What are the criteria?

An intellectual disability (ID, formerly also called mental retardation) is characterized by limitations in both intellectual functioning and adaptive behaviour. This is expressed in conceptual, social and practical skills. ID occurs before the eighteenth year of life. There are three diagnostic criteria:

• Age criterion: an ID occurs before the age of 18, which means that it is a developmental disorder.
• Limited intellectual functioning: the score on an intelligence test must be at least two standard deviations below the average (i.e. the IQ is 70 or lower).
• Limitation in adaptation skills : the score on standardized tests of conceptual, social or practical skills must be at least two standard deviations below average.

According to the American Association on Intellectual and Developmental Disabilities (AAIDD) model, the way in which ID is expressed depends on how it works on the five dimensions;

1. Intellectual skills
2. Adaptive behaviour
3. Health
4. Participation, interactions, social roles;
5. Context and the support that the person receives.

Intellectual limitations are not seen as an absolute trait, because sufficient support can lead to an improvement in functioning.

Initially, the AAIDD made a distinction between four levels of ID: light, moderate, serious and profound. The AAIDD, however, no longer uses this approach because it would not be appropriate for making decisions about the care of individuals with ID. Instead of the ID level, an assessment must be made for each individual of the level of support he or she needs. This approach recognizes that the need for support may be different in different areas of functioning and may change over time. In addition, it emphasizes the perspective in which an ID is seen as dynamically related to the social environment rather than as static quality of the individual.

The DSM approach

The DSM approach to diagnosis has many similarities with the AAIDD approach. A diagnosis requires both intellectual and adaptive limitations, and the disorder must occur during the development phase. The IQ score is usually around 70. The criteria also state that there must be deficits in at least one area of ​​adaptive behaviour.

The DSM in 2013 still classified individuals on the basis of the intelligence level: light (IQ 50-70), moderate (IQ 35-50), severe (IQ 20-40) and profound (IQ <20). About 85% of the cases were only lightly mentally limited. In the past, a light and moderate ID in education in the US was called 'educatively mentally limited' and 'trainable mentally limited' respectively. This classification was the basis for placement in schools. Today, this classification is no longer common and school placement decisions are based more on functional descriptions of the child's needs.

There have also been notable changes in the diagnostic criteria of ID in the past. The AAIDD stated in 1992 that an IQ of 75 or lower could be considered as ID. This took into account the standard error of intelligence tests, which is approximately 5 points. However, this change meant that the number of people with ID doubled. Changes in and controversy regarding the definition of intellectual disability shows that an ID is a socially constructed category.

How are intelligence and intellectual disability related?

Binet's intelligence test is based on the psychometric approach. This approach focuses on individual differences and on the idea that underlying assets provide an explanation for differences in intellectual functioning. Intelligence is often regarded as a construct consisting of a general factor, called g , and a number of specific skills, such as verbal and motor skills. The intelligence is measured by testing both general and specific capabilities. According to critics, these tests mainly measure skills and not thinking processes.

Nowadays, information processing theories are becoming increasingly important, focusing on the processes through which individuals perceive sensory stimuli and store and manipulate this information. With this approach, intelligence is measured based on a person's performance on processing tasks.

In 1905, Binet and Simon developed an intelligence test, with the aim of identifying children who were eligible for special education. The score on their intelligence test led to the assignment of a mental age: the age corresponding to the chronological age of children with a corresponding performance. A 7-year-old student who achieves an average that is equal to the average of 7-year-old students therefore has a mental age of 7 years, while a 7-year-old student has a mental age of 5 years if he or she achieves the same score as the average 5-year-old student.

Terman, who works at Stanford University, adapted the intelligence test from Binet to the Stanford-Binet test. The score on this test was the intelligence quotient (IQ), which was calculated by dividing the mental age by the chronological age and multiplying this by 100. The IQ made it possible to compare children of different ages. Nowadays we still use the term IQ, but we calculate it in a different way, so that the IQ is no longer a quotient, but age comparisons are still possible.

The assumptions regarding the nature of intelligence of Binet on the one hand and Goddard and Terman on the other differ considerably. While Binet believed that intelligence is somewhat malleable and can be influenced by the social environment, Goddard and Terman stated that it is inherited and stable. Goddard and Terman believed that eugenics is needed: the improvement of the human race by exercising control over what is inherited. These ideas had social implications, which led to heated discussions about the assumptions and use of intelligence tests and about the treatment of people with ID.

