Intellectual impairment is classified as a neurodevelopmental disorder that begins in childhood and is characterised by deficits in cognition and adaptive functioning, with onset during the developmental period.
Research has identified causes of intellectual impairment to include genetic abnormalities, in addition to prenatal, perinatal and postnatal environmental factors.
Several other non-genetic factors have been identified that lead to intellectual impairment, including:
- Congenital infections;
- Psychosocial deprivation;
- Acquired hypothyroidism;
- Exposure to toxins;
- Intracranial haemorrhaging;
- Central nervous system infection.
How is Intellectual Impairment Diagnosed?
Intellectual impairment is diagnosed when comprehensive clinical and cognitive assessments indicate an IQ of less than 70 (mean IQ=100), that is accompanied with significant deficits in adaptive functioning with regard to self care, communication, safety and socialisation. Intellectual impairment occurs in approximately 3% of the Australian population.
Several other neurodevelopmental disorders commonly co-occur with intellectual impairment. When assessing for intellectual impairment, these disorders are considered in making differential diagnoses:
- Autism spectrum disorder (ASD), with a comparable prevalence to intellectual impairment, and is characterised by impaired social communication, restricted interests, and repetitive behaviours. At least 1 in 4 children with ASD have intellectual impairment.
- Language disorders, which can present in a similar fashion to intellectual impairment, featuring language delays. The rate of language delay should prompt investigation of other delays, so that intellectual impairment is not disregarded.
- Epilepsy may present with delays and regression in core developmental domains, such as language. Behaviours indicative of epilepsy including shaking episodes, staring spells and changes in levels of consciousness with related automatisms (eg lip smacking, blinking).
Problematic Behaviours, Comorbidity, and Management
Problematic behaviours are exceedingly common in intellectual impairment and can be more strongly related with parental stress than level of delay.
These behaviours may include:
- property destruction;
- tantrums or meltdowns;
- and physical aggression towards themselves or others.
While problematic behaviours occur in up to 9% of typically developing children, they occur in 25% of children with an intellectual impairment.
A major contributing factor of problematic behaviours in those with intellectual impairment is communication impairment, which limits the individual’s ability to express frustration and/or explain external factors or underlying physical or emotional distress.
Children and adolescents with intellectual disability are also identified as having a higher prevalence of psychiatric disorders, including ADHD, mood disorders, anxiety disorders, and psychotic disorders.
The types of cognitive impairment are divided into four categories:
- mild intellectual disability;
- moderate intellectual disability;
- severe intellectual disability;
- and profound intellectual disability.
The degree of impairment from an intellectual disability varies extensively.
Mild Intellectual Disability – IQ between 50-70
- Minimal impairment to daily functioning;
- No differing physical features;
- Slower development across all domains;
- Capable of learning practical life skills;
- Able to integrate socially.
Approximately 85% of people with intellectual disabilities fall into the mild category and many even achieve academic success. Someone with a mild intellectual impairment for example, may be able to read, however has difficulty comprehending what they are reading.
Moderate Intellectual Disability – IQ 35-49
- Evident developmental delays (ie speech, motor skills);
- May have physical signs of impairment (ie thick tongue);
- Can travel alone to nearby, familiar places;
- Can complete self-care activities;
- Communication skills are often simple and basic;
- Can learn basic health and safety skills.
Approximately 10% of people with intellectual impairment fall within the moderate category.
People with moderate intellectual disability have fair communication skills, but cannot typically communicate on complex levels. They may have difficulty in social situations and problems with social cues and judgment. These people can care for themselves, but might need more instruction and support than the typical person. Many can live in independent situations, but some still need the support of a group home.
Severe Intellectual Disability – IQ 20-34
- Considerable delays in development;
- Understands speech, but little ability to communicate;
- Able to learn daily routines;
- May learn very simple self-care;
- Needs direct supervision in social situations.
Approximately 3-4% of those diagnosed with intellectual disability fall into the severe category. These people can only communicate on the most basic levels. They cannot perform all self-care activities independently and need daily supervision and support. Most people in this category cannot successfully live an independent life and will need to live in a group home setting.
Profound Intellectual Disability – IQ less than 20
- Significant developmental delays in all areas;
- Obvious physical and congenital abnormalities;
- Requires close supervision;
- Requires attendant to help in self-care activities;
- May respond to physical and social activities;
- Not capable of independent living.
Approximately 1-2% of people diagnosed with an intellectual impairment fall within the profound category. People with profound intellectual disability require 24 hour support and care. They rely on others for all aspects of day-to-day life and have extremely limited communication ability. Often, people in this category have other physical limitations as well.
Author: Tara Pisano, BA (Psych) (Hons), M Psych.
Tara Pisano is a registered psychologist with a special interest in early intervention in adolescents and young adults, as this is when three quarters of mental health conditions emerge. In her practice, she draws on a range of evidence-based therapies such as CBT, DBT, IPT, ACT and Motivational Interviewing, to promote recovery and positive outcomes.
Tara is not currently taking bookings, however, we have a number of clinicians available for bookings. To make an appointment for counselling please visit our webpage here to learn about our highly qualified clinicians, or call M1 Psychology Loganholme on (07) 3067 9129.
- American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders.5th. Arlington, VA: American Psychiatric Association; 2013.
- Einfeld SL, Ellis LA, Emerson E. Comorbidity of intellectual disability and mental disorder in children and adolescents: a systematic review. Journal of intellectual & developmental disability. 2011;36(2):137–43.
- Gluck, S. (2014, May 21). Mild, Moderate, Severe Intellectual Disability Differences, HealthyPlace. Retrieved on 2021, June 7 from https://www.healthyplace.com/neurodevelopmental-disorders/intellectual-disability/mild-moderate-severe-intellectual-disability-differences
- Harris JC. Intellectual Disability: A Guide for Families and Professionals.New York, New York: Oxford University Press; 2010.
- Leonard H, Wen X. The epidemiology of mental retardation: challenges and opportunities in the new millennium. Ment Retard Dev Disabil Res Rev. 2002;8(3):117–34.
- Toth K, deLacy N, King BH. Intellectual disability. In: Dulcan MK, editor. Dulcan’s textbook of child and adolescent psychiatry.2nd 2016.