In a Population with Severe Mental Illnesses, do Substance Use Disorder and Multimorbidity Influence Hospitalization Length of Stay?

Background: One in two people with a severe mental illness also has a substance use disorder. This frequent combination highlights the complexity of care required for these individuals, as conditions often interact and exacerbate each other. The accumulation of chronic conditions (either psychiatric or somatic) is defined as multimorbidity (two or more conditions). Multimorbidity is known to increase hospitalization length of stay but there is poor information about the impact of substance use disorder in the population of severe mental illness. Objectives: The goal of the present study was to determine the prevalence of multimorbidity in people with severe mental illness and substance use disorder, and also to determine whether it has an influence on hospitalization length of stay. Results: The prevalence of multimorbidity among individuals with severe mental illness and substance use disorder was 82%, and the presence of substance use disorder was associated with a shorter hospitalization length of stay. Sensitivity analyses showed that individuals with severe mental illness and a diagnosis of substance use disorder stayed 8.58 days less in hospitalization compared to those with only a severe mental illness. Conclusions: In specialized psychiatric services, the prevalence of multimorbidity is elevated in individuals with severe mental illness, and that it was comparable among those with a concomitant substance use disorder, yet this does not appear to impact the hospitalization length of stay. The additional presence of substance use disorder did seem to reduce the length of stay and that shows that substance use disorder has an impact on hospitalization length of stay, as well as increasing the presence of chronic physical and mental conditions in the same way as other severe mental illness.