The dysmyelinating diseases comprise a group of disorders in which myelin is either not formed or, once formed, is not maintained. The dysmyelinating disorders which solely affect the white matter are called the leukodystrophies. Those due to a known specific enzyme deficiency are: metachromatic leukodystrophy, Krabbe's disease, and adrenoleukodystrophy. Alexander's disease, Canavan's disease, Pelizaeus- Merzbacher disease, Cockayne's disease and the sudanophilic leukodystrophies are classified by clinical and morphological data, with research currently directed toward elucidation of their respective biochemical abnormalities.

Symptoms are usually nonfocal and rapidly progress from weakness and mild intellectual impairment to spasticity, dementia, and unresponsiveness. The clinical presentation is usually within the first 2 years of life, with the exception of metachromatic leukodystrophy, adrenoleukodystrophy, and Leigh's disease, which may present later in the first decade. The pathologic findings may be nonspecific, but extensive demyelination is present in all. Several disorders can be recognized microscopically by the presence of distinctive cell types.

For many of these disorders CT and MR findings are nonspecific, especially in the later stages, demonstrating cerebral atrophy and widespread white matter changes with decreased attenuation on CT and increased signal intensity on T2-weighted MR scans. As with other white matter diseases the extent of involvement on MR is much broader than the subtle hypodensities visualized on CT.

Krabbe's disease has additional findings which are characteristic. CT shows increased attenuation in the thalami, corona radiata, and body of the caudate nuclei with corresponding low signal intensity lesions on T2-weighted MR sequences, a possible paramagnetic effect. Adrenoleukodystrophy usually begins in the central occipital white matter and advances anteriorly through the internal and external capsule and centrum semiovale, as well as centripetally to involve the subcortical white matter. Contrast enhancement by CT and MR is seen along the anterior margin of progression presumably representing areas of ongoing demyelination. With proper clinical correlation a specific diagnosis of adrenoleukodystrophy can be made. Alexander's disease predominantly involves the centrum of the frontal and temporal lobes and is associated with megaloencephaly. In Leigh's disease, decreased attenuation in the putamen is a characteristic CT finding. MR shows increased signal intensity on T2- weighted images in the striate nuclei, caudate, centrum semiovale, cerebral cortex, midbrain, and pons, and shows more lesions than CT.

Canavan's disease is a recessively inherited disorder of Jewish infants of unknown etiology, demonstrating megaloencephaly, blindness, and initially hypotonia followed by spasticity and progressive psychomotor degeneration with onset usually between the second and ninth months of life. The occipital lobes are most severely affected with the temporal lobes being least involved. The cerebellum is generally atrophic. Microscopically, there is spongy change (vacuolization) and demyelination. The arcuate fibers in the peripheral white matter are extensively involved, in contrast to other leukodystrophies. MR shows bilateral and symmetric white matter hyperintensity on T2- weighted sequences which extends to the corticomedullary junction (top row, arrows) and is most prominent in the occipital lobes, although all lobes may be affected. T1- weighted sequences show corresponding areas of faint hypointensity (bottom row, arrows).