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Classification of demyelinating diseases
Many diseases of the nervous system result in damage to, or destruction of, the myelin sheaths of nerve fibres. The appearance of areas of demyelination, which may be situated either in white matter of the brain and spinal cord, or in peripheral nerves, including the optic nerves, varies. They vary in size, shape and distribution, in the speed with which they appear, and with their ability to recover.
The aetiology of most demyelinating disorders is unknown. In some cases genetical factors are involved; others may follow viral infections such as measles and small pox or vaccination, and demyelination or destruction of nerve fibres and surrounding myelin occurs in a wide variety of disorders involving the nervous system
Demyelination can be regarded as either primary or secondary. The primary form destroys or damages myelin or myelin-forming cells and axons are relatively normal, at least in the early stages. Secondary demyelination, on the other hand, follows damage to neurons or axons, followed by breakdown of myelin. Many pathological processes such as cerebrovascular disease, infarcts, haemorrhages, trauma and tumours result in the breakdown of both together.
The visual pathways, from the optic nerves to the visual cortex contain myelinated nerve fibres and these may be damaged at any point.
Too little is at present known about the aetiology of many demyelinating disorders to enable a precise pathological classification. Similarly, a clinical classification is impossible in that there is marked overlap in the clinical presentation of disorders that can clearly be distinguished by other techniques. Current classification, therefore, depends upon a mixture of clinical and pathological features, including genetics, biochemical and immunological abnormalities. Some metabolic defects influence the formation of myelin rather than damage to myelin once it has been formed. The group of diseases associated with specific metabolic defects influencing the formation of myelin have now been separated under the general term "dysmyelinating disorders or leukodystrophies".
Demyelination may also be caused by a variety of drugs, toxins, vitamin deficiencies and viruses. For a more detailed description of these the reader is referred to books listed in the reading list at the end of the script.
The following list includes the main demyelinating diseases at the present time:-