The cause of Tourette's syndrome is unknown. However, it is thought to be linked to problems with an area of the brain known as the basal ganglia.
The basal ganglia are a group of specialised brain cells located deep inside the brain. The cells help regulate the body's movements.
Research suggests that the basal ganglia may also play a role in higher brain functions such as motivation and decision making.
In people with Tourette's syndrome, it appears that tics are the result of a temporary problem that occurs inside the basal ganglia and disrupts the decision-making process.
The person suddenly develops an unconscious urge to perform an action (the tic) that the conscious mind regards as both unwanted and unexplained.
It is not known what actually goes wrong with the basal ganglia. One theory suggests that excessive levels of a naturally occurring chemical called dopamine, which can have powerful effects on the brain, could be responsible.
Alternatively, the dopamine levels could be normal in people with Tourette's syndrome, but they may be particularly sensitive to its effects.
Brain imaging studies have also shown that the structure of the basal ganglia is different in people with Tourette's syndrome. However, it is unclear whether these changes are due to a dopamine imbalance or sensitivity, or the cause of it.
As with the cause of Tourette's syndrome, it is not known what triggers it. There are several theories, which are outlined below.
Genetics appears to play a part in some cases of Tourette's syndrome, as it often runs in families.
Further evidence suggests that if one identical twin develops Tourette's syndrome, there is about a 1 in 2 chance that the other twin will also develop it.
It may be that a genetic mutation disrupts the normal development of the brain, triggering the symptoms of Tourette's syndrome. A genetic mutation is where the instructions contained in all living cells become scrambled in some way.
Read more about genetics.
Another theory is that Tourette's syndrome may be linked to a childhood infection by streptococcal bacteria (bacteria that usually cause a sore throat).
In an attempt to fight off the infection, the immune system produces antibodies (proteins). The antibodies may interact with brain tissue, affecting the brain's functioning.
Following a throat infection, some children have developed the symptoms of Tourette's syndrome, as well as the symptoms of obsessive compulsive disorder (OCD).
Some doctors believe that this may be a separate condition in its own right and have called it 'paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections', or PANDAS for short.
However, the concept of PANDAS is controversial because research findings have been inconsistent. It may be that it is only responsible for a small number of Tourette's syndrome cases. Further research is needed to clarify the issue.
In the meantime, the same treatments are recommended as those used in ordinary tic disorders and Tourette's syndrome.