Sarcopenia in Chronic Kidney Disease: A Scoping Review of Prevalence, Risk Factors, Association with Outcomes, and Treatment

Sarcopenia, a condition characterized by loss of skeletal muscle mass and function, has important clinical ramifications. We aimed to map the existing literature about prevalence, risk factors, associated adverse outcomes, and treatment of sarcopenia in individuals with chronic kidney disease (CKD). A scoping review of the literature was conducted to identify relevant articles published from databases’ inception to September 2019. Individuals with CKD, regardless of their disease stage and their comorbidities, were included. Only studies with sarcopenia diagnosed using both muscle mass and function, based on published consensus definitions, were included. For studies on treatment, only randomized controlled trials with at least one sarcopenia parameter as an outcome were included. Our search yielded 1318 articles, of which 60 from were eligible for this review. The prevalence of sarcopenia ranged from 4 to 42% according to the definition used, population studied, and the disease stage. Several risk factors for sarcopenia were identified including age, male gender, low BMI, malnutrition, and high inflammatory status. Sarcopenia was found to be associated with several adverse outcomes, including disabilities, hospitalizations, and mortality. In CKD subjects, several therapeutic interventions have been assessed in randomized controlled trial with a muscle mass, strength, or function endpoint, however, studies focusing on sarcopenic CKD individuals are lacking. The key interventions in the prevention and treatment of sarcopenia in CKD seem to be aerobic and resistance exercises along with nutritional interventions. Whether these interventions are effective to treat sarcopenia and prevent clinical consequences in this population remains to be fully determined. Supplementary Information The online version contains supplementary material available at 10.1007/s00223-021-00898-1.


Introduction
The prevalence of chronic kidney disease (CKD) increases with age. Αccording to a meta-analysis [1], global CKD prevalence is estimated between 11 and 13%. The prevalence classified by stage is 3.5%, 3.9%, 7.6%, 0.4%, and 0.1% for CKD stages 1, 2, 3, 4, and 5, respectively. Sarcopenia also increases with age, while its development may be associated with conditions that are not exclusively seen in older people, the coexistence of age and sarcopenia creates a potential common phenomenon.
Nowadays, sarcopenia constitutes a major public health problem and will be even more important in the future given the aging of the population. Depending on the definition used, its prevalence is estimated to increase between 63.8 and 72.4% from 2016 to 2045 [2].
Sarcopenia was originally defined as an age-related loss of skeletal muscle mass. Currently, we lack a consensus definition for sarcopenia and four main international groups have released general sarcopenia definitions and guidelines: European Working Group on Sarcopenia in Older People (EWGSOP), Foundation for the National Institutes of Health (FNIH), International Working Group on Sarcopenia (IWG), and Asian Working Group for Sarcopenia (AWGS) ( Table 1). In 2010, the EWGSOP recommended the presence of low appendicular lean mass (ALM) [measured by dual-energy X-ray absorptiometry (DXA) and adjusted for Varvara Chatzipetrou and Marie-Josée Bégin have contributed equally to this work.

