Delirium, long-term conditions, and incident dementia in older adults admitted to hospital for emergency care in Lothian, Scotland: a population-based cohort study

BACKGROUND: Delirium is strongly associated with subsequent dementia, but this is often assumed to reflect underlying associations of baseline health with dementia. We examined the associations of delirium on admission with incident dementia across the spectrum of long-term conditions (LTCs). METHODS: We conducted a retrospective population-based cohort study using linked primary care and hospital data for emergency admissions of patients aged 65 years or older in Lothian, Scotland, from April 1, 2017, to April 1, 2020. Delirium on admission was assessed at the bedside for all patients using the 4AT, categorised as delirium (4AT ≥4) or no delirium (4AT 0-3). The primary outcome was incident dementia, with all-cause mortality as a secondary outcome. Associations of delirium, number of LTCs (categorised as 0-1 LTCs; 2-4 LTCs; 5-6 LTCs; and ≥7 LTCs), and their interaction with incident dementia and mortality were examined using unadjusted and adjusted Fine-Gray subdistribution hazards regression and Cox proportional hazards models. FINDINGS: Of 23 558 people without pre-existing dementia, 4135 (17·6%) had an admission with delirium. Mean age was 78·9 years (SD 8·1) and 12 826 (54·4%) patients were female. Delirium was associated with higher incident dementia risk. The relative risk was highest in those without multiple LTCs (MLTCs; adjusted subdistribution hazard ratio aHR 3·38 95% CI 2·46-4·63) and decreased with an increasing number of LTCs. Delirium was also associated with increased mortality, regardless of the number of LTCs. In those without MLTCs, delirium was associated with higher early mortality (≤90 days: aHR 4·23 95% CI 3·27-5·49) and late mortality (>90 days: 1·64 1·33-2·03). INTERPRETATION: Delirium is strongly associated with incident dementia in older adults across the LTC spectrum, with the highest relative risk in people without MLTCs. Findings support routine delirium assessment on hospital admission for all older adults and highlight the need to further investigate neurodegenerative mechanisms in delirium. FUNDING: Wellcome Trust.