The researchers at the University of Edinburgh say the tissue loss is greater in people who suffer multiple episodes of the illness and is associated with a decline in some areas of mental ability.

This study shows for the first time that bipolar disorder - a condition characterised by periods of depression and periods of mania - is associated with a reduction in brain tissue and proves that the changes get progressively worse with each relapse.

The researchers believe the discovery has implications for future research into the disease, and also on the way the condition is treated.

The researchers compared MRI scans of the brains of 20 patients with bipolar disorder and an equal number of volunteers without the condition and saw that everyone loses a small amount of tissue over time.

But in those with bipolar disorder, the loss of grey matter, where nerve signals are processed, was greater than in the control group and supports the theory that cognitive function is impaired in bipolar patients as they age.

The research team found that MRI's carried out four years apart on those with bipolar disorder showed a loss of brain tissue in the areas controlling memory, face recognition and co-ordination.

The researchers also found that those who lost the most grey matter had the most episodes of mania and depression and the biggest decline in brain function.

Study leader Dr. Andrew McIntosh senior lecturer in psychiatry, says the study could not show whether the loss of brain tissue was a cause or consequence of the disease but it is possible that repeated episodes of illness harm the brain and lead to the decline.

Dr. McIntosh says the other possibility is that the brain changes are caused by stress or genetic factors, which tend to lead both to more frequent illness episodes and to greater brain loss.

However regardless of which theory is preferred experts agree on the importance of maintaining people in remission and the importance of accessing the best treatment.

Bipolar disorder is a serious mental illness which affects around half a million people in the UK alone and about 5.7 million American adults.

People with the condition experience mania, when they are over-elated or over-active and also need little sleep.

But they may also suffer from delusions or hallucinations, and are at significant risk of suicide; experts say once a person has an episode of bipolar disorder, the condition tends to be life long.

The symptoms of bipolar disorder usually first occur in young adulthood and a diagnosis is based on the person's self-reported experiences, as well as observed behaviour.

Episodes of illness are associated with distress and disruption, and a relatively high risk of suicide.

Research suggests that genetics, early environment, neurobiology, and psychological and social processes are important contributory factors.

Bipolar disorder is usually treated with medications and/or therapy or counseling.

The research is published in the Journal of Biological Psychiatry.