1. Note that the term “intellectual disability” replaces the previous term of “mental retardation,” and that the term intellectual disability covers the same population of individuals who were diagnosed previously with mental retardation in number, kind, level, type, and duration of the disability, and the need of people with this disability for individual services, and every individual who is or was eligible for a diagnosis of mental retardation is eligible for a diagnosis of intellectual disability (Schalock et al., 2010).

2. Significant limitation in intellectual functioning means an IQ (intelligence quotient) score that is more than approximately two standard deviations below the mean, considering the standard error of measurement for the specific IQ test used.

3. Significant limitation in adaptive behavior means an adaptive behavior score that is more than approximately two standard deviations below the mean, considering the standard error of measurement for the specific adaptive behavior test used.

4. Classification of individuals with intellectual disability should not be based solely on the IQ score. Previously used subcategories of mild, moderate, severe, and profound (which were based solely on the IQ score) should not be used, since the IQ score often does not reflect the totality of the individual’s functioning. Instead, classification is based on the types and intensities of supports and services needed by the individual. Instruments such as the Supports Intensity Scale (Thompson et al., 2004) can be used to classify support needs.

1. Biomedical. These are factors that have had a deleterious impact on the child’s CNS (e.g., genetic and metabolic disorders, environmental toxins, infections).

2. Social. Inadequacies in the social and/or family environment (e.g., inadequate stimulation, social unresponsiveness) can diminish cognitive and social growth and functioning.

3. Behavioral. The damaging behavior of others (e.g., trauma, maternal substance abuse) can lead to intellectual disability.

4. Educational. The availability and quality of educational and training programs can affect intellectual development and influence whether or not a child functions in the range of intellectual disability.

5. Interactions between risk factors. In any given case, multiple risk factors may be present and may interact at different ages or stages of development. This concept reflects the transactional approach to human development, in which reciprocal interactions between individuals and their environment influence the developmental outcome. Some risk factors may be more significant (principal or primary cause of intellectual disability), and others may be less significant (contributing or secondary cause), but the interaction between them is almost always important. For example, a child with phenylketonuria (a biomedical risk factor) may function at a lower level because of both environmental deprivation (a social risk factor) and poor parental compliance with the prescribed diet (a behavioral risk factor).

1. An understanding of the etiology of intellectual disability begins with a complete medical and psychosocial history and a complete physical and neurologic examination. This preliminary assessment results in a list of possible causes or differential diagnosis, which should include consideration of any and all potential risk factors.

2. The differential diagnosis should be thought of as a set of hypotheses about the etiology, so that the subsequent workup is designed to test the most reasonable hypotheses. Table 52-1 is designed to help the primary care clinician design an appropriate workup based on the most likely hypotheses in a particular case. This table lists a series of possible hypotheses based on whether the potential risk occurred prenatally, perinatally, or postnatally. A set of possible strategies for testing each of these hypotheses is then suggested.

3. There is no single diagnostic workup that is appropriate to all cases. In some cases, the workup will be very simple (as in chromosomal analysis when Down syndrome is suspected). In most cases, however, the etiology will not be obvious, and a careful workup will be needed. Such an evaluation, however, will result in identification of the principal or primary cause of intellectual disability in only about one-third of cases. Because new diagnostic measures for intellectual disability are emerging rapidly, the etiologic evaluation in “idiopathic” cases should be considered an ongoing process that can take advantage of the newest techniques and research.