Older people with depression face about a two-fold risk of dementia onset compared to people without depression. Several studies have, however, indicated that depression might be an early symptom of dementia itself rather than an actual risk factor for the dementing process. Because the preclinical stage of dementia generally already begins from 10 to 15 years before the clinical stage, the proximity of depression to dementia onset should be considered when analysing the association between the two disorders. Studies with sufficiently long follow-ups are scarce, and many previous investigations rely on self- or surrogate-reported history of depression.
In their recent publication, Kaarina Korhonen, Lasse Tarkiainen (University of Helsinki); Taina Leinonen (Finnish Institute of Occupational Health); Elina Einiö and Pekka Martikainen (University of Helsinki) aimed to investigate, in more detail, the long-term association between depression and dementia. To this end, the authors followed up a cohort of Finnish older adults in population and health care registers from 2001 through 2018. The study used information on depression diagnoses observed in hospital registers in the period from 15 to 30 years before the follow-up for dementia to minimise bias arising from preclinical depressive symptoms of dementia. The study accounted for several risk factors that depression and dementia have in common, including education, marital status and certain medical conditions. By comparing siblings, unobserved characteristics including family background and genetic factors shared by siblings were also considered in the analysis. Additionally, the study analysed whether the association was similar between genders, educational levels and marital status groups.
The results support the hypothesis that depression is a risk factor for dementia, as the analysis showed an increased risk of dementia for people with a history of clinical depression in earlier life. The results indicated that this association does not arise from the higher risk of both depression and dementia among lower educated or unmarried individuals, or among people with medical conditions. Furthermore, the sibling comparison analysis indicated that the association is not attributable to unobserved characteristics shared by siblings. The authors found that the excess risk of dementia associated with depression was not specific to any of the assessed subpopulations, but the excess risk was greater among men than among women. This was mainly explained by comorbid medical conditions among men – especially alcohol-related conditions. Among the widowed, the association between a history of depression and dementia was weaker, which might at least partly stem from selective survival.
The findings demonstrate that clinical depression earlier in life is consistently associated with an increased risk of later life dementia, yet the authors emphasise that an observational study cannot establish any causal relationship between the two disorders. However, the findings of this study together with those from many previous investigations indicate that promoting mental health and providing better monitoring and treatment of depression when people are of working age should be considered as part of dementia preventive interventions.