Defining Intellectual Disability

An intellectual disability is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an intelligence quotient (IQ) score below 70 in addition to deficits in two or more adaptive behaviors that affect an individual's everyday life.

Intellectual disability was known as "mental retardation" until the end of the 20th century. However, this term is now widely seen as disparaging, so "intellectual disability" is preferred by advocates and researchers in most English-speaking countries. In addition, while the definition of intellectual disability once focused almost entirely on cognitive ability, it now also addresses both mental functioning and functional skills. As a result, a person with an unusually low IQ may not necessarily be considered intellectually disabled.

Intellectual disability can be either syndromic (in which intellectual deficits are present along with other medical and behavioral signs and symptoms), or non-syndromic (in which intellectual deficits appear without other abnormalities). Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.

DSM-V Diagnostic Criteria

According to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (the DSM-5), three criteria must be met for a diagnosis of intellectual disability: 

1. deficits in general mental abilities,
2. significant limitations in one or more areas of adaptive behavior (e.g., communication, self-help skills, interpersonal skills) across multiple environments, and
3. evidence that the limitations became apparent in childhood or adolescence.

In general, people with intellectual disability have an IQ below 70, but the diagnosis may also apply to individuals who have a somewhat higher IQ but severe impairment in adaptive functioning.

Etiology

Among children, 30% to 50% of intellectual disabilities are of unknown cause.

Sometimes intellectual disability is caused by genetics—for example, an abnormal gene may have been inherited from the parents, or an error may have occurred during gene combination. The most prevalent genetic conditions include Down syndrome, Klinefelter's syndrome, fragile X syndrome (common among boys), neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria (PKU), and Prader-Willi syndrome. 

Genetic factors in intellectual disabilities

Down syndrome is one of the more common genetic causes of intellectual disability.

Intellectual disability can also result if the fetus does not develop properly during pregnancy. For example, a pregnant woman who drinks alcohol or who gets an infection such as rubella may have a baby with an intellectual disability. If the mother has a difficult labor or birth and the baby does not get enough oxygen, brain damage may lead to the development of an intellectual disability.

Environmental factors, such as exposure to certain diseases (e.g., whooping cough, measles, or meningitis) or toxins (e.g., lead or mercury), can cause intellectual disability if medical care is delayed or inadequate. Iodine deficiency is the leading preventable cause of intellectual disability in areas of the developing world—it affects approximately 2 billion people worldwide.

Treatment

Currently, there is no "cure" for an intellectual disability. However, with appropriate support and teaching, many individuals can learn to live fulfilling, productive lives.

In addition, there are four broad areas of intervention that allow for active participation from caregivers, community members, clinicians, and of course, the individual(s) with an intellectual disability. These include psychosocial treatments, behavioral treatments, cognitive-behavioral treatments, and family-oriented strategies. Core components of behavioral treatments, for example, include working on language and social skills acquisition.

There are also thousands of agencies around the world that provide assistance for people with intellectual disabilities, both state-run and privately run and both for-profit and non-profit. Such agencies include fully staffed residential homes, day rehabilitation programs that are similar to schools, programs and workshops that help people with disabilities obtain jobs, programs that support people with intellectual disabilities who have their own apartments, programs that assist parents who have intellectual disabilities with raising their children, and many more.

Although there is no specific medication for intellectual disability, many people with such disabilities do have further medical complications and so may be prescribed several medications. For example, children with autism who also experience developmental delays may be prescribed antipsychotics or mood stabilizers to help with their behavior. However, use of psychotropic medications such as benzodiazepines in people with intellectual disability requires vigilant monitoring, as side effects are common and are often misdiagnosed as behavioral and psychiatric problems.