What are the classifications?

Descriptions of the functioning of people with ID can be based on the intelligence level:

• Light ID: develops social and communication skills at pre-school age, has minimal sensorimotor limitations, can acquire academic skills at the level of a 12-year-old in the late teens, achieves professional and social skills necessary for self-support, can be (sometimes with guidance) ) successful in functioning in the community.
• Moderate ID: develops communication skills in early childhood, personal care is possible with support, acquires skills up to the level of a 7 or 8-year-old, benefits from social skills and professional training, can adapt to the community under supervision.
• Severe ID: can learn to speak at primary school age, can learn minimal personal care at primary school age, benefits to a limited extent from pre-academic training, can perform simple tasks as an adult under supervision, can adapt to the community in a residential setting.
• In-depth ID: often has a neurological disorder, has sensorimotor limitations in childhood, may show some improvement in motor, communication and personal care skills with training, can perform simple tasks under supervision, needs structure and continuous supervision for optimal development.

When looking at differences in functioning, it can also be useful to compare developmental profiles of individual children with ID. In addition, it may be useful to describe the following: the physical or medical, cognitive and social functioning of children with ID. Some children with an ID, especially with a light ID, show no deviating physical characteristics. However, there is usually a different appearance. In addition, there may be physical problems, such as motor, hearing and eye problems. It are mostly the more severe intellectual disabilities that are associated with physical disorders and medical problems such as heart problems and epilepsy. The life expectancy of someone with ID is below average.

Children with an ID can learn, but there is variation based on the level and the aetiology of the disability. In addition, adjustments to the school/course work are often required. Attention has been paid to the usefulness of operant learning in treating ID. New behaviour can be formed with successive approaches. In addition, desired behaviour can be retained, and unwanted behaviour can be weakened with the consistent application of suitable consequences.

Individuals with an ID have great heterogeneity in social skills, such as eye contact, facial expression and social problem solving. Young people with a mild or moderate ID have deficits in understanding social cues, social situations and other people's perspective. However, it seems that social competence can gradually improve. The social problems of individuals with ID are partly the result of intellectual disabilities, language limitations and physical or medical problems. However, social experiences also have an influence. For example, young people with an ID experience more social isolation, which means they have few opportunities to practice with social interactions and to observe suitable models and social relationships.

Comorbidity

About 30 to 50% of children with ID also have another disorder. In most cases it is ADHD or ODD or CD. The type of problems that accompany an ID may differ for the level of the ID. A light ID, for example, is often accompanied by anxiety, depression and antisocial problems. Children with a moderate to severe ID also have problems such as autism, psychosis and auto-mutilation. Finally, specific ID syndromes are associated with specific problems, such as Lesch-Nyhan syndrome with auto mutilation.

It is difficult to identify comorbid problems in individuals with ID for several reasons:

• It is difficult to distinguish between an ID and the presence of other disorders, because many researchers view symptoms of other disorders as symptoms of an ID. This is also called overshadowing .
• It is difficult to identify comorbid problems because of the cognitive and communicative limitations that characterize an ID. Emotional symptoms in particular are difficult to assess.
• The standard diagnostic criteria for many disorders are not well applicable if the IQ is lower than 50.

Epidemiology

Mental limitations occur in approximately 1-3% of the population. The prevalence of intellectual disability in children of pre-school age is lower than that and those children who are diagnosed with ID often have moderate or lower IQ scores. During this period, only the more serious cases tend to be identified. The prevalence increases during the school period, when more mild cases are diagnosed. Fewer individuals with ID are diagnosed in adolescence and adulthood.

Mental limitations are more common in boys than in girls. This is possibly explained by a reporting bias, other environmental factors and the vulnerability of men to biological influences. Children with an ID often come from families with a low socio-economic status. Mental disabilities can be more common in high-income countries and older parents, where prematurely born babies often survive and people who are difficult to conceive are helped. On the other hand, the prevalence may decrease due to the availability of prenatal screenings and improved care for babies who are at risk of developing an ID.

Development process

Children with a mild ID can, with guidance, learn good intellectual skills and adaptation skills, which means that in some cases they no longer meet the diagnostic criteria. However, most children continue to have intellectual disabilities throughout their lives. The development course and the outcomes of a CB depend, among other things, on the severity and cause of the CB, medical problems, psychopathology and family variables.

