Depression Linked to Parkinson's
May 28, 2002 -- People diagnosed with depression are more than three times as likely to develop Parkinson's disease than nondepressed people, according to a new study. And that dramatic link now has researchers wondering if depression is really an early symptom of Parkinson's rather than a risk factor.
"This raises the question of whether depression is the first symptom of Parkinson's disease -- that appears before patients have other symptoms and a diagnosis," says study author Agnes Schuurman, PhD, of Maastricht University in the Netherlands, in a news release.
Depression is already known to increase the risk of stroke, cancer, dementia, and heart disease later in life. But while depression often affects people diagnosed with Parkinson's disease, until now there has been no scientific proof that depression precedes the symptoms of Parkinson's.
Parkinson's disease is a progressive disorder that affects movement, muscle control, and balance. About 800,000 Americans have it. Although the exact cause is unknown, research has shown that people with Parkinson's disease have a lower level of a brain chemical called dopamine.
In this study, researchers followed 1,358 people who had been diagnosed with depression to determine how many developed Parkinson's disease during a 25-year period. They then compared those rates with a group of 67,570 similarly matched healthy people in the registry who were never diagnosed with depression.
About 1.4% (19) of the depressed people developed Parkinson's compared with only 0.4% of the others.
Researchers say the link may have a biological explanation, known as the "serotonin hypothesis." Not only do people with Parkinson's disease have lower levels of dopamine in the brain, studies have also shown that the brains of these patients also have reduced levels of another brain chemical, serotonin. Low levels of serotonin are known to be a key factor in depression.
The study authors say serotonin plays a role in regulating the release of dopamine in the brain, and because the level of dopamine activity is impaired in Parkinson's, they believe serotonin activity is also reduced, which increases the risk of depression.
"Because the reduced serotonin activity already exists before any motor symptoms begin, the risk of depression is also increased long before any Parkinson's symptoms become apparent," says Schuurman.
The authors conclude that, "The 'serotonin hypothesis' also raises the conceptual issue of whether depression in this context should be seen as a separate disease entity or rather, ... as an intrinsic symptom or first clinical manifestation of [Parkinson's]."