A Swedish population study was recently published in the journal Neurology suggesting that people who experienced depression at one point in their lifetime were at greater risk for also developing Parkinson’s disease.
Of the 140,688 patients in Sweden with depression over a period of 25 years, 1.1 percent developed Parkinson’s — compared to a rate of 0.4 percent risk factor in the control group. That’s an increased risk factor of nearly 3 times.
If you have depression, should you be concerned?
I’d argue — not much. Here’s why.
The study (Gustafsson et al., 2015) found that the more severe the depression, the greater the risk for later developing Parkinson’s up to 25 years later.
The study demonstrates a relationship between depression and being at greater risk for later being diagnosed with Parkinson’s disease (PD). Other studies looking into these two disorders has found that most Parkinson’s disease patients experience depression. That’s to be expected — Parkinson’s is a neurodegenerative illness that eventually takes away all of your fine motor movements. You’d be surprised at how important such motor control is in everyday life, something most of us take for granted.
But I would caution that because of the study’s design, we actually don’t know whether this is a phenomenon unique to depression, because that’s the only mental illness these researchers looked at. Because of that design decision, there are a multitude of alternative hypotheses that may also explain the results:
- This is not something unique to people with depression — that perhaps any mental illness presents a greater risk factor for Parkinson’s disease.
- This is not something unique to people with Parkinson’s — perhaps any mental illness (or depression specifically) presents a greater risk factor for any neurodegenerative disease in the future.
- There is some third factor that was unexamined by the researchers that is common amongst people with depression (but not others) that could explain the increased risk.
It’s not hard to imagine what some of those third factors might be.
For instance, most people with more severe depression take an antidepressant medication. Isn’t it just as possible that it’s the antidepressant medication that puts a person at increased risk for Parkinson’s? Or, as you’ll see below, perhaps a sleep medication (as many people with depression also suffer from sleep problems). Or perhaps not a medication or depression itself, but problems with REM sleep (REM sleep behavior disorder). Constant, ongoing sleep problems are a symptom of depression — but also may be a contributing factor to the cause of depression too.
Depression Is Hardly the First Risk Factor for Parkinson’s
But here’s a problem not brought up in any mainstream media news mention of this new study — one of perspective.
Because depression isn’t the first thing that seems connected to later developing Parkinson’s. In fact, there’s a litany of diseases and disorders we could name:
- Lai et al. (2015) found a 26 percent greater risk for developing Parkinson’s after being diagnosed with cataracts.
- Lin et al. (2014) found people with constipation were 3 times more likely to develop Parkinson’s in a direct dose-dependent manner (e.g., the more severe constipation you suffer throughout life, the more likely it is that you’ll develop Parkinson’s).
- Yang et al. (2014) found that more you use the most commonly-prescribed sleep medicine for insomnia — Zolpidem (Ambien, Zolpimist) — the greater the risk you’ll be for later developing Parkinson’s.
However, Postuma (2014) notes that disturbance of REM sleep — REM sleep behavior disorder (RBD) — “is by far the highest of any clinical prodromal marker of [Parkinson’s disease]. By counterexample, olfaction, constipation and depression have been shown to predict PD, but occur in up to 1/3 of the general population. […] [T]hese markers would probably have positive predictive values of well under 10 percent — compared to 70–80 percent in RBD.”
So while depression may be a tiny predictor for future Parkinson’s, REM sleep behavior disorder (RBD) is a huge predictor.
Lin summed it up nicely:
Some non-motor symptoms, such as depression, constipation, olfactory problems, and rapid eye movement sleep behavior disorder, can occur early in the disease process and may precede motor symptoms by up to 20 years. Research into these non-motor symptoms during [Parkinson’s disease] development may advance our understanding of the pathophysiology of PD and lead to earlier diagnosis and improved management.
In fact, Hawkes et al. (2010) provided a nice graphical timeline of some of the risk factors that show themselves before Parkinson’s is diagnosed:
The key takeaway then is not to worry so much about depression being connected to Parkinson’s. Rather, be aware that Parkinson’s has a lot of precursor symptoms — but most of the people who have these symptoms won’t actually ever get Parkinson’s.
The greatest risk factor that increases the likelihood of later being diagnosed with Parkinson’s is REM sleep behavior disorder (RBD).1
Gustafsson, H., Nordström, A., & Nordström, P. (2015). Depression and subsequent risk of Parkinson disease: A nationwide cohort study. Neurology.
Hawkes, CH, Del Tredici, K., & Braak, H. (2010). A timeline for Parkinson’s disease. Parkinsonism & Related Disorders, 16, 79-84.
Shih-Wei Lai, Cheng-Li Lin, Kuan-Fu Liao, Kao-Chi Chang-Ou. (2015). Increased risk of Parkinson’s disease in cataract patients: A population-based cohort study.
Parkinsonism & Related Disorders, 21, 68-71.
Chin-Hsien Lin, Jou-Wei Lin, Ying-Chun Liu, Chia-Hsuin Chang, Ruey-Meei Wu. (2014). Risk of Parkinson’s disease following severe constipation: A nationwide population-based cohort study. Parkinsonism & Related Disorders, 20, 1371-1375,
Postuma, R.B. (2014). Prodromal Parkinson’s disease – Using REM sleep behavior disorder as a window. Parkinsonism & Related Disorders, 20, S1-S4.
Yu-Wan Yang, Teng-Fu Hsieh, Chia-Hui Yu, Yung-Sung Huang, Ching-Chih Lee, Tsung-Huang Tsai. (2014). Zolpidem and the risk of Parkinson’s disease: A nationwide population-based study. Journal of Psychiatric Research, 58, 84-88.
- What is RBD? RBD is when people who are sleeping try to act out their dreams, instead of keeping them inside their heads like most people. People with RBD physically move limbs or even get up and engage in things they’d normally do while awake. Some engage in sleep talking, shouting, screaming, hittting or punching. [↩]