Purpose — Survivors of critical illness may be at higher risk of developing subsequent mental illness. We sought to determine the risk of new mental illness diagnoses across a large population of intensive care unit (ICU) survivors compared with hospitalized patients.
Methods — Population-based study (2005–2015) conducted in adults hospitalized in Ontario, Canada. The primary exposure was ICU admission for ≥ 48 h; secondary exposures were ICU procedures including mechanical ventilation and duration of ICU. The primary outcome was mental illness diagnosed during the year after hospital discharge. To account for case mix differences between ICU and other hospitalized patients, sensitivity analyses were conducted restricting to six pre-specified diagnoses that can lead to hospitalization with or without ICU.
Results — 1,847,462 patients survived hospitalization, of whom 121,101 were admitted to ICU for ≥ 48 h. ICU patients had a higher rate of new mental illness diagnoses in the year after discharge compared to hospitalized patients (17 vs. 15%, adjusted hazard ratio (aHR) 1.08, 95% CI 1.07–1.10). In analyses restricted to pre-specified most responsible diagnoses, the increased risk associated with ICU was only significant for patients with pneumonia. Among ICU survivors, exposure to mechanical ventilation (aHR: 1.08; 95% CI 1.05–1.12) or longer ICU stays (aHR: 1.004 per day; 95% CI 1.003–1.005) increased the risk of new mental illness diagnosis.
Conclusions — ICU was associated with a marginally increased risk of mental illness diagnosis after hospitalization that was often no longer apparent when reason for admission was considered. Patients exposed to mechanical ventilation or longer ICU stays may be at higher risk of subsequent mental illnesses.