Depression in old age could be an early sign of serious brain disease, a new study says.

Depression occurs more often and earlier in seniors who go on to develop Parkinson’s disease or Lewy body dementia, researchers recently reported in the journal General Psychiatry.

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The risk of depression rises steadily in these folks, peaking around three years prior to diagnosis of Parkinson’s or dementia, researchers found.

Further, patients with Parkinson’s or dementia continue to experience higher rates of depression than people with other chronic illnesses, researchers said.

“These findings are compatible with depression being an early manifestation of the neurodegenerative changes eventually leading to Parkinson’s disease and Lewy body dementia,” concluded the research team led by Christopher Rohde, a postdoctoral researcher with Aarhus University Hospital in Denmark.

About 30% to 40% of people with Parkinson’s or Lewy body dementia have depression, researchers said in background notes.

But up to now it’s not been clear whether that depression serves as a bellwether for an impending brain disease, or is a natural response people have to a disabling illness, researchers said.

For the new study, researchers analyzed data on more than 17,700 Danish people with either Parkinson’s or Lewy body dementia.

The team compared their rates of depression with those of seniors suffering from other chronic illnesses like rheumatoid arthritis, kidney disease and osteoporosis.

The results showed that depression tied to Parkinson’s or dementia could not be fully explained by the emotional burden of living with a chronic illness.

The other long-term diseases did not show the same strong increase in depression risk, suggesting that depression might be linked to early changes in the brain rather than an emotional response to declining health.

The findings were particularly striking for Lewy body dementia, where rates of depression were even higher than in Parkinson’s, researchers said.

"Following a diagnosis of Parkinson’s disease or Lewy body dementia, the persistent higher incidence of depression highlights the need for heightened clinical awareness and systematic screening for depressive symptoms in these patients,” Rohde said in a news release.

"Thus, our main conclusion — that Parkinson’s disease and Lewy body dementia are associated with a marked excess depression risk preceding and following diagnosis when compared with other chronic conditions — remains valid,” he added.

This doesn’t mean that everyone with depression will wind up with a brain disease like Parkinson’s or dementia, researchers emphasized.

Instead, depression might serve as a red flag for seniors who are at risk for developing these brain diseases, the team said.