The exact causes of autistic spectrum disorders (ASD) are unknown. However, it is thought that several complex genetic and environmental factors are involved or, in some cases, an underlying condition that causes symptoms of ASD.
The causes of ASD can be described in two ways:
- Primary ASD (also known as idiopathic ASD) - no underlying medical condition can be found to explain the symptoms of ASD.
- Secondary ASD - an underlying medical condition is thought to be responsible, or partially responsible, for the symptoms of ASD.
90% of cases of ASD are primary, and 10% secondary.
Researchers have studied four possible causes of primary ASD. These are:
- Genetic factors - certain genetic mutations may make a child more likely to develop an ASD.
- Environmental factors - during pregnancy, a child may be exposed to certain environmental factors that could increase the risk of developing an ASD.
- Psychological factors - people with ASD may think in certain ways that contribute towards their symptoms.
- Neurological factors - specific problems with the development of the brain and nervous system could contribute to the symptoms of ASD.
Each of these factors is described in more detail below.
Most researchers believe that certain genes a child inherits from their parents could make them more vulnerable to developing an ASD.
Cases of ASD have been known to run in families. If an older child develops an ASD, there is a 5-6% chance that any new child born to the same parents will also develop an ASD.
If an identical twin develops an ASD, there is a 60% chance that the other twin will develop the condition.
At present, no specific genes linked to ASD have been identified and there are currently no tests that can screen for "ASD genes".
Some researchers have argued that ASDs are not primarily caused by genes, but also by environmental factors. The theory is that a person is born with a vulnerability to an ASD, but the ASD develops only if that person is exposed to a specific environmental trigger.
Some suggested environmental triggers include:
- the mother having a viral or bacterial infection during pregnancy
- the mother smoking during pregnancy
- the age of the father
- air pollution
There is evidence to support the first three factors listed above.
Women exposed to a rubella infection during pregnancy are estimated to have a 7% risk of giving birth to a child with an ASD. Women who smoke daily throughout early pregnancy are 40% more likely to give birth to a child with an ASD.
New fathers who are older than 40 are estimated to be six times more likely to father a child with an ASD than fathers under 40. This is possibly because a man's genetic material is more at risk of developing mutations as he gets older.
Researchers are currently studying the possibility that air pollution and pesticides may cause ASDs, under what is known as the CHARGE study. However, it will probably be several years before there is definitive information on environmental factors.
Much of the research into the possible psychological factors behind ASD is based on a concept known as "theory of mind" (TOM). This is a person's ability to understand other people's mental states, recognising that each person they meet has their own set of intentions, beliefs, emotions, likes and dislikes. To put it simply, it's seeing the world through another person's eyes.
It is thought that most children without ASD have a full understanding of theory of mind by around the age of four. Children with ASD develop a limited understanding or no understanding at all of theory of mind. This may be one of the root causes of their problems with social interaction.
Much research into neurological factors that may be associated with ASD has focused on part of the brain that matches your emotions to the situation you're in (the amygdala).
The amygdala chooses emotional responses from the part of the brain that regulates your emotion (the limbic system) and relays them to the part of your brain that processes sensory information (cerebral cortex).
Brain imaging studies carried out in people with ASD suggest that the connections between the cerebral cortex, the amygdala and the limbic system have become scrambled.
As a result, people with ASD may suddenly experience an extreme emotional response when seeing a trivial object or event. This may be a reason why people with ASD are fond of routines: they have found a set pattern of behaviour that does not provoke an extreme emotional response. It may also explain why they often become very upset if that routine is suddenly broken.
This confusion of emotional responses may also explain why children with ASD are interested in topics that most children would find boring, such as train timetables.
Another area of research into ASD has focused on special types of brain cells called mirror neurones, which were first discovered in the 1990s.
Mirror neurones are thought to enable us to copy other people's actions. At the most basic level, mirror neurones may give a baby the ability to return their mother's smile.
As a person becomes older, the mirror neurones create more complex pathways in the brain that may be involved in many of the higher brain functions, such as:
- the ability to learn from others
- the ability to recognise and understand other people's emotional states (theory of mind)
Brain imaging studies have found that mirror neurones do not respond in the usual way in people with ASD.
Mirror neurone dysfunction may be responsible for the difficulties children with ASD have with language, social interaction and some types of learning.
Some conditions that are known to cause symptoms of ASD are listed below:
- Fragile X syndrome - an uncommon genetic condition that is more common in boys. It is estimated that one in every 3,600 boys and one in every 6,000 girls are born with the condition. Children with fragile X syndrome usually develop certain facial and bodily characteristics, such as a long face, large ears and flexible joints.
- Tuberous sclerosis - a rare genetic condition that causes multiple non-cancerous tumours to grow throughout the body, including the brain. It is estimated that one in every 6,000 children is born with tuberous sclerosis.
- Rett syndrome - a rare genetic condition that almost always affects girls. It causes symptoms of ASD, and difficulties with physical movement and development. It is estimated that one in every 20,000 girls is born with Rett syndrome.