Evoked potentials
These are simple electrical tests, usually carried out on vision, which can detect a delay in messages between the eyes and the brain.An evoked potentials test measures the speed of nerve messages along sensory nerves to the brain.
The most commonly used test is called visual evoked potentials (VEP), which measures messages sent from the eyes in response to being shown a flashing chessboard pattern on a computer screen.
Tests of sensations from the skin (somatosensory evoked potentials), which involve tiny electric shocks, and of hearing (auditory evoked potentials), using clicks, can sometimes also be carried out, though much less commonly.
Evoked potentials work on the basis that it is possible to measure how long it takes for input from the eye, for instance, to get to the visual cortex at the back of the brain where this information is processed. As damage to the optic nerve in MS can slow messages down, evoked potentials tests can indicate the presence of an area of scarring that is not causing any obvious symptoms (described as clinically silent). Delays of as little as 10 milliseconds can indicate that there is damage to the nerve pathway.
Lumbar puncture
This procedure involves drawing off a sample of fluid from around the spine and testing for abnormalities that can indicate MS.A diagnostic test that involves removing and analysing a sample of cerebrospinal fluid (CSF), the fluid that surrounds the brain and spinal cord within the skull and backbone.
The procedure takes about half an hour under a local anaesthetic. A hollow needle is inserted into the base of the spine and a quantity of cerebrospinal fluid is drawn off.
The overall discomfort experienced during the procedure itself is not great. However, the drop in pressure in the cerebrospinal fluid caused by the removal of a sample can produce a splitting headache. This usually lasts for no more than 24 hours but can persist for longer. To reduce the risk of headaches, it is suggested that the individual lie flat for at least six hours after the test and drink plenty of water.
Analysis of cerebrospinal fluid in the diagnosis of MS is looking for:
- White blood cells - the number of white cells in cerebrospinal fluid is usually up to seven times higher than normal in people with MS. If the count is even higher than this, it is probably due to an infection of some sort, such as Lyme’s Disease and not MS.
- Oligoclonal bands – the immune system produces antibodies to fight infection. In MS, antibodies cross the blood-brain barrier and attack the myelin surrounding nerves. As a result, the level of antibodies in the cerebrospinal fluid of someone with MS is higher than it should be and is higher than the level in the blood, a sample of which is also analysed.
One band (monoclonal) in the cerebrospinal fluid is normal. The term oligoclonal bands refers to the presence of two or more bands and shows the presence of disease activity. Whilst this doesn’t necessarily mean that someone has MS, more than 80% of people with MS do have oligoclonal banding in their cerebrospinal fluid.
(published with permission in writing from:http://www.mstrust.org.uk)
