Quality over quantity? Association of skeletal muscle myosteatosis and myofibrosis on physical function in chronic kidney disease

Chronic kidney disease (CKD) is characterized by adverse changes in body composition, which are associated with poor clinical outcome and physical functioning. Whilst size is the 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. 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-60s (STS-60) test, incremental (ISWT) and endurance shuttle walk tests, lower limb and handgrip strength, exercise capacity (VO2peak) and gait speed. A total of 61 patients (58.5 ± 14.9 years, 46% female, estimated glomerular filtration rate 31.1 ± 20.2 mL/min/1.73 m2) were recruited. Lower EI (i.e. higher muscle quality) was significantly associated with better physical performance [STS-60 (r = 0.363) and ISWT (r = 0.320)], and greater VO2peak (r = 0.439). The qualitative rating was closely associated with EI values, and significant differences in function were seen between the ratings. RF-CSA was a better predictor of performance than muscle quality. 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.