This study, called a systematic review and meta-analysis, combined the results of seven studies with 4.3 million participants to examine a potential association between bipolar disorder with a later diagnosis of Parkinson disease of unknown cause.

A sensitivity analysis was performed by removing the studies that had a high risk of bias and also showed an increased risk of PD in people with BD. Preplanned subgroup analyses according to study design and diagnostic certainty failed to show a significant effect.

The findings suggest that a previous diagnosis of bipolar disorder was associated with increased likelihood of a subsequent Parkinson disease diagnosis but subgroup analyses suggest the possibility of an overestimation of the magnitude of the associations.

The main limitation of the study is analysis that suggests a greater likelihood of a Parkinson disease diagnosis in shorter studies which raises concerns over misdiagnosis. These findings highlight the probability that BD may be associated with a later development of PD and the importance of the differential diagnosis of parkinsonism features in people with BD.

https://jamanetwork.com/journals/jamaneurology/fullarticle/2752486