PD Patients Face Nearly Fourfold Greater Risk of Bipolar Disorder, Study Finds

Parkinson’s Disease Patients Face Triple the Risk of Bipolar Diagnosis

Individuals with Parkinson’s disease (PD) face a risk for bipolar disorder that is more than three times higher than those without the condition, according to research published in the Journal of Psychiatric Research. Investigators utilized data from the Korean National Health Insurance Service database covering 2012 to 2023 to evaluate 5158 patients with PD and 25,790 matched control individuals. The findings indicate that the risk is highest within the initial 3 years following a PD diagnosis, persists through subsequent follow-ups, and remains largely consistent across both ages and sexes.

Evaluating the Korean National Health Insurance Service Data

To examine the connection between the two conditions, researchers established a 3-year washout period to guarantee that a PD diagnosis preceded the onset of bipolar disorder. Patients were matched with control participants in a 1:5 ratio using propensity scoring, ensuring no meaningful demographic or clinical differences between the cohorts. The primary outcome focused on incident bipolar disorder, which required at least two outpatient diagnoses by psychiatrists. Out of the 5158 patients with PD analyzed in the cohort—where 53.3% were women and 70% were aged 60 years or older—236 individuals were subsequently diagnosed with bipolar disorder.

The statistical analysis revealed that PD was associated with a nearly fourfold greater rate of subsequent bipolar disorder diagnosis, showing an incidence rate ratio of 3.74. The rate reached 13.01 per 1000 person-years among patients with PD, compared to 3.48 per 1000 person-years in the control group. Medscape covered these results, observing that previous literature had identified bipolar disorder as a predictor for subsequent Parkinson’s, while the new data demonstrates that the reverse pathway can also happen. These findings highlight the need for proactive screening for bipolar disorder in patients with PD, particularly during the early postdiagnosis period, investigator Yong Ju Baik, MD, of the Korean Armed Forces Capital Hospital in Seongnam, South Korea, and colleagues wrote, adding that long-term psychiatric monitoring may be warranted given the sustained and evolving risk over time.

Age and Sex Breakdown of Diagnostic Rates

When researchers stratified the data by age and sex, distinct patterns emerged across different demographics. Among patients aged 60 years or older, the rate of bipolar disorder diagnosis climbed to 14.55 per 1000 person-years, representing a more than threefold higher incidence rate than control individuals. For participants younger than 60 years, the absolute rate was lower at 9.27 per 1000 person-years, though the relative rate was higher with an incidence rate ratio of 5.77. Among individuals with Parkinson’s, male patients exhibited a bipolar disorder incidence rate exceeding four times that of controls, whereas female patients demonstrated an incidence rate more than three times higher than their control counterparts.

After adjusting for variables such as sex, age, smoking, total cholesterol, body mass index, and income, the risk for bipolar disorder peaked within the first 3 years following a PD diagnosis, yielding an adjusted hazard ratio of 3.55. For patients aged 60 and older, that early 3-year risk held strong at an adjusted hazard ratio of 3.56 before declining significantly over a span of 6 to 9 years. Conversely, patients younger than 60 showed an estimated bipolar disorder risk that was higher 6 to 9 years after diagnosis at an adjusted hazard ratio of 13.44, compared to an adjusted hazard ratio of 3.69 in the first 3 years. Investigators cautioned that wide confidence intervals meant these temporal differences were not statistically significant and should not be misconstrued as a late rebound risk. Study limitations included potential misclassification, surveillance bias, and a lack of evaluation regarding how treatments or interventions might affect bipolar disorder risk.

Biological Mechanisms and Broader Psychiatric Comorbidities

While psychiatric conditions like depression and anxiety frequently affect patients with PD—with earlier research indicating that PD patients face nearly double the risk for depression—the incidence of bipolar disorder has historically received less attention. The study authors pointed out that this relationship is biologically plausible, given shared alterations in dopaminergic, serotonergic, and noradrenergic systems, as well as neuroinflammatory processes and cortico-limbic dysfunction. Chronic low-grade neuroinflammation in the brain can lead to dopaminergic neuron loss and the pathological aggregation of alpha-synuclein, contributing to neurodegeneration.

In a related evaluation of the broader literature, a systematic review and meta-analysis published in the PMC database examined cohort studies assessing PD risk in individuals with bipolar disorder. That meta-analysis pooled six cohort studies and identified extreme heterogeneity, concluding that the evidence carries very low certainty and does not warrant a causal interpretation due to uncontrolled medication exposure and outcome misclassification. Meanwhile, in his analysis of the Korean research for Medscape, Dr. Gregory Pontone—a geriatric psychiatrist and the chief of the Aging, Behavioral and Cognitive Neurology Division at the University of Florida in Gainesville—pointed out that overlapping clinical presentations likely lead to underreporting of bipolar disorder within the Parkinson’s community. He suggested that impulse control disorders, which impact up to 20% of PD patients, might represent a bipolar diathesis. If we figure out whether impulse control and bipolar are associated phenomena, that could help inform the treatment of bipolar illness, which is a major problem worldwide, Pontone stated, adding that studies like this are really important because even if these are separate phenomena, we’re going to learn something about mechanisms that will hopefully improve treatment outcomes in the future.