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height meter squared] along with measurements of low muscle function (i.e., strength and/or or physical performance) for the diagnosis of sarcopenia but did not mandate specific measures or cut-off points [3]. The European definition was recently updated [4] and recommends low muscle strength as the primary diagnostic parameter of sarcopenia. Once low muscle strength is detected, low ASM poses the definitive diagnosis of sarcopenia. Then, low physical performance indicates the severity of the disease [4]. For the FNIH Sarcopenia Project, the appendicular lean mass (ALM) was also evaluated after adjustment for body mass index (BMI) and not for height 2 (ALM BMI ) [5]. Comparing the FNIH, EWG-SOP, and IWG criteria in the elderly population, the positive percent agreement (for diagnosing sarcopenia) is low but the negative percent agreement is high (for ruling out sarcopenia), both in men and women [6]. The estimated prevalence of sarcopenia in people aged 65 and older varies between 0.5 and 5.3% in men and 1.8 and 13.3% in women, depending on the criteria used for diagnosis. Due to its more restricted cut-offs, prevalence tends to be lower with the FNIH criteria than with the other proposed criteria [5].
All these diagnostic criteria have greatly contributed to the official recognition of sarcopenia as a disease in 2016, with the attribution of its own International Classification of Diseases, 10th Revision (ICD-10) code [3]. Nevertheless, the remaining problems are the application of these criteria in clinical practice and the lack of recommended cut-off points in specific populations, such as in patients with CKD. CKD is a condition associated with muscle loss [10][11][12][13] for which clinicians should consider assessing sarcopenia [7,9]. At the present time, the adequacy of actual cut-offs in CKD subjects is unknown, especially the performance characteristics of these criteria/cut-offs (e.g., predictive ability for adverse outcomes) derived from the general population may vary in specific clinical conditions. There is a possibility of underestimating the clinical significance of sarcopenia in CKD population [5]. One important step before recommending a definition Gait speed used as a diagnostic criterion Definition of sarcopenia based on the pooled analysis of cohort studies with important number of participants AWGS, 2014 [9] ALM/height 2 Men: < 7.0 kg/m 2 , Women: < 5.7 kg/m 2 (measured by BIA) and < 5.4 kg/m 2 (measured by DXA) over another, in this specific population, is to demonstrate its ability to predict adverse outcomes such as falls, fractures, disability, hospitalizations, and death. In the general population, sarcopenic patients have a higher rate of mortality [pooled Odds Ratio (OR) of 11 studies, 3.6; 95% Confidence Intervals (CI) 2. 96-4.37], disability (pooled OR of 6 studies, 3.03; 95% CI 1. 80-5.12), falls (significant association in two out of two studies), and a higher incidence of hospitalizations (significant association in one out of one study) compared to the nonsarcopenic patients [14]. Sarcopenia also increases the risk of readmission to the hospital [pooled Relative Risk (RR) from 8 studies, 1.75; 95% CI 1.01-3.03] [15] and the risk of fractures (pooled OR in 5 prospective studies: 1.71; 95% CI 1.44-2.03, p = 0.011) compared with non-sarcopenic individuals [16]. Moreover, in the elderly population, sarcopenia is associated with disabilities in instrumental activities for daily living (ΙADL) [17] and mortality [17,18]. In CKD individuals, the ability of different operational definitions and their cut-off points to predict outcomes may vary and the contribution of sarcopenia in those major clinical outcomes still needs to be clarified. In this review, we tried to answer the following questions: Sarcopenia in CKD -Which definition of sarcopenia is most commonly used in CKD population and which is the most suitable for prevalence estimation? -What are the risk factors for sarcopenia in CKD individuals? -Does sarcopenia in CKD population independently increase the risk of adverse outcomes as it does in the general population?

Treatment of sarcopenia in CKD
-What are the therapeutic options intended to reverse sarcopenia in CKD patients?

Protocol
The structure of the review is based on the 'PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation' [19] and the PRISMA statement for systematic reviews [20].

Eligibility Criteria and Rationale
All papers published until September 2019, written in English and referring to sarcopenia in CKD were potentially eligible. The eligibility criteria were as follows: -Study population: any type of study conducted in participants aged ≥ 18 years, with CKD regardless of their disease stage and their comorbidities, including patients in hemodialysis (HD) and peritoneal dialysis (PD), were included in the review process. Studies conducted on kidney transplant patients were excluded to avoid heterogeneity among the study population. -Sarcopenia: only studies with a clear definition of sarcopenia, based on published consensus guidelines were included. We found many studies in the literature, referring to sarcopenia solely by one parameter (low muscle mass or function) and we preferred to exclude them from the qualitative synthesis. -Studies' design: both retrospective and prospective studies with an observational or experimental design were considered in this review. The objective was to obtain a global and large aspect regarding the topic.
Studies not corresponding to the conceptual framework (letters to the editor, case reports, and reviews) were excluded. -Adverse outcomes were defined as physical disabilities, physical dependence, mortality, cardiovascular events, falls, fractures, and hospitalizations.
For studies on the treatment of sarcopenia in CKD, only randomized controlled trials (RCTs) with a sarcopenia parameter (muscle mass, strength, or function) as an outcome were included in the review process.

