Does computed tomography-based muscle density predict muscle function and health-related quality of life in patients with idiopathic inflammatory myopathies?

Objective To investigate the association of low-density (lipid-rich) muscle measured by computed tomography (CT) with skeletal muscle function and health-related quality of life in idiopathic inflammatory myopathies (IIMs). Methods Seventeen patients and 10 healthy controls underwent CT of the midthigh to quantify high- (30-100 HU) and low-density (0-29 HU) skeletal muscle areas. Anthropometric measures, body composition, physical activity level, health-related quality of life, skeletal muscle strength, endurance, and fatigue were assessed. Patients were compared against controls. The relationship of anthropometric, body composition, and disease variables with measures of muscle function were examined using Spearman's test on the patient group. Linear regression was used to assess the age- and disease-adjusted relationship of muscle quality to physical function and muscle strength. Results Patients had higher body fat percentage (P-=-0.042), trunk fat mass (P-=-0.042), android:gynoid fat (P-=-0.033), and midthigh low-density muscle/total muscle area (P < 0.001) compared to controls. Midthigh low-density muscle/total muscle area was negatively correlated with self-reported physical function, strength, and endurance (the Short Form 36 [SF-36] health survey physical functioning [P-=-0.004], manual muscle testing [P-=-0.020], knee maximal voluntary isometric contraction/thigh mineral-free lean mass [P < 0.001], and the endurance step test [P < 0.001]), suggesting that muscle quality impacts function in IIM. Using multiple linear regression adjusted for age, global disease damage, and total fat mass, poor muscle quality as measured by midthigh low-density muscle/total muscle area was negatively associated with SF-36 physical functioning (P-=-0.009). Conclusion Midthigh low-density muscle/total muscle area is a good predictor of muscle strength, endurance, and health-related quality of life as it pertains to physical functioning in patients with IIMs.