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What is a visual evoked potential? (VEP)
A visual evoked potential is an evoked potential caused by a visual stimulus, such as an alternating checkerboard pattern on a computer screen. Responses are recorded from electrodes that are placed on the back of your head and are observed as a reading on an electroencephalogram (EEG). These responses usually originate from the occipital cortex, the area of the brain involved in receiving and interpreting visual signals.
When is the VEP used?
A doctor may recommend that you go for a VEP test when you are experiencing changes in your vision that can be due to problems along the pathways of certain nerves. Some of these symptoms may include:
These changes are often too subtle or not easily detected on clinical examination in the doctor’s surgery. In general terms, the test is useful for detecting optic nerve problems. This nerve helps transfer signals to allow us to see, so testing the nerve allows the doctor to see how your visual system responds to light. The test is also useful because it can be used to check vision in children and adults who are unable to read eye charts.
What does the VEP detect?
The VEP measures the time that it takes for a visual stimulus to travel from the eye to the occipital cortex. It can give the doctor an idea of whether the nerve pathways are abnormal in any way. For example, in multiple sclerosis, the insulating layer around nerve cells in the brain and spinal cord (known as the myelin sheath) can be affected. This means that it takes a longer time for electrical signals to be conducted from the eyes, resulting in an abnormal VEP. A normal VEP can be fairly sensitive in excluding a lesion of the optic nerve, along its pathways in the anterior part of the brain.
How to prepare for a VEP test
You will be given instructions on how to prepare for the test. This will depend on where you are going to get the test done. Some things that you may need to do include:
What happens during a VEP test?
The procedure is very safe and non-invasive.
There are rarely any side effects from this procedure. It is a painless procedure and apart from possible minor skin irritation from the electrodes, there are often no complications. If you have any medical conditions such as epilepsy, the visual stimuli you are exposed to is unlikely to set off seizure activity. This should however be noted with your doctor and technician before you undergo a VEP test.
After the procedure is done, patients normally return home on the same day. You should be able to drive home safely if you are feeling well after the procedure.
Factors influencing VEP
The cells within the part of your brain involved with vision are most sensitive to movement at the edges of your central visual fields. If you have poor vision, this does not seem to have too much effect on the response – larger checkerboard patterns can be used. Your gender, age and the size of your pupils are other factors that can affect the VEP. If you have taken any drugs that make you drowsy, or under the influence of any anaesthetic drugs, your VEP is also greatly affected.
What the results of the VEP may show
The VEP is particularly useful in detecting past optic neuritis. This refers to inflammation of the optic nerve, associated with swelling and progressive destruction of the sheath covering the nerve, and sometimes the nerve cable. As the nerve sheath is damaged, the time it takes for electrical signals to be conducted to the eyes is prolonged, resulting in an abnormal VEP. This may be seen in multiple sclerosis – one of the most common causes of optic neuritis (as above). Abnormal VEP’s are seen in multiple sclerosis patients due to the presence of optic neuritis.
The following are less easily differentiated but may cause abnormal VEPs:
Clinical usefulness of the VEP
The VEP is an important test that is very good at detecting problems with the optic nerve and lesions in the anterior part of our visual pathway, before the optic nerves merge. However, it is a non-specific test and to determine the exact underlying problem in each patient, a good history and examination is also very important.
Article kindly written and reviewed by:
Dr Karl Ng MB BS (Hons 1) FRCP FRACP CCT Clinical Neurophysiology (UK)
Consultant Neurologist and licensed botulinum toxin administrator
Sydney North Neurology & Neurophysiology (download referral form and map)
Conjoint Senior Lecturer, University of Sydney and Editorial Advisory Board Member of the Virtual Neuro Centre