Secondary demyelination disorders and destruction of white matter.

Demyelinating disorders of the central nervous system are characterized by the breakdown of myelin, with or without preservation of the associated axons. Primary demyelinating diseases typically involve loss of myelin with relative sparing of axons. Secondary demyelinating disorders represent a spectrum of white matter disease characterized by damage to neurons or axons with the resultant breakdown of myelin. The pathologic changes seen in secondary demyelinating disorders are varied, ranging from pure demyelination to necrosis with subsequent demyelination. Secondary demyelinating diseases are associated with a wide variety of conditions, including infections/vaccinations, nutritional/vitamin deficiencies, chemical agents, genetic abnormalities, and vascular insult.