Mental Illness Tied to Increased Dementia Risk

Batya Swift Yasgur, MA, LSW

March 02, 2022

Mental disorders in early life are associated with a significantly increased risk of dementia in later years.

Results of a large, longitudinal, population-based study show that individuals hospitalized for a mental health disorder had a fourfold increased relative risk (RR) for developing dementia compared to those who were not hospitalized with a mental illness.

In addition, those with dementia plus a mental disorder developed dementia almost 6 years earlier than those without a mental illness.

The findings were consistent among men and women, in patients with early- and late-onset dementia, in those with Alzheimer's and non-Alzheimer's dementia, and across all mental health disorders — and remained so after accounting for preexisting physical illness and socioeconomic factors.

"Dementia is not typically treated until later in life, but our study suggests that we need to be thinking about dementia prevention much earlier in the life course," study investigator Leah Richmond-Rakerd, PhD, assistant professor, Department of Psychology, University of Michigan, told Medscape Medical News.

"Supporting young people's mental health could be a window of opportunity to help reduce the burden of dementia in older adults," she said.

The findings were published online February 16.

Underappreciated Risk Factor

"Recognition of the outsized influence of dementia on later-life functioning has fueled research into modifiable risk factors and prevention targets," the investigators write.

Previous research suggests mental disorders may "comprise an underappreciated category of modifiable risk factors." However, those studies focused primarily on midlife and older individuals, not on capturing mental disorders during young adulthood, which is the time of "peak prevalence," they add. In addition, most studies have not explored the full range of mental disorders.

Richmond-Rakerd noted that it is well known that mental health disorders peak in adolescence and young adulthood — and are treatable.

"If the same people who have mental disorders when they are young tend to develop dementia when they are older, that would mean that preventing mental health problems in younger people might reduce or delay the burden of dementia in older people," she said.

The investigators assessed records from the New Zealand Integrated Data Infrastructure, which is a de-identified register that includes the entire New Zealand population. They also examined information about hospitalizations and diagnoses from records kept by the New Zealand Ministry of Health.

The researchers followed 1,711,386 individuals born between 1928 and 1967 (50.6% men, aged 21 to 60 years at baseline) for 30 years. The population was subdivided into age groups based on birth years: 1928–1937 (14.8%), 1938–1947 (20.85%), 1948–1957 (29.35%), and 1958–1967 (35.1%).

Earlier Onset

During the study period, 3.8% of individuals were identified as having a mental disorder, and 2% were identified as having dementia. Similar percentages of men and women had a mental disorder, and similar percentages had dementia.

Dementia was "overrepresented" among participants with vs without a mental disorder (6.1% vs 1.8%). This finding held across all age groups.

Those diagnosed with a mental disorder were also more likely to develop dementia compared with their peers without a mental disorder (RR, 3.51; 95% CI, 3.39 – 3.64), which is a larger association than that between physical diseases and dementia (RR, 1.19; 95% CI, 1.16 – 1.21).

These associations were present in both sexes and in all age groups, although the associations were stronger in more recently born cohorts.

A sixfold higher risk for dementia remained even after adjusting for preexisting physical illnesses (HR, 6.49; 95% CI, 6.25 – 6.73); and the elevated risk was evident across different lengths of follow-up from the index mental disorder.

When the researchers focused specifically on individuals diagnosed with dementia, they found that those diagnosed with a mental disorder developed dementia a mean of 5.60 years earlier than those without a mental disorder diagnosis — an association observed across both sexes and all age groups.

"Individuals diagnosed with psychotic, substance use, mood, neurotic, and all other mental disorders and who engaged in self-harm were all more likely than those without a mental disorder to be diagnosed with subsequent dementia, even after accounting for their physical disease histories," the investigators write.

Although there was a link between mental disorders in both Alzheimer's and non-Alzheimer's dementias, the association was larger in non-Alzheimer's.

The researchers note that the study has several limitations, including the fact that it was conducted in New Zealand and therefore the results may not be generalizable to other regions. In addition, inpatient hospital records do not capture less severe mental disorder cases treated in the outpatient setting.

Richmond-Rakerd suggested several potential mechanisms that could account for the link between mental illness and dementia, including poor lifestyle choices and metabolic side effects associated with some psychiatric medications.

"There could also be shared risk factors for both mental disorders and dementia, such as shared genetics, or individuals may experience a lifelong brain vulnerability that shows up as mental health problems earlier in life and shows up as dementia later in life," she said.

An Important Risk Factor

Commenting for Medscape Medical News, Ken Duckworth, MD, chief medical officer of the National Alliance on Mental Illness, said a major strength of the study was its longitudinal scope and large population size.

He described the study as allowing clinicians to "watch the movie," as opposed to looking at a "snapshot" of data.

"Although you can learn things from snapshots, a large, comprehensive public health system looking at 30 years of claims — something not possible in the US because of our more fragmented healthcare system — offers more insight," said Duckworth, who was not involved with the research.

The investigators are "painting a picture of a correlation of risk, and to me, that's the beginning of further inquiry," he added. "Would preventive efforts targeting dementia, such as exercise and socialization, be helpful? It's a great study that raises these interesting questions."

Also commenting for Medscape Medical News, Claire Sexton, DPhil, director of scientific programs and outreach at the Alzheimer's Association, said the study "adds a wealth of data to our understanding" of mental disorders as a dementia risk factor.

However, the study was observational, so "the findings cannot imply causation [and] because someone has depression, that does not mean they will go on to develop Alzheimer's," said Sexton, who also was not involved with the research.

Still, "these data support the idea that taking care of one's mental health is incredibly important for overall well-being. For providers, it's important to have mental health evaluation be a part of your patient's regular checkups," she added.

Richmond-Rakerd noted that even if mental health conditions are not a causal risk factor for dementia, "the presence of a mental health problem is still an important indicator of risk. Mental health providers may wish to target other risk factors for dementia that are more common in individuals with mental health conditions, such as social disconnection."

The study was funded by grants from the National Institute on Aging, the UK Medical Research Council, the National Institute of Child Health and Development through the Duke Population Research Center, and the National Institute on Aging through the Center for Advancing Sociodemographic and Economic Study of Alzheimer's Disease and Related Dementias. Richmond-Rakerd reports no relevant financial relationships. The other investigators' disclosures are listed in the original article. Sexton and Duckworth report no relevant financial relationships.

JAMA Psychiatry. Published online February 16, 2022. Full text

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