A study published in the journal Neurology states that there exists a direct relationship between depression and subsequent development of Parkinson’s disease (PD). In a statement to the American Academy of Neurology Press release, senior author Peter Nordstrom said, “We saw this link between depression and Parkinson’s disease during over a timespan of more than two decades, so depression may be a very early symptom of Parkinson’s disease or a risk factor for the disease.”

Parkinson’s disease is a progressive neurodegenerative disorder. While there are severe motor disabilities present, patients suffering from PD also present with cognitive and psychiatric issues. According to the Parkinson’s Disease Foundation, most people diagnosed with PD experience mild to moderate depression and many suffer from clinical depression.

Studies have been underway to understand the significant correlation between PD and depression. Since both conditions are associated with changes in brain chemistry, a question that needed answering was whether depression was the cause or effect of PD. The study being discussed here presents evidence that PD initially presents itself as depression, long before the characteristic movement disorders are noticed.

A huge nationwide cohort study conducted in Sweden followed 140,688 participants with depression over a period of up to 26 years. The following are the results from the study:

• The percentage of participants with depression who developed PD was twice as high as those without depression (1.1%, compared to 0.4%)
• PD was diagnosed on an average within 4.5 years after the start of the study; the likelihood of developing the disease decreased over time
• Compared to people who did not have depression, those with the condition were 3.2 times more likely to develop PD within one year of the start of the study
• Between 15-25 years after start of study, depressed participants were 50% more likely to develop PD
• There was no correlation between one sibling having depression and another developing PD
• People who were hospitalized for depression were more likely to develop PD than patients who were treated in outpatient facilities
• 5 or more hospitalizations for depression increased the likelihood of developing PD by 40%
• The association between PD and depression remained the same even when the data was adjusted for other conditions that were related to depression (traumatic brain injury, alcoholism, drug abuse, stroke, etc.)

The question arises whether everyone who suffers from depression needs to worry about eventually being diagnosed with PD. The answer is likely, “Probably not,” since the study found that 98.9% of the participants with depression did not develop the disease.

 Therefore, while depression is potentially a risk factor for PD and probably an early warning sign, it should perhaps be assessed as an additional factor in a patient’s likelihood of developing PD, in conjunction with established risk factors for the disease such as advanced age, genetics, repeated head trauma, exposure to certain environmental factors, etc.

Written by Mangala Sarkar Ph.D.