Scientists are learning more and more about the physical signs of depression. Previous studies have shown that persistent depression is linked to a shrinking of the hippocampus, the part of the brain involved in forging new memories. Now, a study published in The Lancet Psychiatry has found that years of depression can leave a telling mark on the brain.

In patients with long bouts of untreated depression lasting 10 years or more, the researchers detected significantly higher levels of inflammation compared to those who had not experienced the condition for as long or at all.

“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," Jeff Meyer, senior author and Canada Research Chair in the Neurochemistry of Major Depression, explained in a statement.

Depression wouldn’t be categorized as a degenerative brain disease like Alzheimer's or Parkinson's, but the results here do seem to suggest that, for many, it is a progressive (rather than a static) condition. If this is true, it could have some important implications when it comes to treatment.

For the study, researchers from Toronto's Centre for Addiction and Mental Health (CAMH) measured translocator proteins (TSPO), which are produced by the brain’s microglia when activated.

The microglia make up 10 to 15 percent of brain cells and are responsible for controlling the immune response in the central nervous system (CNS), removing dead neurons and cellular debris a bit like a vacuum cleaner might clean up dirt. They play an important role in the brain’s healthy inflammatory response to trauma, but can also cause excessive neuroinflammation.

Eighty volunteers were involved in the study: 25 participants had a history of the illness lasting 10 years or more, 25 had depression for nine years or less, and a further 30 did not have depression.

The two groups with depression displayed higher levels of TSPO than the healthy controls, but those with 10 years or more of untreated depression had considerably higher levels of brain inflammation than those with fewer. TSPO levels in this group were between 29 and 33 percent higher in various brain regions than those with shorter periods of untreated depression and 31 to 39 percent greater than the controls.

What does this mean? The study was relatively small-scale, but it has the potential to revolutionize the way we treat later stages of depression or, at the very least, make us re-evaluate current treatments.

Right now, it doesn’t matter how long or how persistent a patient’s condition has been, a medic will use the same methods as they would another person with depression. But in the not-so-distant future, doctors could be looking at current medication for inflammation in other diseases to treat persistent major depression.