Dystonia is a movement disorder. Although the causes of dystonia are not fully known it is currently thought that the condition results from a malfunction in a part of the brain called the basal ganglia. The basal ganglia are structures situated deep in the brain. They help to regulate voluntary and involuntary movement by controlling muscle contractions in the body.

The problem may mainly lie in an area of the basal ganglia called the globus pallidus. If this area of the brain is not functioning correctly then the control of another structure in the brain called the thalamus is affected. The thalamus controls the planning and execution of movement and sends nerves to muscles via the spinal cord. The end result is that muscle co-ordination is not regulated properly. The wrong muscles will contract on movement or all muscles will contract unnecessarily causing abnormal movement and posture. Muscles positioned around joints usually work in pairs opposite each other, eg the biceps and triceps muscles of the upper arm bend or straighten the elbow respectively. Usually if one muscle of a pair is contracted the other is relaxed. However, in Dystoniaboth muscles in the pair contract at the same time leading to the abnormal movement or posture.

It is thought that in some cases there may be a chemical imbalance or 'wiring fault' in the basal ganglia. Chemical transmitters, such as dopamine, convey messages from one nerve cell to another within the basal ganglia. If this balance is upset then incorrect signals will be sent out resulting in loss of regulation of co-ordinated movements.

Supporting this theory is the fact that people with Dystonia do not showstructural abnormalities of the brain. The 'wiring fault' theory is therefore more likely because it works at a much smaller scale.

The fault in the basal ganglia may be caused by an inherited factor or be secondary to another problem such as drugs or toxins, or a separate neurological disease. Recently scanning studies using positron emission tomography (PET) in patients with cervical (neck muscle) Dystonia have revealed reduced basal ganglia density of an important dopamine receptor (the D2 receptor).