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Intellectual disabilities and its Types

A person with Intellectual disability may have difficulty to communicate, learn and retain information.

What is an intellectual disability?

An intellectual disability is characterised by someone having an IQ below 70 (NCBI, 2015).

A person with an Intellectual disability may as well have significant difficulty with activities of daily living such as self-care, safety, communication and participation in community.

Also, a person with an intellectual disability may process information more slowly, have difficulty with communication and daily living skills, and also have trouble understanding the handling of money and planning time.

About 3% of Australians have an intellectual disability, making it the most common primary disability. About 60% of people with Intellectual disability have severe communication limitation. (Australian Institute of Health and Welfare).

An intellectual disability may be caused by a genetic condition, problems during pregnancy and birth, health problems or illness, and environmental factors.

Types of Intellectual disability image

Types of intellectual disabilities


Fragile X syndrome

Fragile X syndrome is the most commonly known cause of an inherited intellectual disability worldwide. It is a genetic condition caused by a mutation (a change in the DNA structure) in the X chromosome.

Fragile X can affect people differently. Girls and women are usually less affected than boys and men. A person may be fully affected by Fragile X but not show all of these signs below:

  1. Intellectual disability, slow development and difficulties with communication, coordination, motor skills and learning.

  2. Behavioural and emotional features such as aggression, anxiety, shyness and repetitive speech.

  3. Physical symptoms such as weak muscles and joints, a heart murmur and a high palate (Health Direct, 2020).

Boys are usually more affected than girls – it affects around 1 in 3,600 boys and between 1 in 4,000 – 6,000 girls (NCBI, 2015).

Down syndrome

Down syndrome is not a disease or illness, it is a genetic disorder.  

In 1959 Professor Jerome Lejeune, a geneticist in Paris, discovered that

Down's syndrome occurred as a result of a trisomy of chromosome 21.


This means that instead of the usual 46 chromosomes in the cells of the body,

there is an extra chromosome 21, making 47 chromosomes in all

(Intellectual Disability and Health, 2020).

In Australia, It is estimated that approximately 1 in every 1100 babies born

in Australia will have Down syndrome. This means that each year there are

approximately 290 new babies born each year who have Down syndrome

(Down Syndrome Australia, 2020).


People with Down syndrome can have a range of common physical and

developmental characteristics. Even though people with Down syndrome might act

and look similar, each person has different abilities.


People with Down syndrome usually have an IQ (a measure of intelligence) in the

mildly-to-moderately low range and are slower to speak than other children

(Centre for Disability Studies, 2019).

Some common physical features of Down syndrome include:

  • A flattened face, especially the bridge of the nose

  • Almond-shaped eyes that slant up

  • A short neck

  • Small ears

  • A tongue that tends to stick out of the mouth

  • Tiny white spots on the iris (colored part) of the eye

  • Small hands and feet

  • A single line across the palm of the hand (palmar crease)

  • Small pinky fingers that sometimes curve toward the thumb

  • Poor muscle tone or loose joints

  • Shorter in height as children and adults

trisomy down syndrome
Autism chromosome

Developmental delay

A developmental delay refers to a child who has not gained the developmental skills expected of him or her, compared to others of the same age (SSM Health, 2020).

One or more areas of development may be affected including delay in reaching milestones required in areas of language, thinking, social, or motor skills.

Austism Spectrum Disorder

This is a term used to describe several neurodevelopmental conditions. Autistic people may think, move, communicate, and process senses differently from neurotypical people.

Autism is usually diagnosed in early childhood and includes a noticeable delay in language and social development.

Signs and symptoms of ASD vary, but usually include delayed speech and language skills and challenges communicating and interacting with others.

What cause autism
Prader Willi Syndrome
Prader Willi Syndrome 2

Prader-Willi Syndrome (PWS)

Prader-Willi syndrome (PWS) is a rare genetic disorder which affects around 1 in 10,000 – 20,000 people (Better Health Channel).

This disability is quite complex and it’s caused by an abnormality in the genes of chromosome 15.

This disorder results in a number of physical, mental and behavioural problems. A key feature of Prader-Willi syndrome is a constant sense of hunger that usually begins at about 2 years of age.

People with Prader-Willi syndrome want to eat constantly because they never feel full (hyperphagia), and they usually have trouble controlling their weight. Many complications of Prader-Willi syndrome are due to obesity.

The symptoms of PWS can be quite varied, but poor muscle tone and a short stature are common. A level of intellectual disability is also common, and children can find language, problem solving, and maths difficult.

Fetal alcohol spectrum disorder (FASD)

Fetal alcohol spectrum disorders (FASDs) are a group of conditions that can occur in a person whose mother drank alcohol during pregnancy. These effects can include physical problems and problems with behaviour and learning. Often, a person with an FASD has a mix of these problems. (Centers for Disease Control and Prevention, 2020).

The symptoms can vary however can include distinctive facial features, deformities of joints, damage to organs such as the heart and kidneys, slow physical growth, learning difficulties, poor memory and judgement, behavioural problems, and poor social skills.

Many cases are also often misdiagnosed as autism or ADHD as they can have similarities.

It is recommended by World Health Organisation, that mothers-to-be, or those planning on conceiving, should completely abstain from alcohol.

Fetal alcohol spectrum disorder

Environmental and other causes

An environmental factor can be a cause for an intellectual disability and these may vary greatly however some of the factors are as follows:

  • Problems during pregnancy such as viral or bacterial infections

  • Complications during birth

  • Exposure to toxins such as lead or mercury

  • Complications from illnesses such as meningitis, measles or whooping cough

  • Malnutrition

  • Exposure to alcohol and other drugs

  • Trauma

  • And even unknown causes

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