Background/Purpose: Interstitial lung disease (ILD) is the leading cause of mortality in systemic sclerosis (SSc). Little is known about the association between body composition and lung function in SSc. Thoracic computed tomography (CT) scans are routinely obtained in SSc patients to screen for ILD. Accessory respiratory muscles and subcutaneous fat (SF) measurements can be calculated from these CT scans. Our aim was to examine the association between thoracic body composition and pulmonary function in SSc patients.

Methods: SSc patients who met ACR/EULAR 2013 classification criteria were recruited longitudinally from the Scleroderma Programs at two academic medical centers. We performed a cross-sectional analysis using baseline thoracic CTs and baseline pulmonary function tests. Pectoralis muscle and SF area and density from thoracic CT scans were measured using ImageJ. Pectoralis muscle area to SF area ratio (MFR) was calculated. To evaluate the relationship between pectoralis muscle and SF area and density and MFR (exposures) and the % predicted forced vital capacity (ppFVC) (outcome), we performed unadjusted and multivariable-adjusted linear regression analyses. The multivariable regression model was adjusted for potential confounders of the relationship between body composition and ppFVC, including age, sex, height, weight, disease duration, smoking status, SSc subtype, and Scl-70 positivity.

Results: One hundred-four CT scans from SSc participants were analyzed. The median (interquartile range [IQR]) age was 55.0 (44.0, 62.0) years (Table 1). The mean (standard deviation [SD]) BMI was 27.7 (7.08) kg/m2. The majority were female (80.8%) and white (76.9%). The median (IQR) disease duration was 1.1 (0.8, 2.8) years from first non-Raynaud’s symptom. Men were taller, weighed more, and had larger muscle and fat area than women. Men had lower ppFVC than women (75.0% vs 87.0%, p = 0.03). In unadjusted analyses, the following body composition measures were inversely associated with ppFVC: pectoralis muscle area (PMA) (β coefficient = -0.45, 95% confidence interval [CI] -0.89 to -0.008), SF density (β coefficient -0.56, 95% CI -0.98 to -0.14), and MFR (β coefficient = -6.72, 95% CI -13.18 to -0.28) (Table 2). In a multivariable-adjusted model, SF density remained statistically significantly inversely associated with ppFVC (β coefficient = -0.77, 95% CI -1.20 to -0.35). MFR was inversely associated with ppFVC, but only approached statistical significance (β coefficient = -7.44, 95% CI -14.96 to 0.066). There was no association between PMA and ppFVC in the multivariable-adjusted model.

Conclusion: SF density was statistically significantly inversely associated with ppFVC in a multivariable-adjusted regression model. MFR was also inversely associated with ppFVC in the adjusted model, but only approached statistical significance. High SF density and MFR may be risk factors for diminished ppFVC. However, it remains unknown whether thoracic muscle and fat composition can predict ppFVC over time. Prospective studies are needed to determine whether thoracic body composition can predict outcomes in SSc-ILD.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/association-between-thoracic-muscle-and-fat-area-and-density-and-pulmonary-function-in-systemic-sclerosis/