Information Sources
The primary literature search was conducted through the following electronic databases: PubMed (NCBI, U.S. National Library of Medicine) and Cochrane Central Library (most recent search executed on September 10th, 2019). Furthermore, a secondary search strategy was also used which involved reference tracking of identified full texts.
The search strategies were drafted through teamwork. All retrieved papers were exported to reference manager software (Endnote). The articles were selected, assessed, and verified by the first reviewer (VC). The second reviewers (MH, MJB) verified data from the eligible studies and the third reviewer (AT) resolved all discrepancies.

Data Extraction
The initial search included terms that were used in various combinations and was based only on titles. The main terms searched for sarcopenia, CKD, clinical outcomes, mortality, hospitalization, disability, falls, fractures, and treatment. Additional terms and combinations of all of them were searched thoroughly. The full electronic search strategy for the PubMed database is provided as supplementary material (Online Resource 1). After that, duplicates were removed. Then, the abstracts of potentially eligible articles were evaluated and subsequently, the full texts were studied. The studies were grouped into two categories according to the topics covered: (1) Sarcopenia in CKD and (2) Treatment of sarcopenia in CKD.

Results
The primary database search identified 2926 records from PUBMED and 413 from Cochrane Library. We found an additional number of 30 records through the secondary search. After duplicates were removed, the total search was composed of 1318 records. We excluded 1160 records based on title and abstract screening. Figure 1 illustrates in detail the reasons for exclusion. Of the remaining 158 full texts, 60 full texts were considered eligible and included in this review (all of them correspond to the inclusion criteria and answer the questions of this review). More precisely, we included 22 articles in the first category (Tables 2 and 3),  while 37 articles and 1 Cochrane were included in the second category concerning treatment of sarcopenia in CKD  (Tables 4 and 5).
As mentioned above, all articles included in the first category (i.e., 22 articles included), defined sarcopenia in CKD individuals, based on both low muscle mass and function, according to one of the operational definitions described in Table 1. The skeletal mass was measured by DXA or BIA, while muscle strength and physical performances were based on handgrip strength (HGS) and gait speed assessment, respectively.

Prevalence of Sarcopenia According to the Definition Used
Six studies examined the prevalence of sarcopenia in chronic kidney disease without kidney replacement therapy (CKD without KRT), and 16 in dialysis patients (11 in HD and five in PD patients). As depicted in Tables 2 and 3, the most common definition used was the European consensus guidelines as the EGWSOP1 and the EWGSOP2 criteria were assessed in sixteen and two articles, respectively. The FNIH definition was determined in four articles and the AWGS criteria in five studies. Among them, four studies compared  the prevalence estimated by more than one definition [24,27,30,41]. The prevalence of sarcopenia ranged from 4 to 42% according to the definition used, the population studied, and the stage of CKD. To be more specific, based on EWGSOP1 criteria, the prevalence in CKD without KRT individuals varied from 5.9% [23] to 14% [26], similarly to PD patients (from 4% [30] to 15.5% [27]). A higher prevalence was found in HD patients, ranging from 13.7% [39] to 42.2% [32]. Based on the other sarcopenia definitions, sarcopenia prevalence also varied, ranging from 3.9% [68] to 28.7% [24] when FNIH criteria were used and from 1.9% [34] to 40% [38] when AWGS cut-offs were applied.

