Department of Clinical Neurophysiology
What are visual evoked responses?
This leaflet is designed to give you some information about the tests for which you have been referred. The procedure will be explained again on the say by the physiologist doing the test who will be happy to answer any further questions you may have.
What is involved?
Firstly, the tests are non-invasive and do not need injections, please advise the person performing the test if you have any allergies such as medical tape.
Adults:
To prepare you for the test it is necessary to attach small disc electrodes to the surface of the head with tape or a sticky paste. Depending on which tests you have been referred for it may also be necessary to add a fine thread electrode along the edge of each bottom eyelid. This may initially make the eyes water but are usually tolerated well in most patients. It may also be necessary to use dilating drops to enlarge the pupils, these will need time to work. You may also have to sit in the darkness to allow the eyes to adjust before measurements can be made of how well the eyes work in the dark.
Children:
The process is adapted when testing children and babies to shorten the process.
What tests will I have?
There are 4 main tests available. You have been referred for one or more of the following tests:
Visual Evoked Potential (VEP)
This is a test of the pathway between the eye and the vision area at the back of your brain. We will record the electrical signals produced naturally by your brain following repeated stimulation of each eye. Typically, this takes 30-45minutes.
Pattern Electroretinogram (PERG)
This is a measure of the function of the central part of the retina, which is used for detailed vision e.g. reading. This is usually recorded simultaneously with the VEP above.
Electro-oculogram (EOG)
This is a measure of how the eye gradually adjust to dark conditions and readjusts to normal light conditions. Typically, 45 minutes.
Electroretinogram (ERG)
This is a test of the retina at the back of the eye. Electrical signals produced by the retina are recorded in response to flashes of light. Different types of flash (colour, rate and brightness) are used and both eyes are recorded from simultaneously. Part of the process involves allowing the eyes to adapt to dark conditions. This may take 1-2 hours.
Preparation, what do I have to do?
It is very important that you bring with you all current glasses. If you wear contact lenses please also bring your case and necessary solutions, as it may be necessary to remove them. If you are having ERG’s please bring some sunglasses, you will NOT be able to drive home after the dilating drops are given. Please arrive on time otherwise it might not be possible to perform all of the tests. If you have any further questions please call the department on 01752 430193 and we will do our best to answer them.
Are there any side effects?
The dilating drops will cause your vision to be blurred and be more sensitive to light for several hours after the tests but this will wear off. You may need to wear sunglasses on bright days. You should NOT drive until this has worn off. There is a very small risk of developing a condition called closed angle glaucoma after the dilating drops are given. This is a very rare but serious condition and can cause loss of vision if left untreated. If any of the following occur please seek immediate medical attention: red, painful eye, nausea or vomiting. The condition is mainly seen in the over 55 age group. You will need to wash your hair to remove the paste from your head. Normal shampoo is sufficient.
What happens next?
The physiologist will compile a report which will be forwarded to the doctor that asked for the tests.
Any questions?
If you have any questions or concerns about your appointment, please do not hesitate to contact us on 01752 430193 before your appointment, or talk to us before the test. The department is open Monday to Friday 8am to 5pm. There is an answerphone service to leave a message if we are unavailable.
Date issued: April 2018
For review: April 2020
Ref: B-185/NB/Electro-diagnostic Test v4
