A group of rare genetic neurodegenerative disorders that affect infants and children. These disorders are characterized by metabolic abnormalities in the development of the myelin sheaths in the white matter. Clinical signs and symptoms include developmental delays, mental retardation, dementia, seizures, loss of motor skills, and muscle weakness. Representative examples include metachromatic leukodystrophy, Krabbe disease, Canavan disease, and Alexander disease. [ ]