Opportunistic MRI assessment of skeletal muscle mass in children and adults with Fontan circulation

Background Children and adults with Fontan circulation are vulnerable to reduced skeletal muscle mass because of altered Fontan physiology, exercise limitation, nutritional challenges, and chronic liver disease. Abdominal magnetic resonance imaging (MRI) performed for Fontan-associated liver disease (FALD) surveillance provides an opportunity to assess muscle mass without additional imaging. Objective To determine the frequency of imaging-defined low skeletal muscle mass on clinically acquired abdominal MRI examinations in children and adults with Fontan circulation and, secondarily, to explore associations with available clinical and imaging markers related to FALD. Materials and methods This single-center, retrospective, cross-sectional study included individuals with Fontan circulation who underwent abdominal MRI for FALD surveillance. Pediatric total psoas area and adult total skeletal muscle index were measured using age-appropriate published approaches and analyzed separately. Continuous variables were summarized with medians and interquartile ranges and explored using Spearman correlations and Mann-Whitney U tests. Sparse cardiac and outcome data were treated descriptively. Results The study included 124 individuals (78 male; 63%), including 57 children (age range 3.2-17.4 years, median=14.1) and 67 adults (age range 18.1-43.7 years, median=22.3). Imaging-defined low muscle mass was present in 17/57 children (30%) and 14/67 adults (21%) using pediatric- and adult-specific measurement approaches. Exploratory analyses did not identify a consistent pattern linking low muscle mass with the available FALD-related measures including liver stiffness measurement, liver biochemistries or liver scoring systems. Several comparisons including ejection fractions and aortopulmonary collateral flow were limited by missing data, small subgroups, and few outcome events. Conclusion Clinically acquired liver MRI examinations can be repurposed to identify imaging-defined low muscle mass in individuals with Fontan circulation. The frequency observed in both age groups supports prospective evaluation of standardized MRI measurements combined with muscle function, exercise capacity, and longitudinal outcomes.