Intellectual disabilities are not an actual medical condition. It is a label used to designate children with subnormal intellectual functioning (i.e. IQ < 70). The DSM-5 (1), AAIDD (2) and the educational system (3) are used for classification of intellectual disability. To be diagnosed with ID in any of the three systems, a low IQ (1), deficits in adaptive functioning (2) and age of onset prior to 18 years of age (3) must be present.

1. American Association on Intellectual and Developmental Disabilities (AAIDD)
   They emphasize that classification needs to depend on the degree and nature of support required. It needs to be assessed whether an individual needs services that are intermittent (1), limited (2), extensive (3) or pervasive (4). They believe that support will enhance the functioning and quality of life for individuals with ID.
2. DSM-5
   This states that a significantly subaverage intellectual functioning (i.e. IQ < 70) (1), concurrent deficits or limitations in adaptive functioning (2) and onset before the age of 18 years (3) is necessary for a diagnosis. The IQ is approximately 70 because intelligence tests are not 100% accurate. There are specifiers for mild-, moderate-, severe- and profound support required.
3. Educational system
   While this system makes use of deficits in adaptive functioning, the main focus is on IQ cut-off scores. Financial aid is provided to meet he needs of infants and toddlers with developmental delays. A developmental delay also makes these children eligible for special education.

Global developmental delay refers to a diagnosis which indicates that a child is not meeting developmental expectations in some areas of intellectual development. This diagnosis is only given to children under the age of 5 and can be seen as a temporary category.

A developmental delay refers to a delay of 35% or more in one of five developmental areas (i.e. cognitive; motor; speech and language; social/emotional; adaptive functioning) or 25% in two or more. This definition of developmental delay is used by the educational system. It is a temporary category and requires further validation of definite disability in an area of functioning.

Children who lack stimulation or have been deprived of adequate opportunity to develop their cognitive skills may score very low on IQ tests. This makes it essential to assess whether deficits are the results of lack of opportunities or limitations in capacity. Adaptive functioning may be influenced by factors other than intellectual ability (e.g. comorbid conditions; deprivation).

The prevalence of ID is 1% to 3% with more males than females having ID. 85% has a mild degree of disability. Earlier identification is associated with more severe forms of ID. Mild delays may not be detected until formal schooling begins. Comorbid disorders are common. Diagnosis of comorbid features may be more complex due to the presence of ID. The most common comorbid disorders are ADHD (1), depression (2), bipolar disorder (3), anxiety disorders (4), ASD (5) and disorders of impulse control (6). The diagnosis of ADHD is only made if the symptoms of inattention are excessive for the child’s mental age and not for the chronological age.

The presenting features of ID depend on the severity (1), personality (2) and behavioural characteristics (3). Developmental delays vary with the nature of the disability (e.g. academic achievement or other areas). Self-injurious behaviour (1), aggression (2), stereotypical movements (3), communication problems (4) and overactivity (5) predict more serious problems.

People with mild ID are able to function adequately at a slower pace with modified goals. Cognitive limitations are less noticeable in predictable and structured environments and most noticeable in novel situations or with abrupt changes to a schedule.

ID can be the result of genetic defects (e.g. Down syndrome). The risk for Down syndrome increases with the maternal and paternal age. The Prader-Willi syndrome includes low muscle tone and low reflex responses and includes problems of impulsivity, temper, tantrum, compulsive eating and some degree of ID. William’s syndrome is the result of a random genetic mutation. There often is a developmental delay and some form of ID is common. Strengths are verbal short-term memory, musical talent and hyper sociability. Weaknesses are evident in visual-spatial abilities and visual motor integration.

Environmental factors during prenatal development can cause birth defects (e.g. foetal alcohol syndrome). Teratogens refer to environmental toxins. Alcohol or substance use (1), premature birth (2), lack of oxygen at birth (3), head injuries (4), encephalitis (5), meningitis (6) or environmental pollutants (e.g. lead toxicity) (7) can lead to birth defects such as ID.

Assessment of ID requires developmental and medical history to determine the aetiology. Information regarding when a child achieved developmental milestones is important for the identification of ID. Individual assessment of intellectual and adaptive functioning is also necessary.

Interventions for children and adolescents with ID focus on either behavioural/emotional issues or educational issues. Successful behavioural programmes often break down problem behaviour into component parts and systematically shape behaviours into more socially adaptive skills through contingency management. The most successful programmes use a functional analysis. Increasing deficit behaviour is preferred over decreasing excessive behaviour because it relies on positive reinforcement. A parent component in the behavioural programme can enhance success by increased contingency and transfer of effects between home and school or treatment facility.