Incidence of systemic physical multimorbidity among people with schizophrenia-spectrum or bipolar-spectrum disorders compared with age-matched and sex-matched individuals in Queensland, Australia: a statewide, hospital cohort study

BACKGROUND: People with schizophrenia-spectrum or bipolar disorder frequently have physical comorbidities, but the epidemiology of developing multiple physical conditions is unclear. We hypothesised that this group experiences an earlier accumulation of physical multimorbidity affecting multiple systems. Our aim was to estimate the relative and absolute incidence of physical multimorbidity in people with and without severe mental illness. METHODS: We did a cohort study using statewide hospital registries in Queensland, Australia, between Jan 1, 2000, and Dec 31, 2023. Individuals with a recorded diagnosis of schizophrenia-spectrum or bipolar disorder were age-matched and sex-matched (1:4) to individuals without these disorders. The study outcome was systemic physical multimorbidity, defined as the cumulative number of systems affected by chronic physical disease. We measured the relative incidence of developing systemic physical multimorbidity in at least one to at least five systems using Fine-Gray adjusted subdistribution hazard ratios (HRs); absolute differences were measured using differences in the cumulative incidence function at 20 years. Additional age-stratified and sex-stratified analyses were undertaken. This study featured no lived experience involvement. FINDINGS: A total of 178 665 individuals were included in the analysed sample: 30 189 people with schizophrenia-spectrum or bipolar disorder (11 940 females and 18 249 males; median age 32 years [IQR 24-42]) were compared with 148 476 hospital comparators in the control group (61 365 females and 87 111 males; median age 33 years [IQR 24-44]). Ethnicity data were not available. Relative incidence was higher in the disorder group with adjusted subdistribution HRs of 1·85 (95% CI 1·81-1·88) for at least one affected system, 2·25 (2·19-2·32) for at least two affected systems, 2·65 (2·56-2·74) for at least three affected systems, 2·93 (2·81-3·05) for at least four affected systems, and 3·16 (3·01-3·30) for at least five affected systems. At 20 years, absolute cumulative incidence differences were 9·79 additional cases per 100 persons (95% CI 9·06-10·53) for at least one affected system, 18·43 additional cases (17·58-19·28) for at least two affected systems, 18·57 additional cases (17·73-19·42) for at least three affected systems, 15·71 additional cases (14·92-16·51) for at least four affected systems, and 11·66 additional cases (10·95-12·37) for at least five affected systems. Elevated adjusted subdistribution HRs were consistently observed for younger strata, with the highest observed in females younger than 25 years with at least five affected systems (adjusted subdistribution HR 9·24 [95% CI 7·30-11·71]). INTERPRETATION: People with schizophrenia-spectrum or bipolar disorder experience increased relative and absolute incidence of systemic physical multimorbidity spanning multiple systems, compared with individuals without these disorders. The largest differences were observed among younger cohorts, emphasising the need for early prevention. FUNDING: None.