A new study has found that people with bipolar disorder have a higher risk of developing Parkinson’s disease. Also, having bipolar disorder of a higher severity seems to increase the risk even further.
Recently, however, lead study author Mu-Hong Chen and collagues — from Taipei Veterans General Hospital in Taiwan — decided to investigate.
They have now published their findings in the journal Neurology.
In Parkinson’s, neurons in certain parts of the brain gradually die, which results in symptoms that include tremor, rigidity, slower movement, and difficulties in balancing, swallowing, and speaking.
In the United States, doctors diagnose Parkinson’s in around 50,000 people each year. Currently, about 500,000 people in the U.S. have the condition.
The National Institutes of Health (NIH) say that more people will develop Parkinson’s as we start to live longer, and that the likelihood of developing the condition will increase the older we get.
People with Parkinson’s disease usually take a drug called levodopa to help slow the degeneration of the brain.
Chen and colleagues examined the health records of 56,340 people with diagnosed bipolar disorder in Taiwan. All had received their diagnoses in 2001–2009. The researchers compared these data with the health records of 225,360 people in Taiwan with no diagnosis of either bipolar disorder or Parkinson’s disease.
The scientists followed both groups through 2011. After analyzing their data, they found that 0.7% of the people with bipolar disorder developed Parkinson’s during the study, and that only 0.1% of the control group developed it.
The researchers adjusted their findings to take into account factors such as medication history, age, sex, and history of diseases and injuries affecting the brain, all of which could influence a person’s risk of developing Parkinson’s.
After adjustment, they found that participants were almost seven times more likely to develop Parkinson’s if they had a diagnosis of bipolar disorder at the start of the study, compared with those who did not have bipolar disorder.
The team also noticed some differences between people with bipolar who developed Parkinson’s and people without bipolar who developed it: Those who had bipolar disorder developed Parkinson’s at a younger age (64, on average) than those who did not have bipolar disorder (age 73, on average).
It also seems that the severity of the bipolar disorder influenced the level of risk; people who had to go the hospital due to bipolar disorder most often had the highest risk of developing Parkinson’s at a later stage in their lives.
Specifically, those who had been to the hospital once or twice each year were four times more likely to develop Parkinson’s compared with people whose bipolar disorder resulted in one hospitalization or fewer per year.
For people who had been to the hospital more than twice per year, the increase in risk was even higher; the people in this group were six times more likely to develop Parkinson’s than those who had been to the hospital less than once each year.
There were a number of key limitations to this study, however. Firstly, the scientists only included people who had asked their doctors for medical help for their bipolar disorder. Many individuals never seek help.
Secondly, the health record database they used did not include information about family history of Parkinson’s. It also did not hold information on environmental factors that could have influenced a person’s likelihood of developing Parkinson’s.
Chen says that further studies are required to establish whether bipolar disorder and Parkinson’s share any underlying processes that might explain the association.
“These could include genetic alterations,” he explains, “inflammatory processes, or problems with the transmission of messages between brain cells.”
“If we could identify the underlying cause of this relationship, that could potentially help us develop treatments that could benefit both conditions.”