Parkinson’s is a curiously complex disease. While mutations in a small number of genes are known to lead to the development of the disease, the vast majority of cases have no known cause. But interestingly, the disorder has been associated with a number of different things, including pesticides, a rare sleep disorder and even a particular personality type characterized by introversion, inflexibility and low novelty-seeking. There also seems to be a growing body of evidence that it could be linked with depression, and new research appears to lend weight to this idea.

According to the latest study, those who had a history of depression were significantly more likely to develop Parkinson’s disease later in life. Furthermore, the risk rose with the severity of depression, as those who were hospitalized for the condition were even more likely to develop Parkinson’s. The work has been published in Neurology.

Parkinson’s disease is a movement disorder characterized by a progressive degeneration of nerve cells in a brain region called the substantia nigra. Since these cells synthesize the neurotransmitter dopamine, their death leads to a decrease in the amount of this chemical in the brain. Amongst other things, dopamine helps regulate body movement, so its loss leads to many of the symptoms associated with the disease, like involuntary limb shaking.

While this much is known, the disease is idiopathic in most people. Although, as mentioned, it has been tied to depression previously, earlier work didn’t follow individuals for longer than 10 years, so researchers from Umeå, Sweden, decided to probe this link further. To do this, they enrolled almost 150,000 individuals who had been diagnosed with depression between 1987 and 2012 and then matched them 1:3 with similar people without depression.

After following people for up to 26 years, they found that 1.1% of individuals with a history of depression developed Parkinson’s, compared with only 0.4% of depression-free participants. Within one year of study commencement, those with depression were more than 3 times more likely to develop Parkinson’s than those without this condition, the researchers report.

Interestingly, they found that the more severe a patient's depression was, the greater their risk for Parkinson’s. Patients that had been admitted to hospital for the condition were 3.5 times more likely to develop Parkinson’s than those treated as outpatients. Furthermore, those who had been admitted to hospital five times or more for depression treatment were 40% more likely to be diagnosed with Parkinson’s later on than those who had only been admitted once.

While this study certainly has merits, such as including a very large number of individuals and following them up for a long period of time, it’s important to note that since the researchers were looking at associations, they cannot prove depression is causing the increased risk in developing Parkinson’s. Although it is possible that depression may alter the brain in such a way that contributes to its development, the researchers told Live Science, it may also be that depression is an early symptom of Parkinson’s. Alternatively, antidepressant medication may also somehow raise the risk, but these ideas are all speculation at the moment that require further investigation.   

It's also important not to panic over these results: The researchers point out that Parkinson's is an uncommon disease, and the risk of developing it is still extremely low in those with depression. 

[Via Live Science, WebMD and Neurology]