Although age and pre-existent dementia are robust risk factors for developing delirium, evidence for patients older than 90 years is lacking. Therefore, this study assesses the delirium prevalence rates and sequelae in this age group.

Based on a Diagnostic and Statistical Manual (DSM)-5, Delirium Observation screening scale (DOS), and Intensive Care Delirium Screening Checklist (ICDSC) construct, in this prospective cohort study, the prevalence rates and sequelae of delirium were determined in 428 patients older than 90 years by simple logistic regressions and corresponding odds ratios (ORs).

The overall prevalence delirium rate was 45.2%, with a wide range depending upon specialty: intermediate and intensive care services (83.1%), plastic surgery and palliative care (75%), neurology (72%), internal medicine (69%) vs. dermatology (26.5%), and angiology (14.5%). Delirium occurred irrespective of age and gender; however, pre-existent dementia was the strongest delirium predictor (OR 36.05). Delirious patients were less commonly admitted from home (OR 0.47) than from assisted living (OR 2.24), indicating functional impairment. These patients were more severely ill, as indicated by emergency (OR 3.25) vs. elective admission (OR 0.3), requirement for intensive care management (OR 2.12) and ventilation (OR 5.56–8.33). At discharge, one-third did not return home (OR 0.22) and almost half were transferred to assisted living (OR 2.63), or deceased (OR 47.76).

At age older than 90 years, the prevalence and sequelae of delirium are substantial. In particular, functional impairment and pre-existent dementia predicted delirium and subsequently, the loss of independence and death were imminent.

The prevalence rates and adversities of delirium have not yet been systematically evaluated and are based on selected populations, limited sample sizes, and pooled studies. Therefore, this study assesses the prevalence rates and outcome of and odds ratios for managing services for delirium.

In this prospective cohort study, based on the Diagnostic and Statistical Manual (DSM) 5, the Delirium Observation Screening (DOS) scale, and the Intensive Care Delirium Screening Checklist (ICDSC) construct, 28,118 patients from 35 managing services were included, and the prevalence rates and adverse outcomes were determined by simple logistic regressions and their corresponding odds ratios (ORs).

Delirious patients were older, admitted from institutions (OR 3.44–5.2), admitted as emergencies (OR 1.87), hospitalized twice longer, and discharged, transferred to institutions (OR 5.47–6.6) rather than home (OR 0.1), or deceased (OR 43.88). The rate of undiagnosed delirium was 84.2%. The highest prevalence rates were recorded in the intensive care units (47.1–84.2%, pooled 67.9%); in the majority of medical services, rates ranged from 20% to 40% (pooled 26.2%), except, at both ends, palliative care (55.9%), endocrinology (8%), and rheumatology (4.4%). Conversely, in surgery and its related services, prevalence rates were lower (pooled 13.1%), except for cardio- and neurosurgical services (53.3% and 46.4%); the lowest prevalence rate was recorded in obstetrics (2%).

Delirium remains underdiagnosed, and novel screening approaches are required. Furthermore, this study identified the impact of delirium on patients, determined the prevalence rates for 32 services, and elucidated the association between individual services and delirium.