Untreated depression lasting more than a decade leads to increased inflammation in the brain - a known risk factor for neurodegeneration, scientists have discovered.
Until now, depression had not been considered a degenerative brain disease.
But the researchers writing in The Lancet Psychiatry journal said their findings suggested that "progressive neurodegeneration itself occurs in major depressive disorder".
Positron emission tomography (PET) brain scans were carried out on 50 depressed patients, half of whom had suffered the condition for more than 10 years. Images were also taken from a comparison group of 30 non-depressed individuals.
Levels of translocator protein (TSPO), an inflammation marker made by microglia immune cells, were measured.
Participants with long-lasting untreated depression were found to have 30 per cent more TSPO than those who had experienced depression for shorter periods.
Lead author Dr Jeff Meyer said: "Greater inflammation in the brain is a common response with degenerative brain diseases as they progress, such as with Alzheimer's disease and Parkinson's disease."
The findings suggested that different stages of depression should be treated in different ways, he said.
Currently major depression is treated using the same approach irrespective of how long the patient has been ill.
