Background: Subsyndromal delirium (SSD) is a frequent condition and has been commonly described as an intermediate stage between delirium and normal cognition. However, the true frequency of SSD and its impact on clinically relevant outcomes in the intensive care unit (ICU) remains unclear. Aim of the Study: To evaluate the significance of SSD on adverse clinical outcomes especially mortality and length of hospital stay. Methods: A systematic search was performed in the scientific database particularly MEDLINE (2000–2017), EMBASE (2000– 2017), Cochrane Central Register of Controlled Trials, CINAHL (2000–2017), Google Scholar, and individual journals to identify publications that evaluated SSD in ICU patients. Results: The search yielded five studies involving 2453 patients. SSD was detected in 849 patients (34.6%). Three studies evaluated only surgical patients. Three studies used the Intensive Care Delirium Screening Checklist (ICDSC) and two used the Confusion Assessment Method (CAM) score to diagnose SSD. The meta-analysis showed an increased hospital length of stay (LOS) in SSD patients (0.29 (95% CI 0.11–0.48), p = 0.002; I2 = 33%). Hospital mortality was described in two studies but it was not significant (hazard ratio 0.93 (0.58–1.43), p = 0.88 and (4 (1.0-6.9) vs 9 (3.6-20.4), p = 0.05). The use of antipsychotics in SSD patients to prevent delirium was evaluated in one study but it did not modify ICU LOS (6.2 (4–8) vs 7 (4–9) days, p = 0.63 and 2 (2–3) vs 3 (2–3) days, p = 0.517) or mortality (9 (25.8%) vs 7 (20.4%), p = 0.51). Conclusion: Subsyndromal Delirium is a common and adverse condition that is manifested in almost one-third of ICU patients. According to our findings, SSD has increased the length of hospital stay only with low impact on the other outcomes. Nevertheless, studies on a bigger sample size and larger scale are needed for a better understanding of the relevance of SSD in ICU patients as well as its treatment.
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