May 22, 2015

Large Study Links Depression with Increased Risk of Parkinson's Disease

The internet lit up yesterday with reports of a large new study that reported an association between depression and an increased risk of Parkinson’s disease (PD). The results were published in the May 20 online edition of Neurology, the medical journal of the American Academy of Neurology.

The data revealed another link, too: the more severe the depression, the greater the risk of developing Parkinson’s.

This news is an interesting companion to yesterday's post, in which I discussed loss of smell as a reliable early indicator of Parkinson’s.

Researchers in Sweden identified 140,688 people with depression, all born before 1956. For each of those individuals, the study team identified and assigned three people without any history of depression – but the same age and gender -- to a control group.

The research thus involved a large sample of over half a million participants, some of whom were followed for 28 years. Among the depressed group, 1,485 individuals – or 1.1% -- developed PD. Among the study participants without depression, only .4% -- point four percent – developed PD.

That means that the people with depression were about three times more likely to develop PD than their counterparts with no history of depression.

Time Decreases Risk
The risk -- that people with a history of depression will develop Parkinson’s -- decreased over time.

People with depression were 3.2 times more likely to develop PD within a year after the study started than people without depression history. By 15 to 25 years after the study started, people with depression were only .5 – point five – times more likely to develop Parkinson's disease.

Severity Increases Risk
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 3.5 times more likely to develop Parkinson's disease than people who had been treated for depression as outpatients.

No Familial Connection
The data revealed something else. After examining siblings, researchers found no link between one sibling having depression and the other having PD.

Study author Peter Nordström, PhD, at Umeå University in Umeå, Sweden, said, "This finding gives us more evidence that these two diseases are linked. 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."

Experienced Depression? Don’t Panic
The risk is small, and became apparent only through this large study.

Not involved with this Swedish project, Peter Schmidt, vice president of research for the National Parkinson Foundation, urged people with a history of depression to take a deep breath: "The risk here is that people will overestimate the importance of this increased risk from depression. 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."

Dr. Eugene Lai, chair of Parkinson's disease research and treatment at Houston Methodist Hospital Neurological Institute, said many of his Parkinson's patients report suffering from depression and other non-motor conditions, such as constipation, loss of smell and sleep disorders. He said, "Depression can't be used on its own as an indicator of risk for Parkinson's disease, 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."

While noting that the new study’s findings are completely consistent with the growing evidence linking depression and PD, Dr. Michael Okun -- national medical director of the National Parkinson Foundation – also urged caution:
I don't think depression triggers Parkinson’s. 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.

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Many media outlets around the world reported the results from this study, including:

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