People with depression may be more likely to develop Parkinson's disease

"We saw this link between depression and Parkinson's disease during 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," said study author Peter Nordström, PhD, at Umeå University in Umeå, Sweden.

The researchers also examined siblings, and found no link between one sibling having depression and the other having Parkinson's disease. "This finding gives us more evidence that these two diseases are linked," said Nordström. "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."

For the study, researchers started with all Swedish citizens age 50 and older at the end of 2005. From that, they took the 140,688 people who were diagnosed with depression from 1987 to 2012. These people were then matched with three control participants of the same sex and year of birth who had not been diagnosed with depression, for a total of 421,718 control participants.

The participants were then followed for up to 26 years. During this time, 1,485 people with depression developed Parkinson's disease, or 1.1 percent, while 1,775 people, or 0.4 percent of those who did not have depression, developed Parkinson's disease.

Parkinson's disease was diagnosed an average of 4.5 years after the start of the study. The likelihood of developing Parkinson's disease decreased over time. People with depression were 3.2 times more likely to develop Parkinson's disease within a year after the study started than people who did not have depression. By 15 to 25 years after the study started, people with depression were about 50 percent more likely to develop Parkinson's disease.

People with more serious cases of depression were also more likely to develop Parkinson's disease. People who had been hospitalized for depression five or more times were 40 percent more likely to develop Parkinson's disease than people who had been hospitalized for depression only one time. People who had been hospitalized for depression were also 3.5 times more likely to develop Parkinson's disease than people who had been treated for depression as outpatients.

The link between depression and Parkinson's disease did not change when researchers adjusted for other conditions related to depression, such as traumatic brain injury, stroke and alcohol and drug abuse.

The study was supported by the Swedish Research Council.