Cerebrospinal fluid (CSF) is a clear, colourless liquid that surrounds the entire surface of the central nervous system. This acts to cushion the brain and spinal cord within the bony casing of the skull and backbone and helps support the weight of the brain.
In MS, damage to the myelin sheath around nerves releases proteins into cerebrospinal fluid. These can be detected by analysing a sample of the fluid collected by a lumbar puncture. This test is sometimes used in the diagnosis of MS.