Depressed people -- particularly those 65 or older or with hard-to-treat depression -- are more than three times as likely to develop Parkinson's disease as their peers without depression, a new study indicates.
The new findings don't imply that depression causes Parkinson's disease. Instead, they suggest that depression may precede Parkinson's in some cases, said study author Dr. Albert Yang, an attending psychiatrist at Taipei Veterans General Hospital, in Taiwan.
The researchers looked at information from a national Taiwan health insurance database. During a decade of follow-up, 1.43 percent of about 4,600 people with depression were diagnosed with Parkinson's disease. By contrast, only 0.52 percent of more than 18,500 people with no history of depression were diagnosed with Parkinson's.
After accounting for age and sex, people with depression were 3.24 times more likely to develop Parkinson's disease, according to the study published in the Oct. 2 online issue of Neurology.
"Depression is an important risk for Parkinson's disease, but mainly for [the] elderly and those with long-term and difficult-to-treat depression," said Yang, who is also a visiting assistant professor at Harvard Medical School. "Our study does not implicate all depressions are risky for Parkinson's disease."
Nearly 1 million people in the United States have Parkinson's disease, a chronic and progressive movement disorder. Motor symptoms may include tremor, rigidity and trouble walking.
In the study, difficult-to-treat depression was defined as two or more treatment changes during the first two years after a depression diagnosis.
Depressed seniors were slightly more likely to be diagnosed with Parkinson's disease than their younger counterparts with depression, and those with hard-to-treat depression were more than twice as likely to be diagnosed with Parkinson's disease as those whose depression was easier to treat.
Depression and Parkinson's disease are known to travel together. Up to 60 percent of people with Parkinson's will show some symptoms of depression, according to the Parkinson's Disease Foundation.
"Depression can come after a Parkinson's diagnosis and is highly prevalent in Parkinson's disease patients, but it can also come on years before the actual onset, thus it's difficult to say whether depression causes Parkinson's or Parkinson's causes depression," Yang said.
One theory is that, as with the loss of a sense of smell, depression may be an early symptom of Parkinson's disease. The researchers attempted to control for this possibility by excluding all people who were diagnosed with Parkinson's disease within two to five years of a depression diagnosis. Even so, Parkinson's risk remained elevated, which suggests that depression is a risk factor for the development of Parkinson's, not a symptom.
More study is needed to understand the connection, Yang said. "Screening for Parkinson's disease would only be reasonable among elderly individuals and those with difficult-to-treat depression," he added.
The new study had some limitations. The researchers did not have any information on family history of Parkinson's disease, smoking, coffee drinking and environmental factors such as exposure to pesticides -- all factors that have been linked to the development of Parkinson's disease.
Outside experts said that the findings add a new dimension to the link between depression and Parkinson's disease, but point out that more study is needed before any conclusions can be drawn.
"Overall, a very small number of people with depression will go on to develop Parkinson's disease. It's an important link, but it is not a given," said James Beck, vice president of scientific affairs at the Parkinson's' Disease Foundation.
Dr. Roy Alcalay, an advisor for the Parkinson's Disease Foundation, said, "It is too early to say that hard-to-treat depression should be a red flag for Parkinson's disease, but it is something to think about when we see people with this type of depression who have Parkinson's disease in their family."
"We know that depression and Parkinson's disease are associated, and if depression is a risk factor for Parkinson's, it is possible that treating it could postpone the onset of the disease," added Alcalay, an assistant professor of neurology at Columbia University Medical Center, in New York City.
In any event, it's important that depression be treated: "Depression is bad for the brain, and so is Parkinson's," Alcalay said.
Dr. David Straker, an adjunct assistant clinical professor of psychiatry at Columbia University Medical Center, agreed that depression should be treated promptly and aggressively. But "we always want to rule out other neurologic diseases when a person has symptoms of depression," he added.
Both depression and Parkinson's disease involve some of the same brain chemicals, notably dopamine, Straker said. He added that certain medications used to treat depression can also cause symptoms of Parkinson's disease. However, antidepressants did not appear to increase risk for Parkinson's in the new study.