Comparative incidence rates of physical multimorbidity in a psychiatric inpatient population among people with and without severe mental illness: A retrospective cohort study

Objective: While people experiencing severe mental illness have a greater prevalence of physical multimorbidity, data on incidence are largely limited to the onset of specific physical conditions. We assessed whether people with severe mental illness have increased incidence rates of physical multimorbidity compared to people with other psychiatric conditions. Methods: This retrospective observational cohort study reported on a longitudinal psychiatric inpatient sample (2010–2024) in a metropolitan service in Brisbane, Australia. Within a subgroup of individuals with no pre-existing physical conditions, we compared individuals with and without a history of severe mental illness (schizophrenia-spectrum or bipolar disorder). With a denominator of person-years, we calculated the incidence of different thresholds of physical multimorbidity using adjusted Fine-Gray subdistribution hazard ratios. Results: Among the 3310 individuals with severe mental illness, 298 developed physical multimorbidity (two chronic physical conditions) across 21893 person-years, compared to 52 among the 2850 individuals and 18,112 person-years in the comparison group. When adjusted for clinical and demographic covariates, people with severe mental illness had an increased risk of developing one (subdistribution hazard ratio = 3.36; 95% confidence interval = 2.79, 4.03), two (subdistribution hazard ratio = 4.06; 95% confidence interval = 3.02, 5.46), three (subdistribution hazard ratio = 5.36; 95% confidence interval = 3.35, 8.59), and four (subdistribution hazard ratio = 4.84; 95% confidence interval = 2.49, 9.40) chronic physical conditions. Except for malignancy and genitourinary disease, people with severe mental illness had increased incidence of chronic physical conditions in all other organ systems. Conclusions: People with severe mental illness experienced greater incidence rates of multimorbidity at various thresholds, with a majority of organ systems affected. This highlights the need for holistic prevention and intervention strategies to curb the accumulation of physical multimorbidity.