Presentation (CTI)

OBJECTIVES: Idiopathic inflammatory myopathies (IIM), or myositis, are rare, heterogeneous autoimmune diseases characterised by chronic skeletal muscle inflammation, multisystem involvement, diagnostic delay, and substantial morbidity and mortality. Economic evidence in IIM remains limited, and existing model structures do not adequately capture subtype heterogeneity, multi-organ disease burden, treatment pathways, or long-term consequences. This study aimed to develop a conceptual economic model framework for IIM to support future research.
METHODS: A conceptual framework was developed through targeted review of IIM classification, treatment guidelines, clinical outcomes, economic literature, and model structures in related autoimmune and interstitial lung disease (ILD) conditions. It considered adult and juvenile IIM subtypes, including dermatomyositis, juvenile dermatomyositis, clinically amyopathic dermatomyositis, polymyositis, anti-synthetase syndrome, overlap myositis, immune-mediated necrotising myopathy, and inclusion body myositis. Key model domains were selected based on clinical relevance, measurability, impact on costs and health-related quality of life, and feasibility for future evidence generation.
RESULTS: The proposed framework comprises interconnected modules for diagnosis, active disease, treatment response, remission, relapse, progressive disability, organ involvement, complications, and mortality. Core outcomes include muscle strength and function, skin disease, ILD, dysphagia, cardiovascular involvement, treatment-related adverse events, corticosteroid exposure, healthcare resource use, informal care, productivity loss, and caregiver burden. Symptom progression may be interdependent (e.g. muscle strength and cardiovascular symptoms), which poses higher complexity in the myositis model framework. Total Improvement Score may support response-based modelling where trial data are available; however, subtype-specific manifestations require additional modules, particularly for ILD-dominant disease, skin-dominant disease, progressive disability, and refractory disease. Our proposed framework is designed to allow adaptation by subtype, setting, perspective, time horizon, and decision problem.
CONCLUSIONS: A flexible conceptual framework can improve consistency and transparency in future IIM economic evaluations. Priority evidence gaps include subtype-specific natural history, utilities, resource use, productivity impacts, treatment sequencing, long-term complications, and mapping between clinical endpoints and economic outcomes.