Quality over quantity? Effects of skeletal muscle myosteatosis and fibrosis on physical functioning in chronic kidney disease

Background Chronic kidney disease (CKD) is characterised by adverse changes in body composition, which are associated with poor clinical outcome and physical functioning. Whilst size is key for muscle functioning, changes in muscle quality specifically increase in intramuscular fat infiltration (myosteatosis) and fibrosis (myofibrosis) may be important. We investigated the role of muscle quality and size on physical performance in non-dialysis CKD patients. Methods Ultrasound (US) images of the rectus femoris (RF) were obtained. Muscle quality was assessed using echo intensity (EI), and qualitatively using Heckmatt’s visual rating scale. Muscle size was obtained from RF cross-sectional area (RF-CSA). Physical function was measured by the sit-to-stand-60 (STS-60) test, incremental (ISWT) and endurance shuttle walk tests (ESWT), lower limb and handgrip strength, exercise capacity (VO2peak), and gait speed. Results patients (58.5±14.9) years, 46% female, eGFR 31.1±20.2 mL/min/1.73m2 40 ) were recruited. Lower EI (i.e. higher muscle quality) was significantly associated with better physical performance [STS-60 (r=.363) and ISWT (r=.320)], and greater VO2peak (r=.439). The qualitative rating were closely associated with EI values, and significant differences in function was seen between the ratings. RF-CSA was a better predictor of performance than muscle quality. Conclusions In CKD, increased US-derived EI was negatively correlated with physical performance, however, muscle size remains the largest predictor of physical function. Therefore, in addition to the loss of muscle size, muscle quality should be considered an important factor that may contribute to deficits in mobility and function in CKD. Interventions such as exercise could improve both of these factors.