Mental Retardation

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