Psychiatric disorders do not increase the risk of Alzheimer's disease, according to a recent study from the University of Eastern Finland. However, the prevalence of psychiatric diagnoses increased before the Alzheimer's diagnosis, which might be due to prodromal symptoms of Alzheimer's disease. The results were published in European Psychiatry.
History of mood disorder, such as depression, or any psychiatric disorder were associated with a higher risk of Alzheimer's disease when psychiatric disorders that occurred at least five years before the Alzheimer's diagnosis were taken into account. However, the associations disappeared when this time window was extended to 10 years. The exponential increase in the prevalence of psychiatric disorders before the diagnosis implies that some of these psychiatric disorders might actually have been prodromal symptoms of Alzheimer's disease. This underlines the importance of proper differential diagnostics of Alzheimer's disease. Further, the findings also highlight the importance of using an appropriate time window when assessing the risk factors of neurodegenerative diseases with a long onset period. Otherwise the identified "risk factors" may actually be manifestations of the neurodegenerative disease.
It should also be acknowledged that although psychiatric disorders diagnosed 10-40 years before Alzheimer's disease were not related to a higher risk, the life expectancy of persons with psychiatric disorders was, and is still decreased. Thus, those persons with psychiatric disorders who lived long enough to develop Alzheimer's disease were a selected sample of all persons with psychiatric disorders.
The study was conducted in the MEDALZ-2005 cohort which included all Finnish community dwellers with clinically verified Alzheimer's disease at the end of 2005 and their age, sex and region of residence matched controls (N of case-control pairs 27,948). History of psychiatric disorders since 1972 was extracted from the Finnish Hospital Discharge Register. Chronic disorders and substance abuse were taken into account.
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V. Tapiainen et al. Hospital-treated mental and behavioral disorders and risk of Alzheimer's disease: A nationwide nested case-control study, European Psychiatry (2017). DOI: 10.1016/j.eurpsy.2017.02.486