Brain inflammation is associated with both cognitive decline and depression. A study published in The Lancet Psychiatry now links longer duration of untreated major depressive disorder to greater activation of brain microglia — neural cells involved in the brain’s inflammatory response.

Scientists measured the activity of these cells and determined whether there was an association between that activity level and how long patients had been depressed without being treated for their depression.

The research volunteers were not taking antidepressant medications during the course of the study.

Patients who had not received treatment for depression for 10 or more years had significantly greater microglial activation than those with shorter durations of untreated depression.

Other studies have shown that untreated depression in late life increases risk of developing Alzheimer’s dementia.