In a new study, Swedish researchers have found that people with depression were much more likely to develop Parkinson's disease years later. The new study strengthens the already existing theory that depression and Parkinson's are linked.
This new study suggests that depression comes first, not the other way around.
"We saw this link between depression and Parkinson's disease over a timespan of more than two decades, so depression may be a very early symptom of Parkinson's disease or a risk factor for the disease," says Peter Nordström of Umea University, who led the study.
The study followed the entire population of Swedes who were over 50 by the end of 2005. The team of researchers found more than 140,000 who were diagnosed with depression between 1987 and 2012. They compared them to similar people who were not diagnosed with depression. Next, they checked to see who had Parkinson's disease thanks to an extensive database in Sweden on the health of its citizens.
Over 26 years, 1,485 of the people had been diagnosed with Parkinson's, the team reported in the journal Neurology. Of those people, about one percent had depression at some point, while just 0.4 percent of the population that did not have depression were diagnosed with the disease.
James Beck, vice president of scientific affairs at the U.S. Parkinson's Disease Foundation, said that doesn't necessarily mean that depression causes Parkinson's. He was not involved in the study.
"I think the bigger message is that depression and Parkinson's disease really go hand in hand," Beck says. "We have known that before, but this is a very large study."
The link between depression and Parkinson's hit the U.S. last year after the death of Robin Williams. His wife said he had been struggling with both, although the autopsy revealed he had Lewy body dementia that causes Parkinson-like symptoms.
In a study in the United States, researchers found about 14 percent of Parkinson's patients screened positive for depression, versus 6.6 percent of controls who didn't have the disease.
According to federal statistics, about 8 percent of Americans have depression of some kind, but only a third of those are getting treatment. The Parkinson's Disease Foundation estimates between 7 to 10 million Americans have the condition, marked by tremors, rigid muscles and movement difficulties. While there is no cure, treatment can delay the worst symptoms.
"Understanding how depression occurs still remains a mystery," says Beck. "There is something about the Parkinson's disease brain that leads to depression."
