Out-of-hospital onset versus in-hospital onset for clinical outcomes in spontaneous intramuscular hematoma diagnosed by computed tomography: a retrospective cohort study.

The aging global population and the increasing use of antithrombotic agents have made spontaneous intramuscular hematomas (SIH) a growing concern. The association between the settings of SIH onset and clinical outcomes remains unclear. The aim of this study was to determine these associations. A retrospective cohort study was conducted in a tertiary hospital in Hiroshima, Japan between January 2008 and January 2022. We included consecutive patients aged ≥ 15 years with SIH at any site diagnosed by computed tomography. The subjects were divided into two groups according to onset settings: out-of-hospital onset and in-hospital onset. The main outcome was treatment failure (composite of change in initial treatment and in-hospital death), and in-hospital mortality was also assessed. We used inverse probability of treatment weighting (IPTW) to estimate the causal effects of onset settings on outcomes. Of 84 included subjects with SIH, 63 had out-of-hospital onset and 21 had in-hospital onset. One subject (1.6%) with out-of-hospital onset and four subjects (19%) with in-hospital onset experienced treatment failure. In the IPTW cohort, in-hospital onset was associated with treatment failure [odds ratio (OR) 29, 95% confidence interval (CI) 7.2-270]. In addition, one subject (1.6%) with out-of-hospital onset and three subjects (14%) with in-hospital onset died during hospitalization. In-hospital onset was associated with a high rate of in-hospital mortality (OR 25, 95% CI 6.3-240) in the IPTW cohort. SIH with in-hospital onset had a poorer prognosis than that of SIH with out-of-hospital onset, suggesting that onset setting might be a novel predictor of clinical outcomes for SIH.