More than three times as likely to develop Parkinson's
The Swedish study found that people diagnosed with depression were more than three times as likely as people without a history of the mood disorder to develop Parkinson's disease within the first year of depression.
By 15 to 25 years later, those with depression were about 50 percent more likely to develop Parkinson's disease.
"There's substantial evidence of an association with depression in the last years before a diagnosis of Parkinson's disease," said study author Peter Nordstrom, professor and chief physician in the department of Community Medicine and Rehabilitation at Umea University in Sweden.
Read: Symptoms of depression
But Parkinson's experts warned that the study does not prove a cause-and-effect relationship between depression and Parkinson's. And they noted that the overall risk for developing Parkinson's was still low, even with a history of depression.
The research is published online May 20 in the journal Neurology.
Major depression affects nearly 7 percent of Americans over age 18 in any given year, according to the U.S. National Institute of Mental Health. The mood disorder includes symptoms such as prolonged sadness, apathy, appetite changes, sleep problems and irritability.
1 million US citizens suffer from Parkinson's
About 1 million people in the United States suffer from Parkinson's disease. Another 50,000 to 60,000 are diagnosed with the neurodegenerative disorder each year, according to the National Parkinson Foundation. Symptoms of the disease include shaking, tremor, slowness of movement, stiffness and trouble with balance.
For the study, Nordstrom and his colleagues looked at more than half a million Swedish people who were 50 or older. About 140,000 had a history of depression, while around 421,000 similar participants had not been diagnosed with depression.
The researchers followed the participants for as long as 26 years. The average follow-up time was seven years, according to the study.
Read: Causes of depression
The study revealed that 1.1 percent of those with depression eventually developed Parkinson's. Just 0.4 percent of those without a history of depression later developed Parkinson's.
Noting that the overall risk of developing Parkinson's disease after depression is still tiny, Peter Schmidt, vice president of research for the National Parkinson Foundation, urged those with a history of the mood disorder not to panic.
"The risk here is that people will overestimate the importance of this increased risk from depression," said Schmidt, who wasn't involved in the new research. "Young people with depression should not start to worry they will develop Parkinson's ... your lifetime risk of injury in an auto accident is certainly higher."
People with serious depression at risk
The new study also suggested that people with more serious depression that requires hospitalization are even more likely to develop Parkinson's disease.
Participants hospitalised five or more times were 40 percent more likely to be diagnosed with Parkinson's than those hospitalized for depression only once. Additionally, people hospitalized for depression were 3.5 times more likely to develop Parkinson's than those treated for depression in an outpatient setting, according to the study.
Dr. Michael Okun, national medical director of the National Parkinson Foundation, said, "This finding is completely consistent with mounting evidence showing a relationship between depression and Parkinson's disease.
"I don't think depression triggers Parkinson's," he said. "I think the pathological process of Parkinson's disease is going to impact multiple circuits in the brain and it's likely the circuits underpinning depression get hit before the circuits that underpin motor involvement. Just because you have depression doesn't mean you're going to get Parkinson's disease."
Dr. Eugene Lai, chair of Parkinson's disease research and treatment at Houston Methodist Hospital Neurological Institute, said many of his Parkinson's patients tell him they also suffer from depression as well as a constellation of other non-motor conditions, such as constipation, loss of smell and sleep disorders.
"Depression can't be used on its own as an indicator of risk for Parkinson's disease," Lai said, "but maybe we can develop a group of symptoms that can hopefully predict Parkinson's. We still have a ways to go to be able to predict Parkinson's disease much earlier."
Image: Sad depressed old woman, covering her face from Shutterstock