People who suffer with depression may have an increased risk of going on to develop Parkinson's disease, a new study suggests.

Parkinson's is a progressive neurological disease, the symptoms of which include tremors, stiffness and slow movement. An estimated six to seven thousand people in Ireland are affected and there is currently no cure.

Previous research has suggested a link between these two conditions so Swedish researchers decided to investigate further. They looked at more than 140,000 people over the age of 50 who had been diagnosed with depression between 1987 and 2012. These were matched with over 421,000 control participants and all were followed up for up to 26 years.

While the overall number of people to go on to develop Parkinson's was low, the study found that those with depression were more likely to develop the neurological disease than those without depression.

The researchers noted that the risk of developing Parkinson's decreased over time. Those with depression were more than three times more likely to develop Parkinson's in the year after the study started compared to those without depression.

However 15 to 25 years into the study, people with depression were around 50% more likely to develop Parkinson's than those without depression.

Meanwhile, the study also noted that those with more serious cases of depression were more likely to develop Parkinson's.

Those who had been hospitalised with depression five or more times were 40% more likely to develop Parkinson's compared to those who had only been hospitalised once.

Those who had been hospitalised at all for depression were also found to be 3.5 times more likely to develop Parkinson's than those who had been treated for depression as outpatients.

The results stood even when other depression-related conditions were taken into account, such as alcohol abuse, stroke and traumatic brain injury.

"We saw this link between depression and Parkinson's disease during a timespan of more than two decades, so depression may be a very early symptom of Parkinson's disease or a risk factor for the disease," commented the team from Umeå University.

Details of these findings are published in the journal, Neurology.

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