31.3 Intellectual Disability
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Degrees of Severity of
Intellectual Disability
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.
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.
Comorbidity
Prevalence
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
