People with bipolar disorder suffer from an accelerated shrinking of their brain, UK researchers have found.
Depression is one of the symptoms of bipolar disorder
Imaging studies carried out four years apart showed loss of brain tissue in the areas controlling memory, face recognition and co-ordination.
The findings, published in Biological Psychiatry, back observations that people with the disorder lose brain function over time.
Bipolar disorder, is characterised by periods of depression and mania.
It affects half a million people in the UK.
When people have mania they are typically elated, overactive and need less sleep. They may also suffer from delusions or hallucinations and are at significant risk of suicide.
MRI scans of the brains of 20 patients with bipolar disorder and an equal number of volunteers without the condition showed 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.
The researchers at the University of Edinburgh 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 said the study could not show whether the loss of brain tissue was a cause or consequence of the disease.
"It may be that repeated episodes of illness harm the brain and lead to the decline," he said.
"Another 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.
"No matter which way round it is, it emphasises in my mind the importance of maintaining people in remission and the importance of getting them the best treatment."
Professor Guy Goodwin, head of the psychiatry department at Oxford University said the findings showed that bipolar disorder was a "brain disease".
"It supports the idea that cognitive function is impaired in bipolar patients in middle age and this probably helps to account for problems in a full return to work and evidence that patients with bipolar disorder underachieve."
Dr Philip Timms, honorary senior lecturer in psychiatry at King's College London, said the findings raised many questions.
"The important one is are the brain changes causing the disorder or is the disorder - and its associated stress - causing the brain changes?"
Marjorie Wallace, chief executive of mental health charity SANE, said: "Bipolar disorder is a cruel affliction carrying a high risk of suicide and this research appears to underline the importance of preventing relapse, which is already recognised as one of the most important goals in the long-term management of the condition."