Metachromatic leukodystrophy (MLD): an inherited disease characterized by the abnormal accumulation of certain fats in cells. The abnormally accumulating fats are known as sulfatides, and this process affects cells in the nervous system that produce myelin, a protective covering substance for nerves. The result is worsening destruction of the white matter in the nervous system (leukodystrophy), including both the brain and spinal cord (the central nervous system) and the peripheral nervous system, the nerves that connect the brain and spinal cord to muscles and sensory cells. Damage to the white matter leads to symptoms such as loss of intellectual function, decreased motor skills, loss of sensation in the arms and legs, seizures, incontinence, paralysis, blindness, hearing loss, and loss of speech. The condition is named because the sulfatides form granules that pick up stain differently (metachromatic) when viewed microscopically.
Metachromatic leukodystrophy occurs in 1 in 40,000 to 160,000 people worldwide. Most people with metachromatic leukodystrophy have mutations in a gene known as ARSA, which codes for production of the enzyme arylsulfatase A. This condition is inherited in an autosomal recessive pattern, which means an affected individual must receive a defective copy of the gene from each parent. Other names for metachromatic leukodystrophy include ARSA deficiency; arylsulfatase A deficiency disease; cerebral sclerosis, diffuse, metachromatic form; cerebroside sulphatase deficiency disease; Greenfield disease; metachromatic leukoencephalopathy; sulfatide lipidosis; and sulfatidosis.