DEMYELINATING DISEASES

There is a group of diseases which involve damage to, and destruction of, myelin sheaths that surround many nerve fibres. The axons of these fibres may be damaged as well. The mechanism of myelin damage may be different in different diseases, but they have been grouped together under the general term of "demyelinating diseases". They are predominantly white matter diseases and the demyelination is often perivenous. A well-demarcated area of demyelination is know as a "plaque". They often show a predilection for the subependymal region and the area beneath the pia mater, which covers the surface of the brain. The blood vessel within the centre of the area of demyelination is usually patent. Myelin is broken down into a variety of products and that these can be recognised using stains for lipids. It is important to realise that there are many diseases in which myelin destruction occurs as a secondary phenomenon and that these usually show a pattern that is not particularly perivenous, although this may be seen in some, such as Vitamin B12 deficiency, and following occlusion of small blood vessels following embolisation.

The range of causes is wide and includes various virus infections, immunological disorders, nutritional or metabolic disorders, toxins ( including some used in therapy such as anti-mitotic agents and radiotherapy ), genetic factors, and following prolonged oedema or hypoxia.