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Those with bipolar who developed Parkinson's were nine years younger -- average age 64 -- than others who also developed Parkinson's, the study found.

"When you say there's seven times the risk, it gets scary. But it's still very few [bipolar] patients who get Parkinson's," said Dr. Justin Martello, who reviewed the findings. He's a neurologist specializing in movement disorders and Parkinson's disease at Christiana Care Health System in Wilmington, Del.

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Martello pointed out that while the study was large, it was limited by including only people in Taiwan.

"We don't know how this would apply more globally or broadly," Martello said. "I think it's more interesting for physicians to know this and be more aware of the association."

The study was published online May 22 in the journal Neurology.

Pontone, who cowrote an accompanying editorial, and Martello said scientists have many theories -- still unproven -- about how bipolar disorder might be connected to the development of Parkinson's.

"A depressive or manic episode may do something to the brain that renders it more vulnerable" to Parkinson's over time, Pontone said.

And Martello noted that many medications used to treat bipolar disorder can trigger Parkinson's-like symptoms.

Many experts think Parkinson's is active years or decades before movement problems show up, and mood disorders such as bipolar may actually be an early symptom of Parkinson's, Pontone and Martello said.

Much more research is still needed, they agreed.

"We definitely need to look more at a global population," Martello said. "The researchers here did follow patients for 10 years, but it needs to be extended longer to see how many of these patients convert to Parkinson's down the road."

Although DBS, similar to all PD therapies, cannot cure, modify, or slow progression of the disease, some recent research suggests that the symptom alleviation and quality-of-life improvement it provides might lead to mild improvements in longevity, noted Justin Martello, MD, a PD and movement disorders specialist from Christiana Care Neurology Specialists in Newark, Delaware.

“It does improve function, so it may improve morbidity over time,” Dr Martello told Neurology Advisor. “It can improve a patient’s life so they can be more active, and that can infer a longer life.”

Most research is therefore focusing on comparing different DBS targets, refining DBS methods and tools, and exploring other interventions only in early phase or preclinical trials.