Disability/Physical Dependence
Three studies, two of them regarding CKD without KRT individuals [24,25] and one in HD patients [32], investigated the association between sarcopenia and physical disabilities. It was indicated that sarcopenic patients had significantly worse physical performance in instrumental activities for daily living (IADL) [25], lower walking speed [24], as well as lower scores in Short Physical Performance Battery (SPPB) [25] than non-sarcopenic patients. Similarly, in a prospective analysis in HD patients (in which patients underwent various physical function tests, like an average number of steps per day and one-leg standing time), the sarcopenic group presented higher risk of disabilities than the non-sarcopenic one [32].
All the studies, defined sarcopenia using the European consensus guidelines.

Mortality and Cardiovascular Risk
Two longitudinal studies, one conducted in CKD without KRT [23] and the other in HD patients [36] with a followup of 40 months and 4.5 years, respectively, indicated that sarcopenia was a strong predictor of mortality (HR, 2.89; 95% CI: 1.40-5.96; p < 0.004 [23] and HR, 6.99; 95% CI 1.84-26.58; p = 0.004 [36]) as well as cardiovascular events (HR, 4.33; 95% CI, 1.51-12.43; p = 0.006) [36]. In the study focusing on CKD individuals, the mortality rate was estimated to be 18% at 40 months (41% of deaths were due to cardiovascular complications). After multivariate adjustment for potential confounders such as age, sex, BMI, glomerular filtration rate (GFR), albumin, and Charlson index, sarcopenia (defined with low HGS and low SMMI estimated by BIA) was an independent predictor of mortality [23]. In HD, seven studies have measured the mortality rate in sarcopenic patients (Table 3). Although the majority of them observed that survival was lower in sarcopenic patients than the non-sarcopenics, subjects in the non-sarcopenic group were significantly younger (p = 0.006 for [39], p < 0.01 for [29], and p < 0.001 for [31]). In another study in HD patients, only in older subjects (age ≥ 60 years), sarcopenia was determined as an important predictor of mortality [38]. For Bataille et al., the difference between the mortality rate at 1 year of the two groups (31.4% in sarcopenic vs. 21.4% in non-sarcopenic) was not statistically significant [29]. Moreover, no statistical significant difference was found in another study, after adjustments for potential confounders; however, non-elderly patients (< 65 years) who were classified as having sarcopenia had significantly higher risk of death compared with those without sarcopenia [68]. In PD, the mortality rate for sarcopenic patients was estimated to be 5.9% (mean follow-up 589 days) and sarcopenia was associated with a higher risk of mortality (survival mortality rate per 500 days: 0.667 in sarcopenia group vs. 0.971 in nonsarcopenia group; p < 0.001) [34].

Other Outcomes
According to one multicenter, prospective analysis, sarcopenia was also significantly associated with a higher risk of hospitalization in HD patients, even after multivariable adjustments (RR, 2.07; 95% CI, 1.48-2.88; p < 0.001).
Only one study found in the literature indicated that sarcopenic patients present a higher risk of falls (high risk of falls defined as ≥ 14 s in timed up and go test (TUG) or gait speed ≤ 0.8 m/s in 4-m walk) than non-sarcopenic ones [21].