The speed of intellectual development has been investigated. Normal development takes place gradually with a few growth steps and deteriorations. Children with ID develop slowly and fairly constantly. However, with specific syndromes there may be a different pattern, such as a delay after a number of years of normal development.

The order of development was also considered. According to Piaget, children go through four phases of development. This also appears to apply to children with ID, but this is slower. Moreover, their development does not come as far as that of children without an ID.

What is the aethiology?

It is clear that intellectual disabilities are related to medical and genetic factors, but the precise cause is not clear in 45-50% of the more serious cases. The cause of a light ID is also often unknown.

The two-group approach states that people with an ID can be classified into one of the following categories:

1. Organic group: there is a clear organic cause. This is more common in children with a moderate, severe or profound ID. There are no ethnic or socio-economic differences in this group. This group is more often associated with physical limitations.
2. Cultural-family group: the cause is not clear and sometimes another family member is intellectually limited. This is more common in children with a slight ID. Minority groups and people with a low socio-economic status are over-represented in this group. Often there are no or few physical or medical limitations.

Pathological organic influences

There is a lot of evidence for the role of biological factors in intellectual disabilities. There is evidence for a group of individuals with severe ID who come from all kinds of social classes and who have genetic disorders, birth defects and brain dysfunctions. Genetic processes and prenatal, perinatal and postnatal factors can underlie these cases.

Multigenetic influences

There is evidence for the heredity of intellectual disabilities. It is estimated that half of the variation in intelligence in the general population can be explained by genetic transfer of multiple genes. This also applies to people with a light ID. The heredity factor is smaller for people with a moderate and severe ID. In addition, pathological organic factors are more closely associated with more severe levels of ID, while the reverse applies to multigenetic influences.

Psychosocial influences

A slight ID is more common in the lower socio-economic classes and in some minority groups and may be due to psychosocial risk factors such as a lack of social support, certain attitudes of parents and stressful life events. Psychosocial factors can exert their influence in various ways.

However, there is often an interaction between biological predisposition and environmental influences. In some cases biological factors are the primary cause and in other cases social, behavioural and educational related factors are the primary cause. Even if a known genetic syndrome is strongly related to an ID, the level of disability can be partly determined by a combination of other biological and psychosocial factors.

What are related genetic syndromes?

These are the physical and behavioural characteristics of four syndromes that are associated with intellectual disabilities:

• Down syndrome. Physical: short body length, creases in the corners of the eyes, flat facial features, fissures on the tongue, poor muscle tone, wide hands and feet. Behavioural: Little, socially engaging behaviour. Relatively few (but broad) behavioural problems, such as stubbornness, constant refusal, militancy, inattention, socially withdrawn and depression in adolescence). 
• The fragile X syndrome. Physical: long face, large ears, soft skin, high curved palate, flexible thumb. These characteristics are less common in women. For men, too large testicles are also a characteristic. Behavioural: inattention, hyperactivity, stereotype movements, anxiety, tantrums, social avoidance and little interaction with peers.
• Williams syndrome. Physical: eleven-faced face (for example, narrow lower jaw and protruding cheeks), retardation of growth, often an older appearance in late adolescence and early adulthood. Behavioural: fears and phobias, inattention, hyperactivity, poor social judgment, without discrimination, and overly friendly social interaction.
• The Prader-Willi syndrome. Physical: almond-shaped eyes, downward-facing mouth, short body length, narrow hands and feet, poor muscle tension, underdeveloped glands and obesity. Behavioural: excessive eating and hoarding, obsessions and compulsions, stubbornness, tantrums, aggression, disobedience, fear and impulsiveness.

The term behavioural phenotype refers to the fact that a genetic disorder makes individuals vulnerable to certain behaviours.

Down syndrome

Down's syndrome is caused by an error on chromosome 21. This error occurs randomly and is therefore not inherited. There is an increased risk of this syndrome if the mother is older than 40 years. With Down's syndrome there are various brain disorders, such as a smaller brain, fewer and fewer dense neurons and abnormal dendrites. There is an increased risk of other health problems, such as heart defects and visual problems. Life expectancy has risen to 60 years in recent decades.

Although children with Down syndrome show intellectual growth during the first years of life, the limitation is clear and the development in childhood and adolescence slows down. The disability often varies from moderate to severe. There are often shortcomings in verbal short-term memory and auditory processing. The visual-spatial skills are relatively good. Most children learn to speak but have a delayed speech development. Expressive language development is worse than receptive language development. Although children with Down's syndrome have different social and emotional problems, they experience fewer psychiatric problems than children in other groups with ID.

