Analytic Morphomics in Myositis-Related Interstitial Lung Disease.

Purpose: Interstitial lung disease (ILD) is the most common non-musculoskeletal manifestation of idiopathic inflammatory myopathies (IIM). Identification of body composition change may enable early intervention to improve prognosis. We investigated muscle quantity and quality derived from cross-sectional imaging in IIM, and its relationship to ILD severity.

Methods: A retrospective cohort study assessing IIM of ILD patients (n = 31) was conducted. Two datasets separated in time were collected, containing demographics, biochemical data, pulmonary function testing and thoracic CT data. Morphomic analysis of muscle quantity (cross-sectional area) and quality (density in Hounsfield Units) on thoracic CT were analysed utilising a web-based tool allowing segmentation of muscle and fat. Bilateral erector spinae and pectoralis muscle (ESM&PM) were measured at defined vertebral levels.

Results: FVC and D_LCO decreased but within acceptable limits of treatment response (FVC: 83.7-78.7%, p < 0.05, D_LCO 63.4-60.6%, p < 0.05). The cross-sectional area of the PM and ESM increased (PM: 39.8 to 40.7 cm², p = 0.491; ESM: 35.2 to 39.5 cm², p = 0.098). Density significantly fell for both the PM and ESM (PM: 35.3-31 HU, p < 0.05; ESM: 38-33.7, p < 0.05). Subcutaneous fat area increased from 103.9 to 136.1 cm² (p < 0.05), while the visceral fat area increased but not reaching statistical significance. The change in PM density between time points demonstrated an inverse correlation with D_LCO (p < 0.05, R =  - 0.49).

Conclusion: Patients with IIM ILD demonstrated significant body composition changes on CT imaging unlikely to be detected by traditional measurement tools. An increase in muscle area with an inverse decrease in density suggests poor muscle quality.