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Incidence and associations of hospital delirium diagnoses in 85979 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 - 2017 and used logistic regression to identify associations with delirium diagnosis in a population with severe mental illness.


RESULTS
The cohort included 249,047 people with severe mental illness with median follow-up time in CPRD of 6.4 years. 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 severe mental illness 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.