In MS, the term lesion refers to an area of damage or scarring (sclerosis) in the central nervous system caused by MS. Lesions are sometimes also called plaques, and are caused by inflammation that results from the immune system attacking the myelin sheath around nerves.
An MRI scan can differentiate between active and non-active lesions. Active lesions show up in the scan as white patches when a contrast fluid containing gadolinium is injected. If the lesion does not light up, then it is likely to be an older lesion, and more than 3 months old.
With regular scans, a neurologist can tell how active your MS is, and to what extent your nerves are being damaged. Sometimes, lesions will repair themselves and not be seen on subsequent scans. Persistent lesions may eventually show up as 'black holes', where the underlying neuron has suffered irrepairable damage.
Lesions can be detected in MRI scans before you experience any symptoms, or at your first CIS. These are known as 'silent' lesions. It is difficult to attribute a particular impairment to a specific lesion, and of course lesions can also be caused by other diseases, such as stroke or migraine, and also through ageing.
The presence, distribution and type of lesions seen in an MRI scan are useful clues for a neurologist to be able to give a diagnosis of MS. They form an important part of the McDonald Criteria