Developmental disabilities are birth defects that cause lifelong problems with how a body part or system works. They include Nervous system disabilities affecting how the brain, spinal cord and nervous system function. They cause mental retardation, including Down syndrome and fragile X syndrome . They also cause learning and behavioral disorders, such as autism Sensory-related disabilities, which can cause vision, hearing and sight problems Metabolic disorders such as phenylketonuria , which affect how your body processes the materials it needs to function Degenerative disorders such as Rett syndrome , which might only become apparent when children are older and can cause physical and mental problems Most developmental disabilities have no cure, but you can often treat the symptoms. Physical, speech and occupational therapy might help. Special education classes and psychological counseling can also help.