In the present study, we showed the changes in RF muscle thickness, RF cross-sectional area, GC muscle thickness, CC, and MAC throughout different BMI categories. Therefore, we found out that RF muscle thickness, RF cross-sectional area, and GC muscle thickness measured by the US are independently associated with probable sarcopenia, regardless of age, sex, and BMI. This is the first study, that shows the independent association of muscle US parameters with probable sarcopenia. Low HGS is defined as probable sarcopenia by the European Working Group on Sarcopenia in Older People (EWGSOP2) [4]. It is an easy and inexpensive way of assessing muscle strength. The most important thing is that low HGS is an indicator of poor health outcomes like longer hospital stays, increased functional disabilities, poor life quality and death. It is recommended to use in daily hospital practice and in community healthcare. [4, 21, 22]. To diagnose confirmed sarcopenia, muscle mass assessment is required by magnetic resonance imaging (MRI), computed tomography (CT), dual-energy X-ray absorptiometry (DXA), or bioelectrical impedance analysis (BIA) according to EWGSOP2. [4, 23, 24] They have some advantages and disadvantages. Bioelectrical impedance analysis is a widely available method. It does not measure muscle mass directly. It derives an estimate of muscle mass based on whole-body electrical conductivity, and there are several BIA prediction models. Age, ethnicity, and specific disease conditions need specific cut-off values for different devices. In addition, BIA measurements may be influenced by hydration status of the patient.[25, 26] Muscle US is widely used technique to define muscle wasting and muscle quality, and it is getting to be used especially at bed sight.
Sarcopenic obesity is another important problem as it causes more devastating results than both sarcopenia and obesity. Diagnosing sarcopenic obesity needs altered skeletal muscle functional parameters, and altered body composition including increased fat mass and reduced muscle mass assessed as appendicular lean mass/weight by DXA or as skeletal muscle mass/weight by BIA after screening. [7] Recently, appendicular lean mass and skeletal muscle mass are recommended to be adjusted for weight, BMI, or height square. [27] Therefore, BMI is strongly advised as the ideal representative of the body size for these adjustments. [8] Bahat et al also showed that skeletal muscle mass adjusted for BMI was highly related to functionality, physical performance, and frailty, compared to adjustments made for height squared or weight. [28] Another important issue is that BIA may cause underestimation of fat mass and overestimation of fat-free mass index in patients with obesity. [6]
Obesity, sarcopenic obesity, and sarcopenia are different terms, and it is crucial to classify them. There is a fact like the obesity paradox. There are studies reporting the protective effects of obesity in older patients. [29] In fact, it is associated with higher muscle mass and strength. [30] Besides, current BMI cutoff values may not represent the risk of morbidity and mortality in older subjects compared with younger adults. [31] As we consider all the above data, using muscle US for the diagnosis of both sarcopenia and sarcopenic obesity is rational. In this study, we presented the independent association of muscle US parameters with low HGS regardless of age, sex, and BMI. We examined data according to BMI quartiles for each sex. The higher the BMI, the higher the WC, HC, CC, MAC, fat%, and FFMI in both females and males. Therefore, BMI had a strong positive partial correlation with WC, HC, MAC, fat%, and FFMI, and a moderate positive partial correlation with CC after controlling age and sex. Although there were weak and positive partial correlation between BMI and GC muscle thickness; there was no statistically significant correlation between BMI, RF muscle thickness, and RF cross-sectional area.
Age-related muscle loss is regional and muscles rich in type 2 fibers like the anterior thigh undergo atrophy earlier. And when we measure total muscle mass, we may overlook the sarcopenia. The second striking point is that previous studies have shown higher correlations of rectus femoris muscle measurements with muscle strength when compared with other muscle mass measurements. [11, 32, 33]
A recently published systematic review and meta-analysis revealed the diagnostic test accuracy of ultrasound for sarcopenia diagnosis. The meta-analysis of the five studies for RF muscle thickness demonstrated the pooled sensitivity as 72% (95% CI 62–81%) and the pooled specificity as 72% (95%CI 64–79%). Therefore, the meta-analysis of the five studies for gastrocnemius muscle thickness showed the pooled sensitivity and specificity as 82% (95% CI 71–90%)and 64% (95% CI 48–77%), respectively. Lower extremity muscles were reported to be commonly studied and muscle thickness was defined to be the most widely assessed parameter, followed by CSA. [5] In a study conducted in critical care, rectus femoris muscle thickness measured by US detected the loss of muscle mass in a short time more sensitively than the anthropometric measurements. [34] In our study, although the higher BMI, the higher MAC, CC, WC, and HC, we did not observe the same relationship with HGS, RF muscle thickness, and RF cross-sectional area. Therefore, ın multivariate analysis, we presented the independent association of RF muscle thickness, RF cross-sectional area, and GC muscle thickness with HGS. Calf circumference and MAC was not independently associated with probable sarcopenia regardless of ag, sex and BMI.
Strength and limitations
This is the first study that show the clinic importance of muscle ultrasound for probable sarcopenia regardless of age, sex and BMI in geriatric outpatients. This data has an important role to diagnose sarcopenia especially for patients with obesity, and it will keep light to new studies. Therefore, future studies for standardized cut-off values are needed. On the other hand, a cross-sectional design is a limitation of this study. Besides, we included only cooperated geriatric outpatients without acute ilnessess.