Background/Purpose: Interstitial lung disease (ILD) is the leading causes in patients with systemic sclerosis (SSc). Forced vital capacity (FVC) is routinely used for assessment of severity of SSc-ILD. Since FVC decline is shown to predict poor survival, serial FVC change is used as an outcome measure in clinical practice as well as in clinical trials. In general, FVC is decreased in the presence of restrictive lung diseases such as ILD, but also by neurologic and muscle disorders that affect respiratory muscles¹⁾. Here, we have investigated whether chest wall muscle atrophy influences on FVC change in patients with SSc-ILD.

Methods: This is a retrospective study involving 36 patients with SSc-ILD, who were selected from our SSc database, based on fulfillment of 2013 ACR/EULAR classification criteria, disease duration from non-Raynaud’s symptoms < 10 years, availability of at least 2 simultaneous measurements of pulmonary function test (PFT; FVC, FEV1, DLco) and chest high-resolution computed tomography (HRCT) at an interval of 1-3 years. Two independent examiners measured the extent of ILD according to Goh et al ²⁾ and chest wall muscle volume at the level of 9th thoracic spine on HRCT by manual tracing in a setting of blinded patient identity, and the mean values were used for analysis. Serial changes of FVC, ILD extent, and chest wall muscle volume between two measurements (ΔFVC, ΔILD, and Δmuscle) were statistically assessed using Wilcoxon signed-rank test. Correlation coefficient was calculated by Spearman’s rank sum test. Multiple regression analysis was used to evaluate independent roles of ΔILD, Δmuscle in contributing to ΔFVC, by incorporating various co-variables.

Results: Baseline characteristics of the patients included: 72% female, age of 54.9 ± 13.5 years, disease duration of 2.8 ± 3.0 years (range: 0-10 years), 53% dcSSc, 53% anti-topoisomerase I-positive, FVC of 2.37 ± 0.56 L (%predicted FVC of 86 ± 19%), %predicted DLco of 69 ± 21%, and ILD extent of 20.5 ± 16.1%. An interval between the first and second PFT/HRCT evaluations was 20.8 ± 6.9 months. Inter-examiner correlation coefficient was 0.79 for chest wall muscle volume and 0.61 for ILD extent. Sixteen patients (44%) were treated with cyclophosphamide, mycophenolate mofetil, tocilizumab, and/or nintedanib, while 19 (53%) received corticosteroids. There was no change in FVC (P = 0.30) or ILD extent (P = 0.078), whereas chest wall muscle volume reduced significantly with time (5232 ± 1416 to 4895 ± 1259 mm²: P = 0.0008). ΔFVC was correlated negatively with ΔILD (r = -0.48, P = 0.003) and positively with Δmuscle (r = 0.53, P = 0.01), but ΔILD and Δmuscle were not correlated with each other (r = -0.073, P = 0.67). A variety of multivariate models consistently demonstrated that ΔILD and Δmuscle were independent contributors to ΔFVC (adjusted R-squared 0.50-0.59).

Conclusion: In patients with SSc-ILD, FVC decline might reflect not only ILD progression but also chest wall muscle atrophy, which potentially attribute to skeletal muscle involvement of SSc. Our finding may call our attention upon interpretation of serial FVC changes in SSc-ILD patients.

References:
1) Harlaar L et al. Neuromuscul Disord. 2018;28:246-56.
2) Goh NS et al. Am J Respir Crit Care Med. 2008;177:1248-54.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/contribution-of-chest-wall-muscle-atrophy-to-decline-of-forced-vital-capacity-in-patients-with-systemic-sclerosis-associated-interstitial-lung-disease/