Individuals with bipolar disorder (BD) have a higher incidence of Parkinson disease (PD), according to the results of a study published in Neurology. In addition, more frequent manic/mixed and depressive episodes increase this risk.
Mao-Hsuan Huang, MD, from the Department of Psychiatry, Taipei Veterans General Hospital, Taiwan, and colleagues conducted a longitudinal study using data from the Taiwan National Health Insurance Research Database between 2001 and 2009 and included 56,340 patients with BD and 225,360 age- and sex-matched control individuals in the analysis. Study participants were followed to the end of 2011.
The study found that those with BD had a higher incidence of PD and a shorter duration from enrollment to diagnosis of PD than control individuals during the follow-up period (0.7% vs 0.1%). Patients with a higher frequency of psychiatric admission for BD had an increased risk of developing PD compared with those admitted less than once per year. The hazard ratio for developing PD for 1 to 2 manic/mixed episodes per year was 4.02, whereas for 1 to 2 depressive episodes per year, it was 2.67. For more than 2 episodes per year, the hazard ratios were 6.29 for manic/mixed and 6.19 for depressive.
Sensitivity analyses after exclusion of the first year or first 3 years of observation demonstrated consistent findings of an elevated risk of developing PD among patients with BD, as did sensitivity analyses after exclusion of antipsychotic exposure before PD diagnosis.
The researchers noted that some evidence points to possible shared underlying mechanisms between the 2 diseases, including impaired dopaminergic neurotransmission. In addition, serotonin, which is known to play a pivotal role in BD, may have some role in PD. However, the researchers also state that a single monoamine may not be responsible for the heterogeneous phenotypes of neuropsychiatric disorders.
Limitations of the study included those inherent to the use of a nationwide database.
The researchers called for future studies to investigate the potential common underlying pathophysiology between BD and subsequent PD.
Huang MH, Cheng CM, Huang KL, et al. Bipolar disorder and risk of Parkinson disease: a nationwide longitudinal study. Neurology. 2019;92(24):e2735-e2742.