Patients with bipolar disorder may be at increased risk of Parkinson’s disease in later life, according to a systematic review and meta-analysis published in JAMA Neurology.
Patrícia R. Faustino, MD, from the faculty of medicine at the University of Lisboa (Portgual), and coauthors reviewed and analyzed seven articles – four cohort studies and three cross-sectional studies – that reported data on idiopathic Parkinson’s disease in patients with bipolar disorder, compared with those without. The meta-analysis found that individuals with a previous diagnosis of bipolar disorder had a 235% higher risk of being later diagnosed with Parkinson’s disease. Even after removing studies with a high risk of bias, the risk was still 3.21 times higher in those with bipolar disorder, compared with those without.
“The pathophysiological rationale between bipolar disorder and Parkinson’s disease might be explained by the dopamine dysregulation hypothesis, which states that the cyclical process of bipolar disorder in manic states leads to a down-regulation of dopamine receptor sensitivity (depression phase), which is later compensated by up-regulation (manic state),” the authors wrote. “Over time, this phenomenon may lead to an overall reduction of dopaminergic activity, the prototypical Parkinson’s disease state.”
Subgroup analysis revealed that subgroups with shorter follow-up periods – less than 9 years – had a greater increase in the risk of a later Parkinson’s disease diagnosis. The authors noted that this could represent misdiagnosis of parkinsonism – possibly drug induced – as Parkinson’s disease. The researchers also raised the possibility that the increased risk of Parkinson’s disease in patients with bipolar disorder could relate to long-term lithium use, rather than being a causal relationship. “However, treatment with lithium is foundational in bipolar disorder, and so to separate the causal effect from a potential confounder would be particularly difficult,” they wrote.
One of the studies included did explore the use of lithium, and found that lithium monotherapy was associated with a significant increase in the risk of being diagnosed with Parkinson’s disease or taking antiparkinsonism medication, compared with antidepressant therapy. However the authors commented that the diagnostic code may not differentiate Parkinson’s disease from other causes of parkinsonism.
Given their findings, the authors suggested that, if patients with bipolar disorder present with parkinsonism features, it may not necessarily be drug induced. In these patients, they recommended an investigation for Parkinson’s disease, perhaps using functional neuroimaging “as Parkinson’s disease classically presents with nigrostriatal degeneration while drug-induced parkinsonism does not.”
Two authors declared grants and personal fees from the pharmaceutical sector. No other conflicts of interest were reported.
SOURCE: Faustino PR et al. JAMA Neurol. 2019 Oct 14. doi: 10.1001/jamaneurol.2019.3446.