Depressed people may be at risk of Parkinson's disease

People with depression may be more likely to develop the movement disorder Parkinson's disease, according to new research published in Neurology.

According to the authors of the study, depression is more common in people with Parkinson's disease than those without the movement disorder.

Researchers, at Umeå University in Sweden, saw this link between depression and Parkinson's disease over a timespan of more than 2 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 disorder of the nervous system that affects how a person moves, including how they speak and write. 

As well as problems with movement, Parkinson's disease can also cause cognitive problems, neurobehavioral problems and sensory difficulties.

The authors of the study state that depression is more common in patients with Parkinson's disease than in members of the general population. The mood disorder has a major influence on health-related quality of life and could also be involved in more rapid deterioration of cognitive and motor functions.

However, few studies have investigated this association for periods of longer than 10 years, with any long-term findings so far inconclusive.

For the study, the researchers used a cohort consisting of all Swedish citizens aged 50 years and above as of December 31st, 2005. From this group, they then took the 140,688 people diagnosed with depression between 1987 and 2012.

These individuals were each matched with three control participants (a total of 421,718 controls) of the same age and sex who had not been diagnosed with depression.

The participants were then followed for up to 26 years. A total of 1,485 people with depression (1.1%) developed Parkinson's disease during this time, compared with 1,775 of those who did not have depression (0.4%).

On average, Parkinson's disease was diagnosed 4.5 years after the beginning of the study, with the likelihood of the disorder developing decreasing over time.

No sibling link found for depression and Parkinson's disease

The researchers calculated that participants with depression were 3.2 times more likely than those without depression to develop Parkinson's disease within a year of the study beginning. After 15-25 years, the researchers found participants with depression were almost 50% more likely to develop the condition.

If a participant's depression was severe, their likelihood of developing Parkinson's disease was also higher. For example, those who had been hospitalized for depression five or more times were 40% more likely to develop Parkinson's disease than participants who had been hospitalized for depression just once.

In addition to these observations, the researchers examined siblings. No link was found between one sibling having Parkinson's disease and the other having depression.

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.

The authors suggest there are a number of mechanisms that could explain their findings. Depression or antidepressive treatment could increase the risk of Parkinson's disease, depression could be an early symptom of Parkinson's disease, or that the two conditions could share environmental causative factors.

In the paper, the authors acknowledge that they are unable to evaluate the potential role of substances used in antidepressive treatment as risk factors for Parkinson's disease. The study is an observational one and cannot determine causation.

These findings suggest a direct association between depression and subsequent [Parkinson's disease], supported by a time-dependent hazard ratio, a dose-response pattern for recurrent depression, and a lack of evidence for coaggregation among siblings.

Given that the association was significant over more than two decades of follow-up, depression may be a very early prodromal symptom of or a causal risk factor for Parkinson's disease.

References:

1.  Depression and subsequent risk of Parkinson disease, Peter Nordström, et al., Neurology, published online 20 May 2015, abstract.
2.  American Academy of Neurology news release, accessed 19 May 2015.
3.  Additional source: Mayo Clinic, Parkinson's disease - definition, accessed 19 May 2015.