“This arbitrary distinction between a psychiatric condition and a neurological condition feels a little bit outdated,” said Dr Ruth Morin, a clinical neuropsychologist at Hoag Hospital in Orange County, California, who treats Niederhaus. “It’s all happening in the brain.”
Scientists are working to understand exactly how the two types of conditions are connected, but evidence suggests that having a mental illness, especially in old age, is associated with a greater risk of developing dementia.
How are late-life mental disorders and dementia related?
The association is best established between depression and dementia: a 2020 commission named late-life depression (after age 65) as one of 12 major risk factors for dementia. Building on this, new research suggests that the connection exists for virtually any mental health disorder that is diagnosed later in life. The study examined the medical records of nearly 800,000 people in Sweden and found that, compared to people without dementia, people with the condition were about 70 per cent more likely to develop a new mental illness three years before the onset of their dementia. In the year before the dementia diagnosis, that risk rose to roughly 300 per cent.
Experts have two main theories about what’s behind the connection. One is that the onset of a new psychiatric condition, like substance abuse or anxiety, in a person’s 70s or 80s could actually be an early sign of dementia. In these cases, “the neurodegeneration in the brain is also causing psychiatric symptoms”, said Dr Sara Garcia-Ptacek, a neurologist and assistant professor at the Karolinska Institutet in Sweden, who led the study.
The other theory is that the two diagnoses are independent — a diagnosis of depression later in life, say, may be just that — but the toll the mental illness takes ends up triggering or accelerating a person’s dementia. “Depression is going to reduce our cognitive capacity, and that means that maybe a biological process that has been going on for a number of years suddenly comes to light,” Garcia-Ptacek said.
Not everyone who develops a psychiatric disorder later in life will also develop dementia, but given how often the two types of conditions occur together, Garcia-Ptacek advised that doctors order cognitive testing for older adults with new psychiatric symptoms, as a precaution.
Is mental illness earlier in life also associated with dementia?
The connection between psychiatric health and neurological health is less clear when a person is diagnosed with mental illness earlier on. There is some evidence that a mental health episode, particularly depression, in a person’s 30s, 40s or 50s is linked to an increased risk of developing dementia down the road, but not every study shows that.
For example, one study published in 2017 tracked more than 10,000 adults between the ages of 35 and 55 for 30 years and found that high scores on a test assessing symptoms of depression — not a formal diagnosis — were not associated with an increased risk for dementia if they occurred earlier in life. However, there was an increased risk if the depression symptoms occurred in late adulthood, roughly a decade before a dementia diagnosis.
Backing this up, a 2022 meta-analysis found that depressive episodes after age 60 nearly doubled a person’s risk for dementia, but if they occurred before then, the risk increased by 17 per cent.
One major caveat is that severe mental illness (for example, the kind that requires hospitalisation) in early or mid-adulthood does seem to be associated with a substantially greater risk of dementia. A study published in 2022 that looked at 30 years of health records for 1.7 million people in New Zealand found that those who had been hospitalised for a mental health disorder at any point during adulthood were three to four times as likely to be diagnosed with dementia later.
One way to explain the possible association between mental disorders earlier in life and neurological disorders later is that there could be a direct link in the brain between the two types of conditions. For example, both Alzheimer’s disease and major depressive disorder are associated with decreased volume in an area of the brain called the hippocampus, which is involved in both memory and mood.
The connection could also be attributed to behaviours associated with mental illness that increase the risk for a neurodegenerative disease, said Terrie Moffitt, a psychology professor at Duke University who led the New Zealand study. For example, people with psychiatric disorders tend to be more isolated, not sleep as well, be less physically healthy and have higher rates of chronic conditions like heart disease and diabetes — all things that increase the risk for dementia.
“It’s really tough to say what is driving the association,” Morin said. “But I think the takeaway is you should take this seriously, because we do know that there is an association.”
However, the experts also stressed that having a psychiatric condition is by no means a guarantee that you’re going to develop dementia, and there are things you can do to reduce your risk.
The first priority should be to get treatment for any psychiatric disorder, Morin said. That will help limit the severity of the condition, which can mitigate the risk of developing dementia.
Taking care of your physical health can lower your risk of dementia as well. “Physical activity, eating a healthy diet, staying socially connected, having mental stimulation — all of these are great,” Garcia-Ptacek said.
These are “factors that are within people’s control”, Morin said, and they “can have really a drastic effect” on a person’s future.
Written by: Dana G. Smith
Photographs by: Hiroko Masuike
©2023 THE NEW YORK TIMES
