Newly diagnosed postoperative depression and incident dementia after major surgery: a Korean nationwide retrospective cohort study

Background Depression is a potentially modifiable risk factor for dementia in the general population. Whether newly diagnosed postoperative depression is associated with subsequent dementia, and whether this association differs across patient subgroups, has not been well characterised in large surgical populations. Methods This nationwide retrospective cohort study used Korean National Health Insurance Service data on adults undergoing major cardiac or gastrointestinal surgery (2002–2022), excluding those with depression or major neurocognitive or neurological disorders in the preceding 5 years. Postoperative depression was defined as a new depressive disorder diagnosis with at least one antidepressant prescription within 1 year after surgery; those alive and free of dementia at postoperative day 365 entered the landmark analysis. Incident dementia was ascertained during postoperative years 1–6. After 1:3 propensity score matching on age, sex, and income, associations were estimated using Cox proportional hazards regression with sequential adjustment. Results Among 424,637 patients, 7,633 (1.8%) developed postoperative depression. The matched cohort comprised 30,532 patients: 7,633 with postoperative depression and 22,899 without. During follow-up, 786 dementia cases occurred. Postoperative depression was associated with incident dementia (adjusted HR 1.59, 95% CI 1.38–1.85; P < 0.001). The association was stronger in patients aged < 65 years (HR 2.88, 95% CI 1.81–4.58) than ≥ 65 years (HR 1.56, 95% CI 1.34–1.82; P for interaction = 0.005), and persisted when ascertainment was restricted to years 3–6 (HR 1.57, 95% CI 1.29–1.92). Conclusions Newly diagnosed postoperative depression was associated with increased dementia risk, particularly in patients aged < 65 years, supporting prospective evaluation as a potential marker of long-term cognitive risk. Trial registration: Clinical Research Information Service, KCT0012255. Registered 8 July 2026 (retrospectively registered).