UMEA, Sweden, May 21 (UPI) -- Depression may be an early indicator of Parkinson's disease, or a risk factor for it, researchers said after studying patient records that spanned more than 20 years.
The study found people with depression were 50 percent more likely than those without it to develop Parkinson's.
Researchers in Sweden started by examining the records of more than 140,000 people in the country who were over age 50 in 2005 and were diagnosed with depression between 1987 and 2012, matching each of them them with a control group of three people who had not been diagnosed with depression, for a total of 421,718 people in the control group.
The data showed that over the 26-year period, 1,485, or about 1.1 percent, of people diagnosed with depression later developed Parkinson's, while 1,775, or about 0.4 percent, of people who did not have depression ended up with Parkinson's. Researchers also did not find any link between one sibling with depression and one with Parkinson's.
"This finding gives us more evidence that these two diseases are linked," said Peter Nordström, Professor in the Department of Community Medicine and Rehabilitation at Umeå University, in a press release. "If the diseases were independent of each other but caused by the same genetic or early environmental factors, then we would expect to see the two diseases group together in siblings, but that didn't happen."
Researchers also found more severe depression was correlated with a higher likelihood of developing Parkinson's. Those hospitalized once for depression were about 3.5 percent more likely to develop it, while those hospitalized 4 or 5 times were 40 percent more likely to end up with Parkinson's.
Links between depression and Parkinson's didn't change when considering things like alcoholism or injuries for those diagnosed with depression.
Nordstrom told Live Science that despite the results of the study, the number of people with depression who develop Parkinson's is still very small, and that more research is needed to understand the possible link between the two. It's possible, he said, that the ways depression affects the brain could somehow be setting the brain up for Parkinson's.
The study is published in Neurology.