Patients with mental retardation have subnormal intelligence (as measured by IQ) combined with deficits in adaptive functioning. IQ is defined as the mental age (as assessed using a WISC-R) divided by the chronologic age and multiplied by 100. If mental age equals chronologic age, then the ratio equals one and the IQ is “100.” An IQ ofless than 70 is required for the diagnosis of mental retardation. Severity ranges from mild to profound and is based on IQ (Table 7-2).

Epidemiology
Mental retardation affects 1% to 2% of the population and has a male-female ratio of 2: 1. Milder forms of mental retardation occur more frequently in families with low socioeconomic status (SES); more severe forms of mental retardation are independent of SES. Most patients with mental retardation have mild or moderate forms (see Table 7-2).

Etiology
Mental retardation can be thought of as a final common pathway of a number of childhood or perinatal disorders. The most common cause of mental retardation is Down syndrome (trisomy 21). Fragile X syndrome is the most common cause of heritable mental retardation. Inborn errors of metabolism, perinatal or early childhood head injuries, maternal diabetes, substance abuse, toxemia, or rubella can all cause mental retardation. Overall, there are more than 500 genetic abnormalities associated with mental retardation. In 30% to 40% of patients with mental retardation, no clear etiology can be determined.

Clinical Manifestations
History, Physical and Mental Status
Examinations, and Laboratory Tests Most mentally retarded children have physical malformations that identify them at birth as being at high risk for mental retardation (such as the characteristic facies of the child with Down syndrome).

Infants can show signs of significantly subaverage intellectual functioning. Young children with mental retardation may be identified by parents or pediatricians after failure to meet developmental milestones in a number of functional areas (e.g., delayed speech, social skills, or self-care skills capacity) or on scoring an IQ less than 70 on the Stanford-Binet (usually only for very young children) or WISC-R (standard for school-age children).

The onset of symptoms must be before age 18.

The patient must have both an IQ less than or equal to 70 and concurrent deficits or impairments in several areas of adaptive functioning (e.g., communication, self-care, interpersonal skills). Laboratory findings may suggest metabolic or chromosomal etiology.

Differential Diagnosis
Attention-deficit/hyperactivity disorder (ADHD), learning disorders, depression, schizophrenia, and seizure disorder can all resemble mental retardation.

These disorders can also be comorbid conditions.
Children suspected of having mental retardation should have a thorough medical and neurologic evaluation, including IQ testing, an EEG, and brain imaging (CT or MRI).

Management
Management depends on the degree of retardation, the course, and the particular abilities of the child and the parents. Most children with mental retardation progress through normal milestones (standing, walking, talking, learning to recognize letters and numbers) in a similar pattern to normal children but at a slower rate. Growth and development occur in children with mental retardation. They can have developmental spurts, like normal children, that could not have been predicted at an earlier age.

In mild mental retardation, the child is typically considered educable. The child can usually learn to read, write, and perform simple arithmetic. With family support and special education, most of these children will be able to live with their parents. The long-term goal of treatment is to teach the child to function in the community and to hold some type of job.

In moderate mental retardation, the child is typically considered trainable. With training, the child can learn to talk, to recognize his or her name and a few simple words, and to perform activities of daily living (bathing, dressing, handling small change) without assistance. The long-term goal of treatment is typically to enable the child to live and function in a supervised group home.

Children with severe or profound mental retardation almost invariably require care in institutional settings, usually beginning very early in life. These forms of mental retardation are often associated with specific syndromes (e.g., Tay-Sachs disease) in which there is progressive physical deterioration leading to premature death.