While “dual diagnosis” involving both psychiatric and substance use disorders has long been a focus of schizophrenia research, recent studies have advocated for a shift of focus to multimorbidity, addressing comorbidity from both additional psychiatric disorders and substance use disorders. We hypothesized that more extensive mental health multimorbity would be associated with poorer quality of life (QOL) and functioning, and that additional psychiatric comorbidity in schizophrenia would have similar adverse effects on QOL as substance use comorbidity.
Participants with schizophrenia in the NIMH-funded Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE) were classified using baseline diagnostic data into four groups: 1) monomorbid schizophrenia: 2) additional psychiatric comorbidity, 3) additional substance use comorbidity, and 4) both additional psychiatric and substance use comorbidity. Mixed models compared groups on self-reported QOL (SF-12 and Lehman QOLI) and rater-evaluated QOL (the Quality of Life Scale) using baseline, 6, 12 and 18-month follow-up data.
As hypothesized, patients with schizophrenia alone had a better QOL than those with any multimorbidity; patients with both psychiatric and substance use comorbidities had a worse QOL than those with fewer comorbidities; and patients with comorbid substance use alone were not significantly worse off than those with comorbid psychiatric disorder.
The multimorbidity framework more richly differentiates complex clinical presentations of schizophrenia than the current dual diagnosis concept and deserves further study as to its etiology, consequences, and treatment.