A large-scale study of Danish health records found a strong association between depression in adult life and a later diagnosis of dementia
“There are a number of very high-quality studies on this topic already, most of which demonstrate an association between depression in late life and dementia. What we saw is that this association persisted regardless of whether depression was diagnosed in early, middle, or late adulthood.” Dr Holly Elser, epidemiologist, University of Pennsylvania in Philadelphia
Adults diagnosed with depression are more than twice as likely to develop dementia as those without depression, a new study has found.
The study, carried out jointly by the University of Pennsylvania in Philadelphia and Aarhus University, and published in Jama Neurology, used the Danish National Health Register to look at the health records of 1.4 million people, covering a period from the 1970s to the present.
The researchers identified about 246,500 individuals with a new diagnosis of depression between 1980 and 2018. Of these, about 14,000 were later identified as having dementia. They also examined data on 1.2 million individuals without depression, randomly sampled from the Danish general population. About 39,000 from this group were eventually diagnosed with dementia. In both groups, the average age was about 50, and almost two-thirds were women.
A comparison of the two data sets shows that the people who had previously been diagnosed with depression were 2.41 times more likely to develop dementia later in life.
When looking at age groups of 18 to 44 (early life), 45 to 59 (middle life), and or 60 and above (later life), the authors of the study found that the risk of getting dementia remained almost the same regardless of when a person was diagnosed with depression.
They found that about two-thirds of those with depression were diagnosed before the age of 60.
Lead author Dr Holly Elser, a neurology resident and epidemiologist at the University of Pennsylvania in Philadelphia, said: “There are a number of very high-quality studies on this topic already, most of which demonstrate an association between depression in late life and dementia. What we saw is that this association persisted regardless of whether depression was diagnosed in early, middle, or late adulthood.”
The study found that men with depression were about three times more likely to be diagnosed with dementia, while women with depression were 2.2 times more likely to be diagnosed.
“We know that depression is much more frequently diagnosed in women than it is in men,” Elser said, “but it’s the association with dementia that is stronger in men.”
Although the study did not explore the reasons for difference, Elser speculated that men may be presenting with more severe depression, which in turn confers greater risk for subsequent dementia.
“I think this sex difference warrants further exploration to try and tease out why exactly this is happening,” she said.
The association between depression and dementia was also greater in individuals who had multiple inpatient hospitalisations for depression. The risk increases with each subsequent hospitalisation, the research found.
Elser stressed, however, that while the study shows an association between depression and dementia, the research does not prove a causative link.
The researchers also found that being prescribed an antidepressant within six months of a depression diagnosis had no effect on the rate of dementia diagnosis later in life.
Elser said, however, that there is evidence to suggest that treatment with an antidepressant decreases dementia risk. “It would be interesting to consider whether behavioural therapy combined with antidepressants further mitigates the risk of dementia,” she says. “For individuals who have depressive symptoms now, I think the priority is managing the depressive symptoms rather than worrying about what might be coming in the future.”
Dr Heather Snyder, vice president in charge of medical and scientific relations at the US-based Alzheimer’s Association, speculated that distress from depression takes a toll on heart health, which in turn could have a negative impact on the brain: “We see there is an association with depression and increased vascular disease, which has also been thought to increase the brain’s vulnerability to the diseases that are causing dementia. Depression is also known to increase the production of glucocorticoids, which are a steroid in the brain that is linked to the immune system and perhaps increases inflammation in the brain.”
The findings of this large-scale study demonstrate a strong and inarguable link between a diagnosis of depression and later dementia. Whether there is a causative link between depression and dementia, or whether the association is caused by some other factor, is impossible to say at the moment. The reasons need to be explored further. But at a time when diagnoses of depression are rising, the findings are further evidence of the importance of parity of esteem between physical and mental health.