Evaluating skeletal muscle mass with ultrasound in patients with systemic sclerosis

Patients with systemic sclerosis (SSc) have an increased risk for loss of skeletal muscle mass. Ultrasonography (US) is a safe and promising method to evaluate muscle mass. In this study, we aimed to assess the frequency and clinical associations of low muscle mass status in patients with SSc, investigate the correlations between muscle mass sonographically measured regional muscle thicknesses (MTs), and explore the utility of US in predicting low muscle mass.

A total of 93 patients with SSc (86 women) were included in the study. Appendicular skeletal muscle mass (ASM) was calculated using a bioelectric impedance analysis and adjusted for height² (ASM index, ASMI). Low muscle mass was defined as an ASMI of <7.26 kg/m² for men and <5.50 kg/m² for women. MT of the gastrocnemius medialis (GM), rectus femoris (RF), rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transverse abdominis (TA) muscles were assessed by US. Correlations between ASMI and individual MTs were evaluated. Receiver operating characteristic analysis was used to determine the optimal cutoff values of MTs in predicting low muscle mass.

Low muscle mass was present in 13.9% of patients. Diffuse disease subset (53.8% vs 17.5%), antitopoisomerase-1 antibody positivity (76.9% vs 47.5%), and malnutrition (61.5% vs 8.8%) were more frequent in patients with low muscle mass (P < 0.05 for all). MTs of RA (0.54 vs 0.75 cm), TA (0.30 vs 0.34 cm), and GM (1.23 vs 1.51 cm) muscles were significantly lower in patients with low muscle mass (P < 0.05 for all). RA (r = 0.322; P = 0.002), external oblique (r = 0.310; P = 0.002), TA (r = 0.205; P = 0.049), and GM (r = 0.513; P < 0.001) MTs were positively correlated with ASMI. Selected cutoff values for GM and RA MTs showed the highest sensitivity (92.3% for both) and negative predictive value (97.9% and 97.6%, respectively) in predicting low muscle mass status (area under the curve: 0.846 and 0.760, respectively) in the receiver operating characteristic analysis.

Low muscle mass is prevalent in SSc and patients with diffuse disease are at particular risk for this condition. US measurement of abdominal and calf MTs may be used as a screening method to detect low muscle mass due to its high sensitivity and negative predictive value.