Recently, researchers in Taiwan found that individuals with bipolar disorder may have a significantly elevated risk of acquiring Parkinson’s Disease (Huang et al., 2019). The authors followed a cohort of more than 50,000 patients with bipolar disorder at baseline, and compared them over the time frame to healthy controls. What they found was that individuals with bipolar disorder, had a nearly sevenfold greater incidence of eventually acquiring Parkinson’s Disease than those without bipolar disorder.
Furthermore, individuals with a history of manic and depressive, or mixed episodes, as well as a history of hospitalizations for bipolar disorder, had a much higher likelihood of acquiring Parkinson’s later on in life.
The notion of a bipolar-Parkinson’s link is not new. Prior literature has suggested a link between the two conditions at the level of neurotransmitters. One prime example is that of serotonin, which plays a major role in bipolar disorder pathophysiology, particularly with respect to depression. As it turns out, serotonergic activity is also notably reduced in Parkinson’s (Novaretti, Novaretti, & Tumas, 2016).
Altered Activity of Serotonergic Neurons Plays Role
One such study highlighting a potential mechanism is that by Kato et al. (2010), which suggested that bipolar disorder may involve altered activity of serotonergic neurons because of mitochondrial dysfunction. Similarly, these serotonergic neurons have also been found to deteriorate in a region of the brain known as the dorsal raphe, a region also markedly affected in Parkinson’s disease.
Moreover, while it is well known that dopamine is the most well documented neurotransmitter implicated in Parkinson’s, there is research suggesting that this dopaminergic pathway is also implicated in bipolar disorder. Pharmacological studies have indicated that there is increased dopamine drive during manic episodes, and a corresponding decrease in depressive states.
In Parkinson’s, giving high doses of dopamine analogues or precursors can elicit a manic-like pathology and symptomology, which, upon withdrawal, can elicit a depressive pathology and symptomology (Okasha, 2007). So, there are a multitude of possible neural and molecular pathways which are, or may be implicated in both conditions, and not just by chance, but also potentially linking the two to one another.
As it turns out, a recent case-control study in Sweden found that of those who suffered depression, were more than three times as likely to be diagnosed with Parkinson’s. This was also dose-dependent, hence, the more severe the depression, the more likely, and the earlier one would eventually be diagnosed with Parkinson’s (Gustafsson, Nordström, and Nordström, 2015).
Building off of the Swedish study, this most recent Taiwanese study has gone one step further, implicating not just depression, but a condition which consists of periodic depressive states in bipolar disorder.
REM Sleep Behavior Disorder is Strong Prognostic Factor
However, even in light of these findings, alarm bells should not ring for bipolar patients with regards to Parkinson’s, at least not just yet. There have been a myriad of other conditions that are closely linked (even more so) to Parkinson’s than bipolar disorder.
One such condition is REM sleep behavior disorder, which has been found to be among the strongest prognostic factors for developing subsequent Parkinson’s, with three in every 20 individuals with REM sleep disorder prone to developing Parkinson’s (IDIBAPS, 2019; Postuma, 2014).
Additionally, the Taiwanese study has the caveat of only including individuals with bipolar disorder who wound up seeking medical assistance for their condition. It must also be noted that, in the process of recruiting participants, the database generated did not consist of the participants’ family history with regards to Parkinson’s. It also did not take into account other potential environmental risk factors that could predispose individuals to Parkinson’s Disease.
However, a sevenfold risk is not one to ignore. The fact that this discovery suggests not just comorbidity, but also a very strong potential link between bipolar disorder and Parkinsons’ means that we should delve further in investigative research to substantiate this finding, and ascertain the exact molecular underpinning of this relationship, should it actually turn out to exist.
Ultimately, if there is a strong association between the two, then perhaps, in the unfortunate scenario that someone suffers from bipolar disorder, we can act accordingly via treatment to minimize their risk of acquiring Parkinson’s as well.
Gustafsson, H., Nordström, A., & Nordström, P. (2015). Depression and subsequent risk of Parkinson disease: A nationwide cohort study. Neurology.
Huang, M., Cheng, C., Huang, K., Hsu, J., Bai, Y., Su, T., . . . Chen, M. (2019). Bipolar disorder and risk of Parkinson disease. Neurology, 92(24).
IDIBAPS – Institut d’Investigacions Biomèdiques August Pi i Sunyer. (2011, July 29). REM sleep behavior disorder is a risk factor for Parkinson’s disease. ScienceDaily. Retrieved June 24, 2019 from www.sciencedaily.com/releases/2011/07/110729175617.htm
Kato, T., Suzuki, H.,Itohara, S., Masada, A., Funke, S.,Saitow, F. ,Fujimori-Tonou, N., & Kubota-Sakashita, M. (2010). Mitochondrial Dysfunction and Bipolar Disorder. Behavioral Neurobiology of Bipolar Disorder and Its Treatment Current Topics in Behavioral Neurosciences, 187-200. doi:10.1007/7854_2010_52
Novaretti, T. M., Novaretti, N., & Tumas, V. (2016). Bipolar disorder, a precursor of Parkinsons disease? Dementia & Neuropsychologia, 10(4), 361-364. doi:10.1590/s1980-5764-2016dn1004018
Okasha, T. (2007). Faculty of 1000 evaluation for Dopamine dysregulation syndrome: Implications for a dopamine hypothesis of bipolar disorder. F1000 – Post-publication Peer Review of the Biomedical Literature. doi:10.3410/f.1092557.544416
Postuma, R.B. (2014). Prodromal Parkinson’s disease – Using REM sleep behavior disorder as a window. Parkinsonism & Related Disorders, 20, S1-S4.