision becomes blurred temporarily. This is normal.

What happens next?
Patches are placed over both your eyes for about 30 minutes so that your eyes get adjusted to darkness. You will wait in a designated waiting area. After the 30 minutes you are guided into a testing room where you will sit on a chair for the test. The test is performed in darkness so the tester will wear a red light on her/his head (just like a miner’s lamp). The patches will be removed from one eye. A small area on your forehead will be cleaned with an alcohol swab and a small sensor or electrode will be placed there with some paste and tape.

Next an anaesthetic eye drop will be placed in your eye to numb it. Then a special contact lens will be placed gently on the front of your eye and secured with some tape on your skin. You will be asked to sit with your chin on a chin rest and watch some flashing lights. An electrode in the contact lens will pick up signals from the retina responding to the flashing lights.

After your first eye is tested the process may be repeated on the second eye, or both eyes may be tested together. There is no electric current going to you. You cannot get shocked or electrocuted.

Does it hurt?
The eye drops may sting for a few seconds. When the contact lens is placed at the front of your eye it may feel strange, like a pushing sensation, but it will not hurt. You will be asked to sit still and not blink while the lights are flashing. The contact lens is bigger than a normal one so it helps to keep your eyelids open for you.

After the test is finished your vision may be blurred up close for several hours from the eye drops and you may be light sensitive until your pupils return to their normal size again.

Can my child be put to sleep for this test?
Children between the ages of 10 months and four years are not usually able to cooperate with ERG testing as it is done in a dark room and small children may find it scary. If your child is in this age group and is medically well enough to have the test, the ERG can be done under outpatient sedation or general anaesthesia.

Do you have to be a certain age to get an ERG?
No. This test can be performed on young infants through all ages.

Can any place do this test?
Most hospitals and eye doctors do not do this specialized testing. It is done usually in a designated centre, often at a university hospital.

What is a focal ERG?
A focal ERG is very similar to a regular ERG, but there are a few differences. This test measures the electrical activity present in a small part of the retina, which is called the macula. The macula is the part of the retina which lets us see straight ahead, gives us detailed vision and allows us to see colours.You will not have to sit in the dark and instead of sitting in the testing room with your chin on a chin rest, you will sit in a reclining chair. Instead of watching flashing lights, a bright light will be shone directly into your eye by the tester. You will be asked to look into the light and hold your gaze still. Otherwise the testing procedures are very similar. After testing one eye, the other eye may be tested.

Can anyone do this test ?
The accuracy of this test depends on your/your child’s ability to cooperate. From experience we do not expect children under 12 to be able to complete this test.

What is a VEP?
A VEP (visual evoked potential) is a test which is used to measure the electrical activity of the visual system, similar to the way in which EKGs are used to measure the heart’s activity. By analyzing the electrical signals that we record from the vision centres in the brain we can find out how well the visual system is working (that is, how well the eyes and brain work together). It is a way of telling if the vision messages are getting from the eye to the brain properly.

What will happen when my child and I come for a VEP test?
Usually your vision is tested first. Adults may be asked to read a regular eye chart like the one your eye doctor uses. Babies and young children who cannot recognize letters may be encouraged to look at some stripes or pictures on cards. By watching the child’s reaction to these cards it can be determined how well he/she can see.

For the VEP test, very small metal discs attached to a wire, (sensors), are placed on your/your child’s skin. There is no electricity going through the wires. You cannot get shocked or electrocuted. Three or five electrodes will be taped to the back of you and your child’s head and one on each earlobe, rather like a clip-on earring. Then you will sit on a chair in front of a computer and watch striped or checkerboard patterns on the monitor in front of you. If your child is being tested he/she can sit on your lap.

Each eye may be tested separately and/or both together. We ask parents with young children to encourage them during the test by talking to them, singing to them, and making them feel secure.

