Developmental Disabilities

Types of Developmental Disabilities

Developmental disabilities can be categorized into nervous system disabilities, sensory-related disabilities, metabolic disorders, and degenerative disorders.

Nervous system disabilities refer to primary impairments of the brain and/or spinal cord, thereby impacting multiple aspects of learning and intelligence. The most common result of a nervous system disability is mental retardation (or low intelligent quotient [IQ]), which is a hallmark characteristic of many developmental disabilities. The most common nervous system disabilities include Down syndrome, Fragile X [Page 499]syndrome, and autism spectrum disorders. Autism, for example, is characterized by impairments in social interaction, communication, and restricted repetitive patterns of behavior, interests, or activities.

Sensory-related disabilities refer primarily to impairments in visual and auditory processing. They occur, not only as a primary symptom (such as deafness), but are also common comorbid problems with other developmental disabilities.

Metabolic disorders are often caused by genetic deficiencies in an enzyme, thereby affecting a person's ability to synthesize and break down substances in the body. For example, phenylketonuria (PKU) is caused by a deficiency of a certain enzyme, creating a toxic level of one amino acid and too little of another, resulting in brain damage and severe mental retardation.

Degenerative disorders refer to disabilities that are not apparent at birth but that manifest at an older age, causing the child who was developing normally to lose previously acquired skills. Functioning may regress or be lost in physical, mental, and/or sensory modalities. An example of a degenerative disorder is Rett syndrome, where persistent and progressive degeneration throughout life occurs in five areas: head circumference, hand movements, social engagement, gait, and language development.

Incidence

Approximately 17 percent of children under the age of 18 have some type of developmental disability, including more mild conditions such as speech and language disorders and learning disabilities. About 2 percent of children under the age of 18 have a more serious developmental disability including mental retardation, autism, cerebral palsy, hearing loss, and vision impairment. Of these serious developmental disabilities, mental retardation is the most common.

Causes

The exact cause of many developmental disabilities is unknown. Some factors that contribute to developmental disabilities include trauma or infection to the brain, prenatal nutrition problems, prenatal drug or alcohol exposure, extreme premature birth, genetic and chromosomal abnormalities, and poor nutrition and medical care. Some disabilities are believed to be caused by an interaction of these biological and environmental factors.

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