Paris: People over 54 who suffer from steadily-worsening depression may run a higher risk of developing dementia, according to new research published on Saturday that suggested it may be an early symptom.

Other types of depression, such as one-off or recurring episodes, did not appear to pose a similar threat.

‘Only the group whose symptoms of depression increased over time was at an increased risk of dementia,’ said a statement by The Lancet Psychiatry, which published the results.

Doctors have previously noted a high correlation between depression and dementia in patients, though the nature of the relationship is not known.

The new study claims to be the first to differentiate between types of depression.

Some people experience a one-off depressive event and others have regular relapses. Some see their symptoms ease over time, others worsen, and some are chronic sufferers.

While some depressive events are a response to an adverse life event, others may be caused by a malfunction of brain chemicals or signalling.

For the latest study, researchers analysed data collected on 3,325 people aged 55 and over in the Netherlands over an 11-year period, from 1993-2004, and tracked them for 10 years thereafter.

When the study started, all had symptoms of depression but none of dementia. By its end, 434 people had developed dementia, including 348 cases of Alzheimer’s disease.

The team divided the participant data into five different categories of depression.

In the group of 255 people whose depression symptoms worsened over time, 55 developed dementia.

At 22 percent, this was ‘significantly higher’, nearly 1.5-times, than the other groups — all of which had a dementia risk of about 10 percent.

According to the World Health Organisation, between five and 8 percent of people aged 60 and over are estimated to suffer from dementia at any given time.

The findings suggested that one-off severe depression, or recurring episodes, did not boost dementia risk, the authors said.

It also meant that ‘increasing symptoms of depression in older age could potentially represent an early stage of dementia,’ according to the statement.

And it implied that dementia and some forms of depression may have a common cause.