We should be grateful to be able to see many things in the world. The ability to see the world through the eyes is something that is often taken for granted. There are many kinds of diseases or conditions that can cause a person to have trouble seeing. These can be a great difficulty for a person to live and may be reasons for them to not be happy or content as much as other people who can see well. In this DoctorOnCall’s article, we will learn more about the VEP test and its function in diagnosing diseases.
A VEP test or visual evoked potential test is a test that measures signals from the vision pathway. It is a test that evaluates the function of the visual system that includes the retina, optic nerve and visual pathways in the brain. In general, the VEP test is very useful to evaluate optic nerve problems.
VEP test is often recommended in patients who experience symptoms of changes in vision that is suspected due to any part of the vision pathways. Symptoms such as loss of vision, double vision, blurred vision, flashing light, changes in colour saturation of vision or weakness to the body. VEP tests can detect changes that may be difficult to be seen or detected in clinical settings or even in surgery.
VEP measures the time taken for a visual stimulus (visual input) to reach the eye and to the occipital cortex (area of the brain that plays a great role in visual processing). Depending on the characteristic of the stimulus, VEP can provide different information of the various parts of the visual field affected aside from the integrity of the visual pathways.
VEP test is commonly used to diagnose multiple sclerosis (autoimmune disease affecting brain and spinal cord). Other diagnosis that can be made with VEP test are optic neuritis (inflammation of the optic nerve), ocular hypertension (high pressure inside the eye), glaucoma (eye damages due to high pressure inside the eye), amblyopia (lazy eyes), lesion such as tumours compressing the optic nerve and ischemic optic neuropathy (ION). These diseases often show symptoms of visual disturbances that may cause permanent vision loss when it is not treated. Beside diagnosing these diseases, VEP is also used in monitoring visual function in babies, predicting outcome for poor vision prior surgery and to rule out malingering (falsification or profound exaggeration of physical or mental illness with the goal of receiving specific perks or benefits). VEP test may also be used to detect early neurodegenerative disease such as Alzheimer’s and Parkinson’s disease. VEPS may also be combined with an MRI scan known as functional MRI (fMRI) that is useful in identifying injury of the brain and mapping the visual cortex.
Patients who are planning to go for a VEP test may want to make sure they are prepared before the procedure. What patients can do is to ensure they already wash their hair the night before the test and avoid using chemicals, oils or lotions on the hair. Those who already wear glasses or contact lenses are advised to bring and use these tools as some tests may require these tools. If a patient already has eye conditions such as cataracts or glaucoma beforehand, do notify the doctor so they are aware of this condition as these eye conditions may affect the results. Any medication that can cause drowsiness should be avoided before the test and discuss with the doctor if this medication is a must. Last but not least, patients should arrive on time for the test and ensure they had a good rest the night before. This can help patients feel relaxed for the VEP test day.
VEP test is performed by a trained electrophysiology technician or medical professional. Electrode paste is used to place the scalp electrodes on the patient’s head. This electrode paste is water soluble which makes it easy to be removed with shampoo and water after the test ended. During the test, patients are asked to concentrate on the centre of the screen and flashing lights. Each eye will be tested separately by covering one eye at a time. The VEP test should take around 45 minutes to 1 hour. It is important to follow all instructions or directions given during the test. There is no known side effect or risk involved although in some cases patients might complain of minor skin irritation from the electrodes. The VEP test should not be painful at all. People with epilepsy can be assured that the test would not evoke any seizure activity.
It can be concluded that a visual evoked potential (VEP) test is a test used to evaluate the visual pathways. There are many diagnoses that can be made with VEP test but commonly it is used to diagnose multiple sclerosis. Patients who need to undergo VEP test need to follow all advice by their doctor and technician before and during the test to ensure all goes well.
