Apathy, not depression, could be an early warning sign of dementia, a study by the University of Cambridge has suggested.
Scientists found that individuals with higher baseline apathy, as well as those with increasing apathy over time, had a greater risk of dementia.
But neither baseline depression nor change in depression had any detectable influence on dementia risk, the study concluded.
The research suggests the belief that depression is a risk factor for dementia may be because some depression scales used by clinicians and researchers partially assess apathy.
The study, published in the Journal of Neurology, Neurosurgery and Psychiatry, is based on analysis over several years of more than 450 patients in the UK and the Netherlands with cerebral small vessel disease (SVD) – the narrowing of the small blood vessels deep inside the brain.
Apathy, defined as a reduction in "goal-directed behaviour", is a common neuropsychiatric symptom of SVD and is distinct from depression, which is another symptom of SVD.
SVD may occur in one in three elderly people and is the most common cause of vascular dementia, which is caused by reduced blood flow to the brain. This damages and eventually kills brain cells.
Lead author Jonathan Tay, from Cambridge's Department of Clinical Neurosciences, said: "There has been a lot of conflicting research on the association between late-life depression and dementia.
"Our study suggests that may partially be due to common clinical depression scales not distinguishing between depression and apathy. Continued monitoring of apathy may be used to assess changes in dementia risk and inform diagnosis.
"Individuals identified as having high apathy, or increasing apathy over time, could be sent for more detailed clinical examinations, or be recommended for treatment."
The study's participants, recruited from three hospitals in south London and Radboud University's Neurology Department in the Netherlands, were assessed for apathy, depression and dementia over several years.
In the UK group, nearly 20 per cent of participants developed dementia, while 11 per cent in the Netherlands cohort did, in likelihood due to the more severe burden of SVD in the UK cohort.
In both data sets, patients who later developed dementia showed higher apathy but similar levels of depression at baseline, compared with patients who did not.
It is the first study to examine the relationships between apathy, depression, and dementia in individuals with SVD, Cambridge University said.