Fragile X syndrome

Fragile X syndrome is the most common inherited form of intellectual disability. This syndrome is more common in boys and is caused by a wrong repeat of gene mutations. There are structural abnormalities in various areas of the brain, including the cerebellum and the frontal and parietal lobes. The larger head circumference of children with fragile X syndrome indicates enlarged brain structures, which in turn indicates insufficient pruning of cells in early development.

The pattern of heredity is complex. A man with an affected X chromosome passes it on to all his daughters, but not to his sons (because the sons only receive the Y chromosome from father). A woman with an affected chromosome has a 50% chance of passing it on to her sons and daughters. Daughters, however, are somewhat protected by their second X chromosome. Moreover, the premutation extends to the complete syndrome when women, but not men, transfer it. In families with an affected X chromosome there can therefore be different symptoms.

Almost all men with fragile X syndrome have ID, usually moderate to severe.

From the fifth year of life, cognitive development slows down, reaching a ceiling around late childhood or early adolescence. There are problems with information processing, visual-spatial cognition, motor coordination, math skills and executive functions. Autism spectrum disorders are also more common among boys with fragile X syndrome. The verbal long-term memory and acquired information appear to be relatively strong points.

Women with fragile X syndrome have an ID less often and if they have it, it is usually a light form. They often have learning disabilities, behavioural problems and social disabilities.

Williams Syndrome

Williams syndrome is rare and is the result of a random mutation on chromosome 7. Often there are heart and kidney problems and a mild to moderate ID. The syndrome is associated with both reduced and enlarged areas of the brain and a different pattern of brain activity during tasks related to reaction inhibition, visual processing and auditory processing.

Children with Williams syndrome are often relatively good at auditory processing and music. There are shortcomings in the visual-spatial skills, which differ significantly from the language skills. The intelligence profile is often disharmonic, in favour of the verbal intelligence. Despite certain weaknesses in some aspects of language, such as syntax and reading, children with Williams syndrome are often good at grammar and have a wide vocabulary.

Prader-Willi syndrome

Prader-Willi syndrome is the result of errors on chromosome 15. In this syndrome, the hypothalamus and the serotonin neurotransmitter function abnormally. The intellectual functioning of people suffering from Prader-Willi syndrome varies considerably, but the IQ is often around 70. They often show deficits in motor, auditory and visual short-term memory. Some are relatively strong in visual processing and spatial perceptual perception. Children with Prader-Willi syndrome can have the urge to eat a lot between the ages of 2 and 6. This urge does not go away anymore and can lead to death. There are also obsessions and compulsions, just as everything wants to be clean, tidy and perfect. There is some evidence that the characteristics of Prader-Willi syndrome may be different for genetic subtypes.

What are family factors?

If a child is born with an ID, the family must learn to deal with this. There are several factors that influence how parents deal with the fact that their child has an ID, such as the child's behavioural problems, marital interactions, the intellectual functioning of parents, daily conflicts, the social class and social support. Ethnic and racial factors can also influence.

Parents indicate that having a child with ID has negative consequences, such as limited career options, but also acknowledge its positive effects, such as viewing life from a new perspective. They emphasize the importance of a balance between difficult and positive aspects of the experience.

In general, these families sometimes show negative results, but many families do well and indicate that there are also positive aspects. Siblings also have to adapt to a child with an ID. They take care of the child with the ID more than usual. The research findings regarding the outcomes of the brothers and sisters are inconsistent. The outcomes are likely to be influenced by various factors. Some say that having a brother or sister with an ID has positive consequences, such as more patience, more empathy and helpfulness.

How do you assess and intervene?

Assesment

There are various tools that can be useful in the assessment of potentially mentally impaired individuals. Standardized intelligence tests, such as the Stanford-Binet, Wechsler and Kaufman, are particularly important. For babies and toddlers and children with severe ID, development tests can be used, which results in a development quotient (developmental quotient; DQ). These tests emphasize sensorimotor functioning and put less emphasis on language and abstract thinking.

Adaptive behaviour can be investigated by conducting interviews with parents or by observing the child. Sometimes self-reporting is also possible. An example of a standardized test for adaptive behaviour is the Vineland adaptive behavioural scale. The scale is focused on communication, socialization, motor skills and daily skills.

