DSM-IV-TR Criteria

• A. Significantly subaverage intellectual functioning: An IQ of approximately 70 or below on an individually administered IQ test ( for infants, a clinical judgment of significantly subaverage intellectual functioning).
• B. Concurrent deficits or impairments in present adaptive functioning (i.e., the person’s effectiveness in meeting the standards expected for his or her age by his or her cultural group) in at least two of the following areas: communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.
• C. The onset is before age 18 year
• Code based on degree of severity reflecting level of intellectual impairment:
  317 Mild Mental Retardation: IQ level 50-55 to approximately 70.
  318.0 Moderate Mental Retardation: IQ level 35-40 to 50-55
  318.1 Severe Mental Retardation: IQ level 20-25 to 35-40
  318.2 Profound Mental Retardation: IQ level below 20 or 25
  319 Mental Retardation, Severity Unspecified: when there is strong presumption of Mental Retardation but the person’s intelligence is untestable by standard tests.

Associated Features

Associated descriptive features and mental disorders. No specific personality and behavioral features are uniquely associated with Mental Retardation. Some individuals with Mental Retardation are passive, placid, and dependent, whereas others can be aggressive and impulsive. Lack of communication skills may predispose to disruptive and aggressive behaviors that substitute for communicative language. Some general medical conditions associated with Mental Retardation are characterized by certain behavioral symptoms (e.g., the intractable self-injurious behavior associated with Lesch-Nyhan syndrome). Individuals with Mental Retardation may be vulnerable to exploitation by others (e.g., being physically and sexually abused) or being denied rights and opportunities.

Individuals with Mental Retardation have a prevalence of comorbid mental disorders that is estimated to be three to four times greater than in the general population. In some cases, this may result from a shared etiology that is common to Mental Retardation and the associated mental disorder (e.g., head trauma may result in Mental Retardation and in Personality Change Due to Head Trauma). All types of mental disorders may be seen, and there is no evidence that the nature of a given mental disorder is different in individuals who have Mental Retardation. The diagnosis of comorbid mental disorders is, however, often complicated by the fact that the clinical presentation may be modified by the severity of the Mental Retardation and associated handicaps. Deficits in communication skills may result in an inability to provide an adequate history (e.g., the diagnosis of Major Depressive Disorder in a nonverbal adult with Mental Retardation is often based primarily on manifestations such as depressed mood, irritability, anorexia, or insomnia that are observed by others). More often than is the case in individuals without Mental Retardation, it may be difficult to choose a specific diagnosis and in such cases the appropriate Not Otherwise Specified category can be used (e.g., Depressive Disorder Not Otherwise Specified). The most common associated mental disorders are Attention-Deficit/Hyperactivity Disorder, Mood Disorders, Pervasive Developmental Disorders, Stereotypic Movement Disorder, and Mental Disorders Due to a General Medical Condition (e.g., Dementia Due to Head Trauma). Individuals who have Mental Retardation due to Down syndrome may be at higher risk for developing Dementia of teh Alzheimer’s Type. Pathological changes in the brain associated with this disorder usually develop by the time these individuals are in their early 40s, although the clinical symptoms of dementia are not evident until later.

Associations have been reported between specific etiological factors and certain comorbid symptoms and mental disorders. For example, fragile X syndrome appears to increase the risk for Attention-Deficit/Hyperactivity Disorder and Social Phobia; individuals with Prader-Willi syndrome may exhibit hyperphagia and compulsivity, and those with William’s syndrome may have an increased risk of Anxiety Disorders and Attention-Deficit/Hyperactivity Disorder

Gender and cultural differences in presentation

Mental Retardation is more common among males, with a male-to-female ratio of approximately 1.5:1.Care should be taken to ensure that intellectual testing procedures reflect adequate attention to the individual’s ethnic, cultural, or linguistic background. This is usually accomplished by using tests in which the individual’s relevant characteristics are represented in the standardization sample of the test or by employing an examiner who is familiar with aspects of the individual’s ethnic or cultural background. Individualized testing is always required to make the diagnosis of Mental Retardation. The prevalence of Mental Retardation due to known biological factors, is similar among children of upper and lower socioeconomic classes, except that certain etiological factors are linked to lower socioeconomic status (e.g., lead poisoning and premature births). In cases in which no specific biological causation can be identified, the Mental Retardation is usually milder (although all degrees of severity are represented) and individuals from lower socioeconomic classes are overrepresented. Developmental considerations should be taken into account in evaluating impairment in adaptive skills because certain of the skill areas are less relevant at different ages (e.g., use of community resources or employment in school-age children).

Epidemiology

The prevalence rate of Mental Retardation has been estimated at approximately 1%. However, different studies have reported different rates depending on definitions used, methods of ascertainment, and population studied.

Etiology

The diagnosis of Mental Retardation requires that the onset of the disorder be before age 18 years. The age and mode of onset depend on the etiology and severity of the Mental Retardation. More severe retardation, especially when associated with a syndrome with a characteristic phenotype, tends to be recognized early (e.g., Down syndrome is usually diagnosed at birth). In contrast, Mild Retardation of unknown origin is generally noticed later. In more severe retardation resulting from an acquired cause, the intellectual impairment will develop more abruptly (e.g., retardation following encephalitis). The course of Mental Retardation is influenced by the course of underlying general medical conditions and by environmental factors (e.g., educational and other opportunities, environmental stimulation, and appropriateness of management). If an underlying general medical condition is static, the course is more likely to be variable and to depend on environmental factors. Mental Retardation is not necessarily a lifelong disorder. Individuals who had Mild Mental Retardation earlier in their lives manifested by failure in academic learning tasks may, with appropriate training and opportunities, develop good adaptive skills in other domains and may no longer have the level of impairment required for a diagnosis of Mental Retardation.