Myelin Disorders Definitions

'Demyelination' means, literally: loss of myelin and the literal interpretation of 'demyelinating disorders' is: disorders characterized by loss of myelin. The term demyelination is commonly used to indicate the process of losing myelin, which is caused by primary involvement of oligodendroglia or myelin membranes. Myelin loss that is secondary to axonal loss and simultaneous loss of axons and myelin sheaths is not usually included under the heading of demyelina-tion.

However,there is considerable confusion about the meaning of the terms demyelination and demyelinating disorders. Sometimes demyelination is used to mean all conditions in which loss of myelin occurs, irrespective of whether the myelin membrane was primarily affected or was broken down secondary to or at the same time as axonal loss. This is probably partly because it is not always clear whether the loss of myelin is primary or secondary in nature. The mutual dependence of axons and myelin sheaths is an important factor in this respect. Demyelination will eventually lead to axonal loss, and in the end axonal degeneration will lead to loss of myelin. Hence, using histological examination it may be very difficult to differentiate between primary and secondary myelin loss. Another confusing factor is that some disorders show evidence of simultaneous primary neuronal degeneration and primary demyelination. The random use of related terms, such as dysmyelination, myelino-clastic disorders, white matter disorders, leuko-encephalopathies and leukodystrophies add to the confusion.

Poser (1957) introduced the concept of'dysmyeli-nation.' He proposed dividing the disorders characterized by primary myelin loss into 'myelinoclastic disorders' and 'dysmyelinating disorders' (1961, 1978). He considered the myelinoclastic disorders to be the true demyelinating disorders, in which the myelin sheath is destroyed after having been normal ly constituted. Examples are multiple sclerosis and acute disseminated encephalomyelitis. The dysmyeli-nating disorders comprise those 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." Examples are metachromatic leukodystrophy and adrenoleukodystrophy. The idea behind the concept of dysmyelinating and myelinoclastic disorders is to distinguish between inherited disorders, especially inborn errors of metabolism, leading to disturbed myelination and myelin loss, and acquired disorders characterized by primary myelin loss. However, the definition of dysmyelinating disorders, as formulated by Poser, does not exclude all acquired disorders. There are many conditions characterized by a disturbance of myelination, and most of these are caused by external factors. Moreover, myelin may have been constituted normally in inherited disorders, only to be lost after many years.

There are several definitions of the term 'leukodystrophy.' Seitelberger (1984) defines leukodystrophies as degenerative demyelinating processes caused by metabolic disorders. Morell and Wiesmann (1984) state that leukodystrophies are disorders affecting primarily oligodendroglial cells or myelin. The disorders have to be of endogenous origin with a pattern compatible with genetic transfer of a metabolic defect. The clinical criterion is a steadily progressive deterioration of function. Menkes (1990) defines leukodystrophies as a group of genetically transmitted diseases in which abnormal metabolism of myelin constituents leads to progressive demyelination. Common concepts in these definitions are demyeli-nation and inborn errors of metabolism. Heritability is implied. As such, the leukodystrophies are identical with inherited demyelinating disorders.

The terms 'white matter disorders' and 'leukoen-cephalopathies' comprise all disorders that selectively or predominantly involve the white matter of the CNS, irrespective of the underlying pathophysiologic mechanism and histopathologic basis. 'White matter disorders' is a literal translation of leukoencephalo-pathies. Sometimes these terms are used as if they are interchangeable with 'demyelinating disorders,' but usually they are used in the context of a wider range of disorders, characterized by either primary myelin loss or nonselective damage to myelin, axons and supportive tissue of the white matter. For instance, when the terms white matter disorder and leukoen-cephalopathy are applied in elderly people, ischemic white matter lesions are also implied,which do not involve or do not only involve a selective loss of myelin.

In this book the following definitions are used: - 'Demyelination' is reserved for the process of myelin loss caused by primary and selective abnormality of either oligodendroglia or of the myelin membrane itself. 'Demyelinating disorders' are conditions characterized by demyelina-tion. Examples: metachromatic leukodystrophy, multiple sclerosis.

- 'Hypomyelination' is reserved for conditions with a significant permanent deficit in myelin deposited. The most extreme variant of hypomyelination is amyelination. Example: Pelizaeus-Merzbacher disease.

- 'Dysmyelination', as the literal translation of the name implies, is reserved for conditions in which the process of myelination is disturbed, leading to abnormal, patchy, irregular myelination, sometimes but not necessarily combined with myelin loss. Examples: some amino acidopathies, damaged structure of unmyelinated white matter after perinatal hypoxia or encephalitis.

- 'Retarded myelination' is reserved for disorders in which the deposition of myelin is delayed, but progressing. Examples: inborn errors of metabolism with early onset, malnutrition, hydro-cephalus.

- 'Myelin disorders' comprise all the above-mentioned conditions.

- 'White matter disorders' and 'leukoencephalo-pathies' can be defined as all conditions in which predominantly or exclusively white matter is affected. Either myelin or a combination of myelin and other white matter components is involved. Hence, white matter disorders comprise all myelin disorders,but also,for instance,white matter infections and infarctions, which may affect various white matter components nonselectively.

- 'Gray matter disorders' comprise all disorders in which neurons and axons are predominantly or exclusively affected.

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