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VEP

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What is visual evoked potentials (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 Visual Evoked Potentials (VEPs) testing measures the signals from your visual pathway. Small gold cups called electrodes are pasted to your head to allow us to record those signals.

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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:

  • Loss of vision (this can be painful or non-painful);
  • Double vision;
  • Blurred vision;
  • Flashing lights;
  • Alterations in color vision; or
  • Weakness of the eyes, arms or legs.

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.

What happens during a VEP test?

The procedure is very safe and non-invasive.

  1. Firstly, some wires will be glued to the top of your head to detect the brain waves.
  2. A technician will give you further instructions on what to do during the test. Normally, each eye will be tested separately.
  3. It is very important that you co-operate with the technician who conducts the test and be able to fix your vision in a certain spot. You will be asked to look at a screen similar to a television screen, with various visual patterns.
  4. Readings will be recorded through the wires on top of your head.
  5. After the procedure, the glue and wires are removed from your head. The doctor may discuss the results of the test with you after they have been analysed; otherwise the referring doctor will.
  6. Usually the procedure takes about 45 minutes.

What the results 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.

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