Physical Activity
Despite several RCTs on exercise intervention reporting at least one sarcopenia parameter as an outcome in CKD individuals (Table 4), only one study [45] has focused on CKD subjects with sarcopenia at baseline based on a consensual definition. Sarcopenia was defined using AWGS diagnostic thresholds and the prevalence of sarcopenia in their HD study population was 13.2%. Sarcopenic patients were randomized to intradialytic resistance exercise for 12 weeks (n = 21) or usual care (n = 20). In the exercise group, a significant improvement in maximum grip strength, daily stride rate, and physical activity level were noted, without any significant differences in skeletal muscle mass. Two other RCTs reported the prevalence of sarcopenia in their studies, using EWGSOP criteria [44,60]. Lopes et al. [44] reported after 12 weeks of high (HLG) or moderate load (MLG) resistance intradialytic training three times per week, a reduction of 14.3% and 25%, respectively, in the prevalence of sarcopenia in the exercise groups compared to an increase of 10% in the control group. However, Zhou et al. [60] did not find any significant decrease in sarcopenia prevalence in stages 3-5 CKD without KRT individuals with either strength or balance in combination with endurance training for 12 months.
A Cochrane of 45 RCTs on physical activity (cardiovascular, resistance, mixed exercises, and yoga) in 1863 participants with CKD, with a duration of 2-18 months, have shown an improvement in physical capacity measures (aerobic capacity, walking capacity) [69]. Both aerobic and resistance training exercise (alone or in combination) appeared to be favorable for global health and physical function in CKD individuals [70]. With aerobic exercise training, significant improvement in physical performance as assessed using the sit-to-stand test was noted in several studies [48][49][50]52]. Leg lean mass was improved in obese CKD stages 3-4 men with aerobic exercises [49] but not in HD patients [63]. Furthermore, in another RCT, aerobic exercise training in HD patients failed to alter Six-Minute Walk Test (6MWT), TUG, and grip strength [51].
The addition of resistance exercises is considered an important part of training programs in sarcopenic patients, either alone or in combination with aerobic exercise (Table 4). Improvement in lean mass has been reported [43,44,47], as well as a significant increase in muscle strength [42,43,[45][46][47]61] with resistance training programs in HD patients. In addition, two RCTs assessed SPPB, and concluded that resistance training programs significantly improved physical performance [43,44]. No RCT in CKD without KRT population was included for resistance training alone, but four studies have reported the impact of the combination of resistance and aerobic training in this population [54,56,57,60]. An improvement in the sit-to-stand test (STS) [57], SPPB [56], and 6MWT [54] was observed in CKD without KRT individuals with combination therapy, but not in lean body mass [60] or handgrip strength [54].
The combination of a nutritional intervention to resistance exercise programs was studied in three RCTs [45,63,71]. No additional benefit of resistance exercise to intradialytic oral nutrition on lean body mass was found in a RCT [45]. In subjects with moderate-to-severe CKD (serum creatinine concentrations between 133 and 442 μmol/L), Castaneda et al. [71] reported an improvement in muscle strength with resistance training despite low protein diet.

Nutritional Interventions
Compared to intradialytic parenteral nutrition, oral nutrition imparts anabolic benefits in the post-dialytic period by improving skeletal muscle homeostasis [72]. Regarding sarcopenia specifically (Table 5), two studies have reported improvement in lean body mass with oral nutritional supplements over 6 months; one with polycose, a glucose polymer, and the other using oral branched-chain amino acids (BCAA) [64,66]. Furthermore, Eustace et al. have recorded a significant increase in handgrip strength (+ 2.45 kg) in HD patients with essential amino acids (EAAs) supplementation for 3 months but no impact on mid-arm muscle circumference [65]. In another RCT, supplemental nutritional drinks for 6 months were not associated with improvement of midiliopsoas muscle CSA [67].

