This study was the first to report the relationship between ultrasound derived EI of the skeletal muscle and physical function in patients on hemodialysis. Our results showed that high EI was closely related to low physical function including HGS, gait speed, STS-60, and IADL disability in patients on hemodialysis.
The pathophysiological mechanisms of decreased muscle quality include muscle fiber reduction, mitochondrial dysfunction of myocytes, intramuscular fat infiltration, and increased fibrosis . The existing assessment methods of muscle quality mainly include CT, MRI, and ultrasound. Although CT and MRI directly evaluate the degree of intramuscular fat infiltration, they are expensive and require patients go to a specific place or have a certain degree of radiation, which affects patient compliance with the examination. Numerous studies in the elderly have confirmed the close relationship between ultrasound derived EI and physical function. However, in the field of kidney diseases, only one small-scale study of stages 3–5 CKD pre-dialysis patients has reported that EI is moderately and negatively correlated with the STS-60 test, the incremental shuttle walk test (ISWT) (n=61), and peak oxygen consumption (VO2 peak) results (n = 32), but not with the gait speed and HGS (n = 29); EI was not correlated with any index of physical function when the CSA was corrected . In this study, EI was moderately and negatively correlated with HGS, gait speed, and STS-60 in patients on hemodialysis. Moreover, after adjusting for muscle area, high EI in patients on hemodialysis remained associated with worse HGS, low gait speed, reduced STS-60, and IADL disability. Thus, this is the first report showing the value of ultrasound derived EI in muscle evaluation of patients on hemodialysis.
In addition to muscle quality, muscle quantity is also an important indicator for muscle evaluation of patients. A wealth of evidence from multiple studies has shown that muscle size is closely related to the physical function of patients on hemodialysis [2, 3, 18]. When we further corrected for factors such as age, gender, and the duration of dialysis, only CSA was related to the level of physical function of the patients. Thus, in the clinical setting, the combined evaluation of muscle quantity and muscle quality may provide more information for accurately assessing muscle strength and physical function in patients .
Interestingly, analysis of the ultrasound parameters of the rectus femoris showed that the CSA of the male group was significantly higher than in the female group, and the EI value of the male group was significantly lower than in the female group. These phenomena are consistent with our expectations. Male muscles are stronger, with larger CSA of the rectus femoris muscle. In addition, the proportion of body fat in the females was generally higher than that of males. Thus, the EI value of the female skeletal muscle was higher.
Loss of muscle quantity and quality in patients on hemodialysis may be related to factors such as increased oxidative stress, accumulation of uremic toxins, reduced exercise activities, and malnutrition [20, 21]. Exercise training is an effective way to improve muscle function. In patients with CKD, ultrasound measurement of CSA has been used to effectively assesses muscle mass gain after exercise intervention . Impendence movement reduced the accumulation of fat in the muscles of the elderly . However, there is still a lack of research on the improvement of muscle quality through exercise in patients on hemodialysis. In addition, further research is needed to verify whether EI can be used as an evaluation index of the improvement of muscle quality.
Ultrasound-based evaluation of muscle quantity and quality of patients on hemodialysis provided more information for assessing the level of physical function in patients. The ultrasound method has the advantages of simplicity, convenience, non-invasiveness, and without radiation exposure. It has important clinical value and wide application prospects in the muscle evaluation of patients on hemodialysis.
This study had some limitations: First, no standardized measurement method for EI is currently available, and the measurement of EI is also affected by different machine parameters. Thus, EI values may differ between different studies and are not comparable. Second, this was a cross-sectional study, and therefore it is impossible to clarify the etiological inference of the decline in muscle quality represented by EI and the decreased physical function of patients on hemodialysis. In addition, this study was carried out in a single dialysis center in China, and the generalization of our conclusions requires international, multi-center studies.