Hypomyelination: an abnormally low amount of myelination (the process by which an insulating layer of fatty tissue develops surrounding nerve cells). A number of conditions (hypomyelinating disorders) are characterized by an abnormally low amount of myelination. These may involve the central nervous system (brain and spinal cord) only or both the central and peripheral nervous systems. Examples of hypomyelinating disorders include Pelizaeus-Merzbacher disease, Tay Syndrome (trichothiodystrophy with hypersensitivity to sunlight), Salla disease (free sialic acid storage disease), hypomyelination with congenital cataract, 4H syndrome, and Cockayne syndrome.
Barkovich AJ1, Deon S2. "Hypomyelinating disorders: An MRI approach." Neurobiol Dis. 2016;87:50-8.