An association between bipolar disorder and Parkinson disease

Disorders of mood, especially depressive disorders, are the most common psychiatric disturbances associated with Parkinson disease (PD)1 and may present before or after the onset of motor symptoms. Numerous studies have linked depression and other mental illnesses to an increased risk of a subsequent diagnosis of PD.2 Among the most compelling was a nested case-control study in a Swedish nationwide cohort that found that the odds ratio of being diagnosed with PD within the first year of depression was 3.2 (95% confidence interval 2.5–4.1).3 Furthermore, this study demonstrated a persistent association between depression and PD for >2 decades, a "dose-response" relationship between severity of depression and risk of PD as inferred by recurrent need for hospitalization, and a lack of familial coaggregation of depression and PD in sibling pairs, all of which suggest a direct and independent relationship between depression and subsequent PD. Although both major depression and bipolar disorder are mood disorders that may be characterized by recurrent episodes of low mood (depression), bipolar disorder also requires the presence of ≥1 manic or mixed episodes. Despite the clinical similarities and possibility of etiologic overlap, major depression and bipolar disorder have differing genetic risk. Therefore, the issue of whether bipolar disorder is associated with an increased risk of subsequent diagnosis of PD should be further explored.