Nordström and his colleagues used a huge cohort of study participants, beginning with nationwide Swedish health-registry data, and eventually examining a group of 140,688 patients in Sweden with depression over a period of 25 years. They found that about 1.1 percent of those people developed Parkinson’s in that time. That's compared with a 0.4 percent risk factor for Parkinson’s among people not diagnosed with depression during the study period. In other words, the people with depression were more than twice as likely to develop Parkinson’s. (And the margin of error was tiny—less than 0.001—because of the study’s huge sample size.)

People who were hospitalized for depression were more likely to develop Parkinson’s, with the likelihood increasing with the number of hospitalizations related to depression. The risk of Parkinson’s associated with depression decreased over time, but remained “significant” for two decades, the study said.

“What is interesting is this somewhat reverses the relationship between Parkinson's and depression,” said Alessandro Di Rocco, the director of the Parkinson’s and Movement Disorders Center at New York University's Langone Medical Center. “Is it that people who have depression—or some people who have depression—are really people who are developing the first manifestation of Parkinson's disease before the tremors or other motor physical aspects of the disease become obvious?”

For the vast majority of patients with depression, the answer is no. Remember, only about 1 percent of the group of people with depression in the study went on to develop Parkinson’s. But the research implications are potentially huge.

“One difficulty is that depression is so prevalent in the general population that it's hard to parse out who is going to get Parkinson’s and who isn’t,” said Melissa Nirenberg, the associate director of clinical services at the Parkinson’s and Movement Disorders Center at New York University’s Langone Medical Center. “From a researcher’s perspective, it’s important because it’s possible that intervening in depression early on might alter the course of disease progression or the development of Parkinson’s.”

Depression, then, becomes one more biomarker that might help doctors stratify who’s most at risk for Parkinson’s. “We’re looking for a number of different types of biomarkers,” Nirenberg told me. “It may be that it's not one biomarker that allows you to identify Parkinson's, but it's a cluster. Are there blood markers? Genetic markers? Other clinical markers like REM sleep disorders?”

Nordström is already thinking along these lines, he told me. The next thing he wants to investigate is whether people who eventually develop Parkinson's disease are more accident prone. “Since we also have found lower muscle strength already 30 years before diagnosis,” he wrote, “it would be of interest to investigate whether those who eventually are diagnosed with Parkinson's disease have a higher risk of fractures.”

In the meantime, Parkinson’s specialists like Nirenberg say that the study is a reminder that patients with depression may be suffering in other ways, too. “This is not just a reaction to chronic illness,” Nirenberg said, “but a part of having it.”

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