Incidence and associations of hospital delirium diagnoses in 85,979 people with severe mental illness: A data linkage study

Introduction Delirium is an acute neuro‐psychiatric disturbance precipitated by a range of physical stressors, with high morbidity and mortality. Little is known about its relationship with severe mental illness (SMI). Methods We conducted a retrospective cohort study using linked data analyses of the UK Clinical Practice Research Datalink (CPRD) and Hospital Episodes Statistics (HES) databases. We ascertained yearly hospital delirium incidence from 2000 to 2017 and used logistic regression to identify associations with delirium diagnosis in a population with SMI. Results The cohort included 249,047 people with SMI with median follow‐up time in CPRD of 6.4 years. A total of 85,979 patients were eligible for linkage to HES. Delirium incidence increased from 0.04 (95% CI 0.02–0.07) delirium associated admissions per 100 person‐years in 2000 to 1.05 (95% CI 0.93–1.17) per 100 person‐years in 2017, increasing most notably from 2010 onwards. Delirium was associated with older age at study entry (OR 1.05 per year, 95% CI 1.05–1.06), SMI diagnosis of bipolar affective disorder (OR 1.66, 95% CI 1.44–1.93) or other psychosis (OR 1.56, 95% CI 1.35–1.80) relative to schizophrenia, and more physical comorbidities (OR 1.08 per additional comorbidity of the Charlson Comorbidity Index, 95% CI 1.02–1.14). Patients with delirium received more antipsychotic medication during follow‐up (1–2 antipsychotics OR 1.65, 95% CI 1.44–1.90; >2 antipsychotics OR 2.49, 95% CI 2.12–2.92). Conclusions The incidence of recorded delirium diagnoses in people with SMI has increased in recent years. Older people prescribed more antipsychotics and with more comorbidities have a higher incidence. Linked electronic health records are feasible for exploring hospital diagnoses such as delirium in SMI.