Evoked Potentials
What are Evoked Potentials?
These are recordings of electrical signals produced naturally by your brain following repeated stimulation of the eyes, ears or arms and legs.
The tests take about an hour. A neurophysiologist starts by making measurements of your head, and after gently rubbing over the marks, attaches small metal discs using a sticky paste. Please ensure that your hair is clean and free from grease, hair spray and lacquers etc. Loose clothing will be most comfortable. You may wear jewellery but it may have to be removed if it gets in the way. After the test the discs are removed and any paste left in the hair can be washed out with shampoo.
What are Visual Evoked Potentials (VEPs)?
These are tests of the pathway between the eye and the back of the brain. You will be asked to look at the centre of a television screen that has a moving black and white chequer board pattern. Each eye is tested separately, with a patch covering the eye not being investigated. This is a painless procedure and there are no after effects. When the test is finished you will be able to return home or to work/school. If you wear glasses please bring them along for the test.
What are Somatosensory Evoked Potentials (SEPs)?
These are tests of the pathways from the arm or leg through the spinal cord to the brain. An electrical stimulator is used to pass brief electrical currents to your nerves. This causes an unusual prickling sensation and there may be small, uncontrollable twitches of your hand and feet. This can be uncomfortable, but should not be painful. There are no after effects.
What are Brainstem Auditory Evoked Potentials (BAEPs)?
These test the pathways from the ears. A pair of headphones is used to give a clicking sound to one ear at a time and a continuous noise to the other ear. This is a painless test and there are no after effects.
What is an Electroretinogram (ERG)?
This is a test of the electrical activity from the eye. It is usually necessary to have a very fine metal electrode looped between the lower eyelid and the eyeball for this test, but it can sometimes be done with a sticky pad on the lower lid. You will usually have to be in the dark for 20-30 minutes before the tests start, and then you will be asked to look at flashing lights of different colours and brightness. The electrode on the eye can feel unusual and sometimes causes irritation during the test and you might be given eye-drops to prevent this. Very rarely the front of the eye (cornea) may be scratched, causing irritation and watering of the eye after the test has finished. If this happens you should visit your family doctor. If you do have eye drops you will have blurred vision for a few hours after the test and so will not be able to drive home: otherwise there are no after effects of the test. If you suffer from epilepsy there is a very small, theoretical risk that the tests could cause a seizure. It is not known whether this has ever happened, but if you suffer from epilepsy you should let the doctor or neurophysiologist know at the start of your test.
If you have and questions about the issues raised above, or any other concerns, please talk to the neurophysiologist before the start of your test.
