Neuroradiology

Demyelinating disease

(as opposed to dysmyelinating diseases), conditions in which normally formed myelin is later destroyed by different pathological agents; the definition thus literally means loss of myelin. The pathological condition is that of loss of myelin and oligodendrocytes, with relative sparing of axons; conditions in which myelin is lost secondarily to axonal damage should not be included in this group. It may not, however, always be clear whether the loss of myelin is primary or secondary in nature since the two structures, axons and myelin, are closely related.

This is a group of diseases difficult to classify because of both terminological confusion and lack of knowledge of the precise aetiology of many of them. Terms like leukodystrophies, white matter disorders, myelinoclastic disorders and dysmyelination are sometimes used randomly as if they were all synonyms. The true demyelinating disorders are those in which the myelin sheath is destroyed, having been normally formed. This contrasts with dysmyelinating disorders in which "myelin is not formed properly, or in which myelin formation is delayed or arrested or in which the maintenance of already formed myelin is disturbed."

The most common primary demyelinating diseases are: acute disseminated encephalomyelitis, multiple sclerosis in its various forms, demyelinating disorders associated with systemic disease, central pontine myelinolysis (osmotic demyelination), progressive multifocal leukoencephalopathy and Marchiafava Bignami syndrome.

With a few exceptions, the causes of demyelinating diseases are unknown. The known pathogenic agents and mechanisms include viral infection, postinfectious or postimmunization immune responses, and toxic chemical agents.

GS