Does it hurt?
A VEP test is painless. Eye drops are not needed for this test. Some babies may not like strangers attaching sensors to them, but it does not hurt. They are attached with paste and tape after the skin has been cleaned. This may cause some mild discomfort to the area being cleaned. After the test the sensors will be removed and the paste will be cleaned off. Sometimes small amounts of paste remain in the hair, but this is easily removed when you wash the hair.

Can my child be put to sleep for this test?
No. We need your child to be wide awake for this test, to get the best results.

Do you have to be certain age to get a VEP test?
No. This test can be performed on young infants of all ages.

Can any place do this test?
Most hospitals and eye doctors do not do this specialized testing. It is done usually in a designated centre, often at a university hospital.

What is an EOG?
An EOG (electro-oculogram ) is another test we use to measure the electrical response of the retina. The EOG is used to test how well the cells under the retina are working when certain diseases are known or suspected. Sometimes an EOG is performed on the relatives of people who have a retinal disorder.

What will happen when I come for an EOG test?
Your vision may be tested first to see how well you can see in each eye. We will ask you to read letters on a chart, similar to the one your eye doctor uses. Then we will test your colour vision, by asking you to pick out coloured shapes or colour chips or reading coloured numbers or symbols.

What happens next?
Two small metal disks attached to a wire (sensors ) will be placed on either side of the right and left eye, and a fifth sensor will be placed on the forehead. There is no electricity coming to you through the wires. You cannot get shocked or electrocuted. You will sit on a chair in front of a white dome. You will be asked to put your chin on a chin rest and look at a background light for a few minutes. You will be shown a red light that moves to the left and right, once a second for six seconds. You will be asked to follow the light with your eyes, while your head is still. Then the room lights will be turned off, and once a minute you will be asked to follow the lights. The room lights will remain off for 15 minutes, and will then be turned on. You will be asked to continue following the red lights for six seconds once a minute, for a further 15 minutes.

Does it hurt?
The EOG test is painless. Eye drops are not needed for this test. The face on either side of the eyes will be cleaned with a mild abrasive paste. This may cause mild discomfort to the areas being cleaned. After the test, the sensors will be removed and the gel cleaned off.

Do you have to be a certain age to get an EOG test?
The accuracy of this test depends on your/your child’s ability to cooperate. From experience we do not expect children under six to be able to complete this test.

Can any place do this test?
Most hospitals and eye doctors do not do this specialized testing. It is done usually in a designated centre, often at a university hospital.

Volunteering

The Visual Electrophysiology Unit in the Eye Clinic at The Hospital for Sick Children is looking for participants aged 8 weeks to 80 years with no known eye problems to take part in our ongoing research projects. If you have two hours to spare and are willing to help us, please contact Carole Panton at 416-813-6133 or by e-mail at carole.panton@sickkids.ca

Although you will not benefit directly from participating in this research, the results will provide data essential to define normal limits according to age. Compromised visual development may be identified by comparison of data with these normal limits.

Reimbursement: We can offer you $20 towards your transportation or parking costs to come to the hospital and participate in our studies. Even if you change your mind about participating in the study during the procedure, we will still reimburse you $20 towards your costs.

Useful Resources

Assessing Children's Vision - A Handbook
Susan J. Leat, Rosalyn H. Shute, Carol A. Westall
Butterworth-Heinemann Oxford, Auckland, Boston, 1999
Can be ordered through academic bookstores - ISBN 0 7506 05847

"This concise yet comprehensive handbook gives a very practical approach to assessing children's vision. It ranges from simple visual acuity assessment to the most contemporary technical testing techniques, their expected outcomes and implications." Butterworth Heinemann

Chapters:

  1. The developing child and visual system.
  2. The importance of early visual assessment.
  3. Opening the consultation.
  4. Ocular health.
  5. Refraction.
  6. Psychophysical testing of infants and children.
  7. Visual Acuity - by Kathryn Saunders.
  8. Contrast sensitivity.
  9. Binocular vision.
  10. Visual fields.
  11. Colour vision.
  12. Eye movements.
  13. Electrophysiological testing.
  14. Taking action