Kaplan + Sadock's Synopsis of Psychiatry, 11e

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31.3 Intellectual Disability

Degrees of Severity of Intellectual Disability

Epidemiology The majority of population-based prevalence estimates for intellectual disability in developing countries range from 10 to 15 per 1,000 children. The prevalence of intellectual disability at any one time is estimated to range from 1 to 3 percent of the population in Western societies. The incidence of intellectual disability is difficult to accurately calculate because mild dis- abilities may be unrecognized until middle childhood. In some cases, even when intellectual function is limited, social adap- tive skills may not be challenged until late childhood or early adolescence, and the diagnosis is not made until that time. The highest incidence of intellectual disability is reported in school- age children, with the peak at ages 10 to 14 years. Intellectual disability is about 1.5 times more common among males than females. Epidemiological surveys indicate that up to two thirds of chil- dren and adults with intellectual disability have comorbid psy- chiatric disorders; and this rate is several times higher than that in community samples without intellectual disability. The prevalence of psychopathology appears to be correlated with the severity of intellectual disability; the more severe the intel- lectual disability, the higher the risk for coexisting psychiatric disorders. An epidemiological study found that 40.7 percent of intellectually disabled children between 4 and 18 years of age met criteria for at least one additional psychiatric disor- der. The severity of intellectual disability influenced the risk for particular comorbid psychiatric disorders. Disruptive and conduct-disorder behaviors occurred more frequently in those diagnosed with mild intellectual disability, whereas those with more severe intellectual disability were more likely to meet criteria for autism spectrum disorder and exhibited symptoms such as self-stimulation and self-mutilation. Comorbidity of psychiatric disorders with intellectual disability in children in this study was not correlated with age or gender. Children diag- nosed with profound intellectual disability were less likely to exhibit comorbid psychiatric disorders. Psychiatric disorders among persons with intellectual dis- ability are varied, and include mood disorders, schizophrenia, attention-deficit/hyperactivity disorder (ADHD), and conduct disorder. Children diagnosed with severe intellectual disability have a particularly high rate of comorbid autism spectrum dis- order. Approximately 2 to 3 percent of those with intellectual disability meet diagnostic criteria for schizophrenia, which is several times higher than the rate for the general population. Up to 50 percent of children and adults with intellectual disabil- ity are found to meet criteria for a mood disorder when instru- ments such as the Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS), the Beck Depression Inventory, and the Children’s Depression Inventory were used in studies. How- ever, a limitation of these studies is that these instruments have not been standardized within intellectual disability populations. Frequent psychiatric symptoms that occur in children with intel- lectual disability, outside the context of a full psychiatric disor- der, include hyperactivity and short attention span, self-injurious Comorbidity Prevalence

The severity levels of intellectual disability are expressed in DSM-5 as mild, moderate, severe, and profound. “Borderline intellectual functioning,” a term previously used to describe individuals with a full scale IQ in the range of 70 to 80, is no longer described as a diagnosis in DSM-5. The term is used in DSM-5 as a condition that may be the focus of clinical attention; however, no criteria are given. Mild intellectual disability represents approximately 85 per- cent of persons with intellectual disability. Children with mild intellectual disability often are not identified until the first or second grade, when academic demands increase. By late ado- lescence, they often acquire academic skills at approximately a sixth-grade level. Specific causes for the intellectual disabil- ity are often unidentified in this group. Many adults with mild intellectual disability can live independently with appropriate support and raise their own families. IQ for this level of adaptive function may typically range from 50 to 70. Moderate intellectual disability represents about 10 percent of persons with intellectual disability. Most children with moder- ate intellectual disability acquire language and can communicate adequately during early childhood. They are challenged academ- ically and often are not able to achieve above a second to third grade level. During adolescence, socialization difficulties often set these persons apart, and a great deal of social and vocational support is beneficial. As adults, individuals with moderate intel- lectual disability may be able to perform semiskilled work under appropriate supervision. IQ for this level of adaptive function may typically range from 35 to 50. Severe intellectual disability represents about 4 percent of individuals with intellectual disability. They may be able to develop communication skills in childhood and often can learn to count as well as recognize words that are critical to function- ing. In this group, the cause for the intellectual disability is more likely to be identified than in milder forms of intellectual dis- ability. In adulthood, persons with severe intellectual disability may adapt well to supervised living situations, such as group homes, and may be able to perform work-related tasks under supervision. IQ in individuals with this level of adaptive func- tion may typically range from 20 to 35. Profound intellectual disability constitutes approximately 1 to 2 percent of individuals with intellectual disability. Most individuals with profound intellectual disability have identifi- able causes for their condition. Children with profound intel- lectual disability may be taught some self-care skills and learn to communicate their needs given the appropriate training. IQ in individuals with this level of adaptive function may typically be less than 20. The DSM-5 also includes a disorder called “Unspecified Intellectual Disability” (Intellectual Developmental Disorder), reserved for individuals over the age of 5 years who are diffi- cult to evaluate but are strongly suspected of having intellectual disability. Individuals with this diagnosis may have sensory or physical impairments such as blindness or deafness, or concur- rent mental disorders, making it difficult to administer typical assessment tools. (e.g., Bayley Scales of Infant Development and Cattell Infant Scale ) to aid in determining adaptive func- tional impairment.