Hypomyelinating leukodystrophies are a group of neurodevelopmental disorders in which a permanent deficiency in the amount of myelin occurs in the central nervous system, either due to its abnormal formation or to altered oligodendrocyte differentiation or migration. Generally, this group of diseases is clinically characterized by developmental retardation, hypotonia, ataxia, spasticity, and a variable degree of intellectual disability. In radiologic terms, hypomyelination is characterized by diffuse hyperintensity on T2/FLAIR imaging that often seems to affect uniformly the white matter. On the other hand, unlike demyelinating disorders where the T1-weighed sequence signal is hypointense, changes in this sequence can be isointense or slightly hypo- or hyperintense. This pattern can be overlooked unless several consecutive sequences are examined; thus, it needs to be observed in at least two images taken within at least six months in a patient over age 1.
Among the most frequent disorders is Pelizaeus-Merzbacher disease (PMD), caused by pathogenic variants in the PLP1 gene, as well as a series of diseases with similar clinical and radiologic features, known as PMD-like diseases (PMDL), caused by defects in GJC2, AIMP1, or HSPD. Among the rest of the hypomyelinating diseases, it is worth noting Pol III-associated disorders, Cockayne syndrome, trichothiodystrophy with photosensitivity or Tay syndrome, hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC), and hypomyelination and congenital cataract (HCC).
