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Having depression is known to increase a person's risk of developing dementia, but depression is also an early indicator of dementia – particularly in older people – according to new research led by Queen Mary University of London, published in the Journal of Neurology, Neurosurgery, and Psychiatry.
The researchers found that while people with depression were more likely to develop dementia than those without, the chance of someone experiencing depression increased steadily in the ten years leading up to a dementia diagnosis. If depression were causing dementia, it would be expected that those who have experienced depression for longer would be at greater risk. However, the team at Queen Mary found that the reverse was true, i.e. the link between depression and dementia became stronger just before the time of dementia diagnosis. This suggests that the onset of depression later in life should be viewed as a possible early indicator of dementia to help improve early diagnosis of the disease.
It is well known that depression and dementia are deeply connected, but this study reveals new insights into the complex links between them. Researchers analysed the health records of nearly 500,000 participants from the UK Biobank, comparing those with and without diagnoses of both depression and dementia. They performed a series of analyses to establish whether depression was likely to be causing dementia or whether early dementia was likely to be causing depression, including using MRI brain scans of over 41,000 Biobank participants to examine whether the pattern of changes in the brain associated with depression was different in older adults compared to younger adults with depression.
The researchers found that the link between dementia and depression was highly specific. When they adjusted for other variables that could affect the link between depression and dementia – such as poverty, gender, education, smoking, and social isolation – they still found that depression roughly doubled the risk of developing dementia. The data also showed that more severe depression was associated with greater dementia risk.
In people aged 65 and older, depression was associated with changes in brain regions linked to Alzheimer's disease, particularly the hippocampus. These changes in brain structure suggest that depression in older adults might reflect early Alzheimer's disease changes in the brain. However, younger people with depression did not show the same patterns of brain change, suggesting that depression may affect brain health differently at different ages.
Professor Charles Marshall, who led the study, said: "Depression is often viewed as a 'modifiable' risk factor for dementia, but this may need to be reevaluated in light of this study. We hope that the findings will lead to a more nuanced approach when considering whether treating depression could be a way of preventing dementia. Our results suggest that while treating depression earlier in life might have a small benefit on brain health, there may be more to be gained from using depression in later life as a way of identifying people who are in the early stages of dementia. An awareness of this could help to improve access to timely diagnosis and treatment of dementia."
