Takeaway

• Mental disorders are associated with the development of dementia; thus, treating mental disorders early in life may prevent the onset of neurodegenerative conditions.

Why this matters

• Mental disorders may be a modifiable risk factor for the development of dementia; however, current dementia risk indexes do not account for most mental disorders.
• If mental disorders are found to be associated with dementia, it may provide an early warning sign for the development of neurodegenerative disease.

Study design

• Objective: To determine whether mental disorders antedate dementia based on 3 decades of data.
• This population-based, administrative register study included all individuals born in New Zealand between 1928 and 1967 who resided in the country for any time between the observation period of July 1988 to June 2018 (n=1,711,386; 50.6% male; aged 21−60 years at baseline).
• The main outcomes were the presence or absence of mental disorders (such as psychotic, substance use, mood, neurotic, and self-harm disorder) and dementia, such as Alzheimer’s disease.

Key results

• Compared to individuals without a mental disorder, those with a mental disorder had an increased risk of developing subsequent dementia (relative risk [RR] 4.24, 95% CI 4.07−4.42; hazard ratio [HR] 6.49, 95% CI 6.25−6.73).
• Individuals with a mental disorder developed dementia earlier than those without a mental disorder (mean 5.60 years, 95% CI 5.31−5.90).
• Associations were not affected by sex, age, pre-existing chronic physical conditions, and socioeconomic deprivation.
• Associations were consistent between different types of mental disorders and self-harm behavior.
• Associations were also evident for Alzheimer’s disease (RR 2.76, 95% CI 2.45−3.11) and all other dementias (RR 5.85, 95% CI 5.58−6.13).
• The authors concluded that mental disorders are associated with subsequent dementia in the population.

Limitations

• Results cannot be generalized to other countries.
• Data did not include individuals with less severe mental disorders treated outside of a hospital.
• Data did not include individuals with untreated mental disorders.
• Study only included public hospital records.
• Missing data about later-diagnosed dementias and earlier-diagnosed mental disorders in the youngest and oldest individuals, respectively.
• Historical differences in diagnostic practices may impact results.