Leukodystrophies are a group of usually inherited disorders characterized by degeneration of the white matter in the brain. The word "leukodystrophy" comes from the Greek roots "leuko", "white", "dys", "abnormal" and "troph", "growth". The leukodystrophies are caused by imperfect growth or development of the myelin sheath, the fatty insulating covering around nerve fibers. Leukodystrophies may be classified as hypomyelinating or demyelinating diseases, depending on whether the damage is present before birth or occurs after. Other demyelinating diseases are usually not congenital and have a toxic or autoimmune cause.