Intervention

In the mid to late 18th century children were with ID taught in residential schools, after which they returned to the community. However, this practice declined in popularity as more attention was paid to the biological causes of intellectual disabilities, reinforcing the notion that people with intellectual disabilities cannot be helped. This led to institutionalization.

In recent decades, the idea of standardization has been central: the idea that every individual has the right to a normal and free life. Most young people with ID live at home. Others live in small living groups.

Prevention

The universal prevention of intellectual disability includes prenatal care and diets and the prevention of prenatal exposure to alcohol and other teratogens. One can also make use of prenatal screening or early postnatal detection. For example, Down's syndrome can be identified before birth and early postnatal identification of PKU can immediately lead to dietary adjustments, thereby reducing the risk of ID.

Selective prevention programs focus on babies and children of preschool age with an increased risk of ID. Many of these programs emphasize the needs of both child and parents.

Education

In the 60s and 70s, many children with ID were given an education in classes with other students with the same level of ID. Following the 'Individuals with Disabilities Education Act (IDEA)', this changed - individual education programs were set up and more placement options were introduced. As a result, more children with an ID went to regular education. Inclusion, however, is primarily a challenge for students with severe ID, who often also have physical problems and special health needs.

Behavioural intervention

Operant behavioural techniques can be used to improve the adaptive skills of children with ID and to reduce maladaptive behaviour. A distinction is made between two types of techniques:

• Discrete trial learning: the practitioner selects the task that the child must learn and offers clear instructions and consequences for good behaviour. The child performs the task in a calm environment.
• Naturalistic, incidental learning: the educational situation is informal and less structured. The child often takes the initiative to learn and this is done in everyday contexts. For example, the child's request for toys is used to teach the child something. Learning in daily situations promotes the generalization of what the child has learned.

Acquiring daily living skills, such as getting dressed, is an important treatment goal, especially for young people with a serious ID. For children with a mild and moderate ID, learning social skills is important. Training of the parents can also have a positive effect.

Behavioural techniques can be used not only for learning adaptive skills, but also for unlearning maladaptive behaviour, such as auto mutilation. Auto mutilation occurs in 5-16% of young people with ID. It is more common among young people with a less severe ID. Auto mutilation can take various forms, such as cutting themselves or biting, and its severity can vary. The behaviour can be the result of biological and / or environmental factors. The relationship between auto mutilation and genetic syndromes, such as Lesch-Nyhan, indicates an abnormal organic need for sensory stimulation. However, even then it can be influenced by environmental factors.

Auto mutilation is difficult to handle. One of the most effective approaches to improving various behavioural problems, including auto mutilation, is positive behavioural support. This approach is adapted to different settings and is applied to young people with different disorders. The emphasis is on changing the environment and consequences of behaviour. New behaviour is also often taught to replace maladaptive behaviour. Positive behavioural support uses behavioural principles and job analysis. The underlying assumption is that insight into the variables that influence maladaptive behaviour is useful for preventing or reducing this behaviour.

There is a scheme of variables that can influence maladaptive behaviour and that can be applied to positive behavioural support. This model contains three components:

• Setting events : background variables that influence the likelihood that behaviour will occur. Fatigue, for example, increases the chance of auto-mutilation.
• Antecedent stimulus: precedes maladaptive behaviour and therefore actually triggers this behaviour. Examples of antecedent stimuli are requests that the child does not want to carry out any other events that cause stress.
• Positive consequences: ensure that auto-mutilation is maintained. Auto mutilation can be unconsciously rewarded positively by parents by giving the child attention.

A functional assessment studies the behaviour of the child in different situations and the consequences of and motivations for that behaviour. This can be done by observing the child in his or her natural environment or by interviewing adults who interact with the child. An (experimental) functional analysis can also be used, in which variables are manipulated to study what effect they have on behaviour. An individual treatment plan can be developed based on the functional assessment. Research shows that positive behavioural support is effective.

Pharmacological treatment

Medication does not improve the intellectual functioning of children with ID but it can be of use with reducing medical and psychological symptoms. However, the evidence for the effectiveness of medication is limited. The symptoms of ADHD are reduced with stimulants, while antipsychotic drugs are used to treat aggressive and antisocial behaviour.

Psychotherapy

There is little research into the effectiveness of psychotherapy in individuals with ID. If this form of treatment is used, the techniques must be adapted to the level of development of the child or adolescent. In addition, language must be concrete and clear. Non-verbal techniques are needed if there are communication problems.