Prognostic impact of muscle mass in idiopathic interstitial pneumonia: analysis of idiopathic pulmonary fibrosis and other idiopathic interstitial pneumonias

Low skeletal muscle mass has been reported to associated with poor prognosis in patients with idiopathic pulmonary fibrosis (IPF). However, such associations have scarcely reported in idiopathic interstitial pneumonias (IIPs) other than IPF. Quantification of muscle mass obtained from chest computed tomography (CT) is used as a simple screening tool for sarcopenia in patients with respiratory diseases. However, the optimal thoracic site for muscle mass quantification is controversial. Moreover, there have been no reports investigating the association between muscle mass and acute exacerbations. This study aimed to evaluate optimal site for muscle mass quantification in chest CT to predict survival and acute exacerbation in IPF and non-IPF idiopathic interstitial pneumonias. This study included 528 patients diagnosed with IIP at 29 facilities between September 1, 2013, and April 30, 2016, following multidisciplinary discussions with prospective follow-up over a 5-year period. The cohort was divided into two groups: those with IPF and those with non-IPF IIPs. Skeletal muscle mass was quantified using the erector spinae muscle index (ESMI) and pectoralis muscle index (PMI), defined as the respective muscle area on chest computed tomography (CT) divided by height squared at the time of enrollment. Associations between these indices at baseline and both survival and acute exacerbation were analyzed. In both IPF and non-IPF cohorts, Cox regression analysis revealed that patients with low ESMI had a poorer prognosis compared to those with normal ESMI, even after adjusting for age, sex, % forced vital capacity (FVC), and smoking exposure level. The hazard ratios were 0.62 (95% CI 0.40–0.90; p = 0.013) and 0.46 (95% CI 0.26–0.83; p = 0.009), respectively. In contrast, no significant relationship was identified between PMI and survival. Multivariable Cox regression analysis confirmed that ESMI was an independent predictor of survival in both IPF and non-IPF patients. Additionally, acute exacerbations occurred more frequently in the low ESMI group and low PMI group, particularly among non-IPF patients. The ESMI obtained from chest CT is associated with survival in not only in IPF patients but also in the non-IPF patients. The ESMI and PMI also associate with acute exacerbations in non-IPF patients.