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 with 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 pre-existing 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, said in an interview.
“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 Feb. 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.
Dr. 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 “over-represented” among participants with versus 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% confidence interval, 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 pre-existing 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.
Dr. 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.