Hormonal Therapies
Nandrolone decanoate (ND), an anabolic steroid, has shown to increase significantly lean body mass compared to placebo (mean change (SD) + 4.5 (2.3) kg, p < 0.001) after 6 months course in dialysis patients and improved functional performance (walking and stair-climbing test) [73]. ND increased appendicular lean mass in a dose-responsive manner [74]. In a RCT, the addition of resistance exercise training for 12 weeks (intradialytic, 3 times/week) to nandrolone increased in an additive manner the quadriceps muscle cross-sectional area [75]. Oxymetholone is another anabolic steroid, given orally, with higher anabolic effect and lower androgenic activity compared to testosterone. In a RCT in hemodialysis patients (n = 43), oxymetholone for 24 weeks was associated with a significant increase in fat-free mass, handgrip strength, and physical functioning scores compared to placebo. Muscle biopsies exhibited an increase in mRNA for myosin heavy chain and enhanced IGF-I, IGF-II receptor in patients on oxymetholone [76]. No data are available on the role of anabolic steroids in CKD without KRT population.
Several studies have examined the benefits of recombinant human growth hormone (rhGH) in CKD patients in which abnormalities in the physiological axis GH/IGF-1 are found, in particular GH resistance [77]. Growth hormone therapy is associated with increased protein synthesis and decreased protein catabolism in HD patients [78][79][80][81] and in PD patients [82,83]. In a RCT, 139 HD patients with serum albumin < 40 g/l were randomized to subcutaneous rhGH or placebo. With all dosages, rhGH increased lean body mass significantly with improvement in quality of life (QoL) [84]. Similar results were found in other studies [85] and Johannsson et al. [86] found an increase in handgrip strength after 6 months of rhGH therapy in elderly HD patients. However, in the OPPORTUNITY RCT [87], rhGH in hypoalbuminemic HD patients (n = 712) did not increase albumin, lean body mass, physical capacity, or QoL, but the study was prematurely terminated. Few significant adverse effects are reported with rhGH such as soft tissue edema, arthralgia, carpal tunnel syndrome, gynecomastia, and dysglycemia [88].

Correction of Acidosis
In a RCT, 134 adults with metabolic acidosis and eGFR 15-30 ml/min/1.73 m 2 were randomized to oral sodium bicarbonate or standard care for 2 years. Patients receiving bicarbonate presented an increase in mid-arm muscle circumference, protein intake and serum albumin levels [89]. Similar results have been demonstrated in a RCT of 200 PD patients, showing an increased body weight, mid-arm circumference, and decreased hospitalization admission after 1 year of higher alkali dialysate [90].

Sarcopenia in CKD: Definition Used and Prevalence
The majority of studies investigating sarcopenia in CKD were based on EWGSOP consensus, probably due to the fact that these criteria were established first. For Souza et al. [24], the prevalence was underestimated when EWGSOP1 criteria were assessed (11.9% vs. 28.7% when measured with FNIH criteria), while in the general population, the prevalence of sarcopenia is generally lower when FNIH criteria were used [6]. The indexed method used for the muscle mass assessment may explain the reason for these opposite results (height squared vs. BMI). For example, the prevalence of low muscle mass indexed by BMI was 25% while it reached only 8.1% when indexed by height 2 in one study [68]. One explanation is that the prevalence of low muscle mass (defined by ALM indexed by BMI, according to FNIH criteria) may become higher since BMI is frequently increased in CKD individuals. Therefore, ALM indexed by height 2 may not recognize patients with muscle wasting or patients with sarcopenia and obesity occurring together [68]. Recently, the term 'sarcopenic obesity' has been coined in order to describe the coexistence of sarcopenia and obesity, related to increased fat mass and rapid loss of muscle mass. In CKD without KRT, the prevalence of sarcopenic obesity varied from 9.7% [91] to 11.2% [92]. In PD patients, the prevalence was low, up to 3.8% [40] and in HD subjects, it ranged from 12 to 62% in men and 2% to 74% in females [93]. Of note, sarcopenia prevalence increased with the severity of CKD, independently of the sarcopenia definition used.
The great variability in sarcopenia prevalence in CKD may also be due to the following reason: operational definitions refer to the general population and there are no specific methods or cut-offs validated in the CKD population. As reviewed in Tables 2 and 3, many studies were based on operational definitions of sarcopenia but higher cut-offs were established than the ones proposed by the working groups. Also, the prevalence of sarcopenia varied according to methods used for the assessment of muscle mass [23]. Pereira et al. [23] defined sarcopenia as low muscle strength together with low mass, evaluated by three different assessments (SGA, mid-arm muscle circumference, and bioelectrical analysis-BIA). Interestingly, higher cut-offs than the one proposed by the EWGSOP consensus were used for the evaluation of low muscle mass by BIA. The prevalence varied upon the employment of different methods. Similarly, