What is Intellectual Disability? Everything you need to know.

Summary & Definition

The term intellectual disability (also referred to as ID) is a neurodevelopmental disorder defined by an impaired cognitive ability that impacts the way someone may understand and interact with the world. This includes difficulties in learning, communication, and daily functioning. Intellectual disabilities are assessed based on three components: 

1. Low Intelligence Quotient (IQ).
2. Difficulties with practical adaptive learning abilities (reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience) [1].
3. Condition manifests during childhood development (before age 22).

Intellectual disabilities are most commonly caused by underlying genetic conditions, however complications with pregnancies or exposure to environmental toxins may also lead to the development of intellectual disability [2]. People with ID commonly have coexisting conditions that may complicate their overall health and prognosis, such as Autism Spectrum Disorder (ADS) and epilepsy [3]. Most cases of ID are classified as mild, with persons able to live independently and function on their own [4]. However, individuals with severe or profound intellectual disabilities will require daily interventions and support from caregivers. It is important for families with children with diagnosed ID to create the best support system and schooling environment to help individuals live their best possible lives. 

Symptoms & Diagnosis

Signs of intellectual disabilities are usually observed by parents or their doctor by age 5 or when children enter schooling.

Symptoms Include:

1. Delays in reaching developmental milestones, including motor skills
2. Slower language development including speech skills
3. Difficulties with self-care or hygiene tasks
4. Trouble socially and behaviorally
5. Struggles with learning, falling behind in school
6. Poor adaptive skills such as planning and problem solving [5].

Diagnostic Testing

To confirm a diagnosis of intellectual disability, a specialist will assess both intellectual function and adaptive function [5]. Intellectual function is the basic definition of intelligence. This includes the ability to reason, perform logical thinking, learn academic concepts, problem solve, and learn from one's own experiences. This is commonly scored via an intelligence quotient (IQ) test which can determine the severity of disability. An average IQ score is 100, with anything scored two standard deviations (15 points) below being suggestive of intellectual disability. Below are the guidelines for a diagnosis based on IQ results [4]: 

1. Mild ID: 50-69 points (85% of ID cases)
2. Moderate ID: 35-49 (10% of ID cases)
3. Severe ID: 20-34 (4% of ID cases)
4. Profound ID: 19 or below (2% of ID cases)

Adaptive function relates to skills necessary in order to function independently in society, which includes daily practical tasks and social ability. 

In addition to assessing intellectual and adaptive functioning, the condition must present itself during development, which is defined by 22 years old. Intellectual disability is a neurodevelopmental condition, which means it starts in childhood during development. If similar symptoms manifest later in adulthood, this is not from intellectual disability, but likely from another cause such as brain injury/trauma or neurodegenerative disorders such as Alzheimer's or Frontotemporal Dementia which also affect learning and behavior. The age of onset distinguishes intellectual disability from other neurological conditions.

Causes of Intellectual Disability

About 30-50% of all cases of intellectual disability have an unknown cause [4]. Known causes include genetic, congenital disorders, pregnancy or birth issues, or even exposure to environmental toxins that impact normal development. 

Genetic

Genetic causes of intellectual disability are caused by errors in genetic material, like DNA mutations, that impact a cell's normal function. This makes up for 25% of all ID cases [2]. The most common genetic cause of intellectual disability is from Trisomy 21, also known as Down Syndrome. This is an example of a genetic disorder that is caused from duplication of a person's chromosomes, which causes issues in a cell that leads to abnormal development. In addition to chromosomal abnormalities, genetic causes can include single gene mutations that are passed down from parents, or even de-novo mutations which is a new spontaneous mutation and is a rare cause. Genetic conditions that cause intellectual disability usually are coupled with other symptoms and comorbidities that may lead to issues with movement skills and epilepsy disorders.

NGLY1 Deficiency

NGLY1 Deficiency is an example of a rare genetic condition that causes intellectual disability as well as other comorbidities such as movement disorders, hypotonia, epilepsy, and abnormal liver function. Mutations in the NGLY1 gene cause abnormal brain development, impacting the neurological function and other organ systems leading to a constellation of symptoms including intellectual disability, which significantly impacts language skills. 

Problems with Pregnancy or Birth

A complicated pregnancy or birth may result in a child with intellectual disability. When a fetus undergoes excessive stress, exposure to alcohol, or infection from certain viruses, this can impact the embryo development and fetus growth [6]. Similarly, if a child has a complicated birth such as low oxygen, this increases risk of intellectual disability.

To avoid these risks it is important to follow pregnancy guidelines and avoid substances that may impact the normal development of a fetus. It is also recommended for pregnant women to stay up to date with all vaccinations, as this is a way to reduce the risk of infections [7].

Environmental

Lastly, even if a child has no genetic cause or complications with pregnancy/birth, it is still possible for a child to develop intellectual disability through harmful exposure or poor nutrition. This includes infections (measles, meningitis, etc.), toxic levels of certain things (lead and mercury exposure, etc.), or malnutrition (nutrient deficiencies like iodine deficiencies, etc.).

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Prevalence & Statistics

The estimated incidence of intellectual disability is 1-3% of the global population. The majority (85%) consists of mild ID [4]. Interestingly, ID is slightly more common in males than females (1.5:1 ratio) [10]. It has a higher prevalence in low-income countries, likely due to environmental toxins and nutrient deficiencies [10].

Treatment & Management

People with intellectual disability may struggle due to social or behavior symptoms associated with the condition. Most children with intellectual disabilities require special schooling or programs to assist them. Special education programs may be tailored to help with learning disabilities and create a supportive learning environment. In addition to schooling, occupational therapies may assist individuals with daily tasks to promote independence [11]. It is important to assess a child’s strengths and weaknesses and tailor a management plan to their abilities.

Depending on the severity of intellectual disability, life outcomes may range from living independently to requiring significant 24/7 caretaking. Many people with intellectual disabilities are able to work and live independently. Group homes or other supportive living situations may help increase independence when possible.

Living with ID: Challenges & Stigma

Historically those with intellectual disability have been mistreated by society and stripped of human rights. Intellectual disability was seen as a burden, which led to many people with ID to be cast out of society. This dark historical perspective has lingered, although some social progress has been made.The stigma, or negative judgement or association attached to ID, can have a lasting impact on individuals. Misuse of outdated terms as insults continues to contribute to this form of judgement. People with intellectual disabilities may be aware of these differences, which can lead to increased rates of depression in this population.

Recent progress and recognition of human rights for people with intellectual disability has helped reduce the  stigma, but there is more work to be done. It is important to show people with intellectual disabilities that they have value.

Conclusion

Intellectual disability is a developmental condition that impacts intellectual and adaptive function, resulting in academic, social, and behavioral problems. Symptoms usually manifest in early childhood and can be diagnosed by a specialist that holistically looks at IQ scores and adaptive functioning. Most cases of intellectual disability have an unknown cause, but it may be caused by genetic conditions, pregnancy/birth complications, or environmental risks. 

Prioritizing a supportive learning environment, such as special education programs, can help assist with daily life and independence. Children with intellectual disabilities can learn and do many things, but may require more time.

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References

1. American Psychiatric Association (2013). Diagnostic and Statistical Manual of Mental Disorders. doi:10.1176/appi.books.9780890425596. ISBN 978-0-89042-555-8. Retrieved 2024-04-13.
2. Jansen, Sandra; Vissers, Lisenka E. L. M.; de Vries, Bert B. A. (2023-01-30). "The Genetics of Intellectual Disability". Brain Sciences. 13 (2): 231. doi:10.3390/brainsci13020231.
3. Pedersen AL, Pettygrove S, Lu Z, Andrews J, Meaney FJ, Kurzius-Spencer M, et al. (August 2017). "DSM Criteria that Best Differentiate Intellectual Disability from Autism Spectrum Disorder". Child Psychiatry and Human Development. 48 (4): 537–545. doi:10.1007/s10578-016-0681-0.
4. Daily DK, Ardinger HH, Holmes GE (February 2000). "Identification and evaluation of mental retardation". American Family Physician. 61 (4): 1059–67, 1070. PMID 10706158.
5. American Psychiatric Association (2013). "Highlights of Changes from DSM-IV to DSM-5". Diagnostic and Statistical Manual of Mental Disorders (Fifth ed.). Arlington, VA: American Psychiatric Publishing. p. 809. doi:10.1176/appi.books.9780890425596.
6. Denny L, Coles S, Blitz R. Fetal Alcohol Syndrome and Fetal Alcohol Spectrum Disorders. Am Fam Physician. 2017 Oct 15;96(8):515-522.
7. Biellik RJ, Orenstein WA. Strengthening routine immunization through measles-rubella elimination. Vaccine. 2018 Sep 05;36(37):5645-5650.
8. Shenouda, Josephine; Barrett, Emily; Davidow, Amy L.; Sidwell, Kate; Lescott, Cara; Halperin, William; Silenzio, Vincent M. B.; Zahorodny, Walter (2023-02-01). "Prevalence and Disparities in the Detection of Autism Without Intellectual Disability". Pediatrics. 151 (2): e2022056594. doi:10.1542/peds.2022-056594.
9. Robertson, Janet; Hatton, Chris; Emerson, Eric; Baines, Susannah (July 2015). "Prevalence of epilepsy among people with intellectual disabilities: A systematic review". Seizure. 29: 46–62. doi:10.1016/j.seizure.2015.03.016.
10. IACAPAP Textbook of Child and Adolescent Mental Health. 2015. ISBN 978-0-646-57440-0.
11. Mash, E.; Wolfe, D. (2013). Abnormal child psychology (5th ed.). pp. 308–313). Wadsworth Cengage Learning.

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