Bipolar disorder patients may be more likely to get Parkinson’s, a major review of evidence has suggested.
Scientists uncovered the link after they analysed the results of seven studies involving 4.3million participants.
Patients with the mood disorder were more than three times more likely to end up with Parkinson’s, according to the findings.
Scientists at the Faculdade de Medicina da Universidade de Lisboa in Portugal did not prove bipolar disorder causes the crippling condition.
However, they believe drugs bipolar patients have to use for years to control their disorder could be to blame.
Another suggestion is that bipolar disorder may lower levels of dopamine, and a lack of dopamine is the main cause of Parkinson’s.
People with bipolar disorder are up to three times more likely to get Parkinson’s
Charities today welcomed the evidence published in JAMA Neurology – but warned the risk of a bipolar patient developing Parkinson’s is still slim.
Dr Patrícia Faustino and colleagues combined the results of seven existing studies that investigated the prevalence of Parkinson’s in bipolar patients.
One was conducted in the UK, involving more than 3,000 people, and another took place in the US and had more than 3.4million people.
The findings suggest that a previous diagnosis of bipolar disorder was linked with a 3.35 increased likelihood of Parkinson’s.
A second analysis suggested bipolar patients face a 3.21-fold higher risk of getting the condition.
The researchers said the link may be explained by the drugs bipolar patients have to take.
Bipolar causes extreme highs and lows, called manic and depressive episodes, which can interfere with daily life.
It’s usually diagnosed in teenagers, meaning patients are forced to take medication every day for the rest of their life to try and prevent a switch in mood.
The most common mood stabilisers prescribed in the UK contain lithium, taken for at least six months.
It is lithium, which, over long periods of time, may induce Parkinson’s, the researchers said.
About seven per cent of people with parkinsonism – a term that covers several conditions including Parkinson’s – have developed their symptoms following treatment with particular medications, according to Parkinson’s UK.
Any drug that blocks the action of dopamine, referred to as a dopamine antagonist, is likely to cause parkinsonism.
The link may also be explained by the role of dopamine – a chemical responsible for controlling brain functions – in both disorders.
Scientists are still unsure on the causes of bipolar disorder – but some suggest that the manic phases are caused by an imbalance of dopamine.
During a depression phase, dopamine is lower. This is then later compensated with a surge during a manic phase.
It is thought this may lead to an overall shortage of dopamine production – one of the first typical signs of Parkinson’s.
A reduction in dopamine is what leads to the symptoms of involuntary shaking, slow movements and stiffness.
However, Dr Faustino and colleagues made clear the relationship between bipolar and Parkinson’s ‘remains elusive’.
Dr Beckie Port, research manager at charity Parkinson’s UK, said: ‘A number of studies have now shown a potential link between bipolar disorders and an increased risk of Parkinson’s.
‘This analysis gives us the best indication so far of the level of increase in risk, however, even with a three fold increase, the risk of someone with bipolar developing Parkinson’s is still relatively low.
‘The association between the two conditions likely involves dopamine, which plays a complex role in brain signalling, from coordinating movement to regulating our moods.
‘Further research is needed to develop our understanding of this association and to turn that into a treatment that may, one day, prevent Parkinson’s.’
The findings echo that of a study in Taiwan, published in the journal Neurology, in May this year.
Researchers at the Taipei Veterans General Hospital found 0.7 per cent of bipolar patients got Parkinson’s, while only 0.1 per cent of the general population did.
The scientists suspected factors like inflammation, genetics and miscommunications between brain cells may be to blame.
Parkinson’s is caused by a loss of nerve cells in part of the brain called the substantia nigra, which leads to a reduction in dopamine.
But exactly what causes the loss of nerve cells is unclear.
The condition strikes one million Americans and 148,000 Britons, according to figures.
Like Parkinson’s, an incurable neurological disease, there are many gaps in the understanding of bipolar.
The disorder affects some 5.7million people in the US, according to estimates.
It’s fairly common in the UK, the NHS say. One in every 100 people will be diagnosed with it at some point in their life.