Mortality gap continuum in severe mental illness: a nationwide study across healthcare and socioeconomic strata

Background: A comprehensive overview of how premature mortality in severe mental illness varies by socioeconomic position and healthcare utilization is lacking. Methods: We followed all Norwegian residents aged 30-79 years on January 1, 2008, through December 31, 2023. Nationwide registry linkage provided data on mental illness in primary and specialist care, educational attainment, and cause of death. We estimated the mortality gap as excess life-years lost from age 30 for depression, bipolar disorder, and schizophrenia compared with the general population, stratified by sex, healthcare utilization, and educational attainment. All-cause, external-cause, and natural cause mortality were assessed using competing risks. Results: The cohort comprised 2,486,836 individuals, including 483,933 with depression, 40,125 with bipolar disorder, and 21,963 with schizophrenia. Among men, the mortality gap ranged from 1.50 year (95% CI 0.36-3.34) among those with depression, non-inpatient care utilization, and university education to 19.86 years (18.34-21.36) among those with schizophrenia, inpatient care utilization, and less than high school education. Among women, the corresponding range was no gap to 18.16 years (17.03-19.53). Mortality gaps were generally larger with lower educational attainment and inpatient care utilization and increased from depression to bipolar disorder to schizophrenia. This pattern was consistent across sexes, diagnoses, healthcare utilization levels, and educational attainment levels and was most pronounced for natural-cause mortality. Conclusions: Premature mortality among people with severe mental illness forms a continuum across socioeconomic disadvantage, healthcare utilization, and diagnosis. Combining these measures can identify subgroups with the greatest mortality burden and may help target preventive interventions.