Although use of medications for bipolar disorder could overestimate this risk, the researchers believe that a Parkinson’s diagnosis should be considered if patients with the mental health condition also show parkinsonian features.
The study, “Risk of Developing Parkinson Disease in Bipolar Disorder – A Systematic Review and Meta-analysis,” was published in JAMA Neurology.
Biopolar disorder is a mood disorder characterized by cyclic episodes of depression and mania. Although the mechanisms underlying bipolar disorder are still not completely understood, there is a possible role of the dopaminergic system — which is affected in Parkinson’s disease.
In fact, antipsychotic medications that block dopamine receptors can improve manic symptoms in these patients.
Certain medications used to treat bipolar disorder — such as lithium, antipsychotic medications, and antiepileptic medications — could be associated with drug-induced parkinsonism, which cannot be effectively distinguished from Parkinson’s disease in the clinic.
“Since drug-induced parkinsonism is more common among patients with [bipolar disorder], physicians may be more inclined to misdiagnose PD as drug-induced parkinsonism,” the researchers said.
Some studies also have shown that bipolar disorder is more common in people with Parkinson’s.
To understand the possible association of bipolar disorder with a later diagnosis of idiopathic Parkinson’s disease — meaning it’s due to an unknown cause — researchers from the University of Lisbon in Portugal performed an electronic literature search across four medicine-related databases.
Data from seven studies, involving 4, 374, 211 total participants, were used. The selected studies contained data on the likelihood of developing Parkinson’s disease in individuals with bipolar disorder versus those who did not have this condition.
The results showed that those with bipolar disorder were 3.4 times more likely to develop Parkinson’s disease later in life. Even when accounting for studies that had a high risk of bias, this likelihood was still maintained, albeit lower, at 3.21 times greater risk.
Researchers then performed a subgroup analysis in which they divided participants based on the duration of their follow-up. Specifically, they split patients into two groups: one including participants with more than nine years of follow-up versus those who were followed for less time.
Both groups had an increased risk of developing Parkinson’s, but the subgroup with the shorter follow-up was associated with a greater increase in the risk of a PD diagnosis.
However, this could be due to a higher rate of misinterpretation of drug-induced parkinsonism as Parkinson’s disease. “In fact, one study explicitly did not differentiate between the 2 conditions,” the researchers said.
“This review suggests that patients with [bipolar disorder] have a significantly increased risk of developing PD compared with the general population,” the researchers said.
They added that, if patients with bipolar disorder show parkinsonism features, a Parkinson’s diagnosis should be considered “independently of concomitant medication.”
The investigators said further testing could help to determine whether patients presented Parkinson’s versus drug-induced parkinsonism.
“To clinically distinguish parkinsonism from PD in clinical practice, the use of functional neuroimaging methods may be of particular interest,” they said.
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