Depression is an important, potentially modifiable dementia risk factor. However, it is not known whether effective treatment of depression through psychological therapies is associated with reduced dementia incidence. The aim of this study was to investigate associations between reduction in depressive symptoms following psychological therapy and the subsequent incidence of dementia.

National psychological therapy data were linked with hospital records of dementia diagnosis for 119808 people aged 65+. Participants received a course of psychological therapy treatment in Improving Access to Psychological Therapies (IAPT) services between 2012 and 2019. Cox proportional hazards models were run to test associations between improvement in depression following psychological therapy and incidence of dementia diagnosis up to eight years later.

Improvements in depression following treatment were associated with reduced rates of dementia diagnosis up to 8 years later (HR = 0.88, 95% CI 0.83–0.94), after adjustment for key covariates. Strongest effects were observed for vascular dementia (HR = 0.86, 95% CI 0.77–0.97) compared with Alzheimer's disease (HR = 0.91, 95% CI 0.83–1.00).

Reliable improvement in depression across psychological therapy was associated with reduced incidence of future dementia. Results are consistent with at least two possibilities. Firstly, psychological interventions to improve symptoms of depression may have the potential to contribute to dementia risk reduction efforts. Secondly, psychological therapies may be less effective in people with underlying dementia pathology or they may be more likely to drop out of therapy (reverse causality). Tackling the under-representation of older people in psychological therapies and optimizing therapy outcomes is an important goal for future research.

Post-traumatic stress disorder (PTSD) has been identified as a potential risk factor for developing dementia. There are currently, however, no meta-analyses quantifying this risk.

To systematically review and quantify the risk of future dementia associated with PTSD across populations. PROSPERO registration number CRD42019130392.

We searched nine electronic databases up to 25 October 2019 for longitudinal studies assessing PTSD and risk of dementia. We used random- and fixed-effects meta-analyses to pool estimates across studies.

PTSD was associated with a significant risk for all-cause dementia: pooled hazard ratio HR = 1.61 (95% CI 1.43–1.81, I2= 85.8%, P < 0.001; n = 1 693 678; 8 studies). Pooled HR was 1.61 (95% CI 1.46–1.78; I2= 80.9%, P < 0.001; n = 905 896; 5 studies) in veterans, and 2.11 (95% CI 1.03–4.33, I2= 91.2%, P < 0.001; n = 787 782; 3 studies) in the general population. The association between PTSD and dementia remained significant after excluding studies with high risk of bias (HR = 1.55, 95% CI 1.39–1.73, I2= 83.9%, P < 0.001; n = 1 684 928; 7 studies). Most studies included were retrospective and there was evidence of high heterogeneity.

This is the first meta-analysis quantifying the association of PTSD and risk of dementia showing that PTSD is a strong and potentially modifiable risk factor for all-cause dementia. Future studies investigating potential causal mechanisms, and the protective value of treating PTSD are needed.