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People with depression were much more likely to develop Parkinson’s disease years later, Swedish researchers report in a new study that strengthens the theory that depression and Parkinson’s are linked.
This one suggests that the depression comes first, not the other way around, the team at Umeå University in Sweden found.
"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," Peter Nordström of Umea University, who led the study, said in a statement.
"Depression may be a very early symptom of Parkinson's disease or a risk factor for the disease."
The study is very strong because it follows the entire population of Swedes who were over 50 by the end of 2005. Nordstrom’s team 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.
Then they checked to see who had Parkinson’s disease. They could do this because Sweden has an extensive database on the health of its citizens.
Over 26 years, 1,485 of the people had been diagnosed with Parkinson’s, the team reports in the journal Neurology. About 1 percent of those who had depression at some point went on to develop Parkinson’s, while just 0.4 percent of the population that did not ever have depression were diagnosed.
This does not necessarily mean that depression causes Parkinson’s, says James Beck, vice president of scientific affairs at the U.S. Parkinson’s Disease Foundation, who was not involved in the study.
“I think the bigger message is that depression and Parkinson’s disease really go hand in hand,” Beck told NBC News. “We have known that before, but this is a very large study.”
“There is something about the Parkinson’s disease brain that leads to depression.”
Beck says people who are depressed may need to pay close attention if they develop tremor or other early symptoms of Parkinson's. Most people, he says, wait until symptoms are more evident before seeking medical help.
The links between depression and Parkinson’s hit U.S. headlines last year when comedian Robin Williams died. His wife said he’d been struggling with both, although an autopsy suggested he had Lewy body dementia, which can cause Parkinson-like symptoms.
In a recent U.S. study, researchers found about 14 percent of Parkinson’s patients screened positive for depression, versus 6.6 percent of controls who did not have the disease.
Federal statistics show that close to 8 percent of Americans have depression of some kind, but only about a third of those are getting treated for it. The Parkinson’s Disease Foundation estimates that 7 million to 10 million Americans have the condition, marked by tremor, rigid muscles and problems with movement. There is no cure, although early treatment can delay the worst symptoms.
“Understanding how depression occurs still remains a mystery,” said Beck. “There is something about the Parkinson’s disease brain that leads to depression.”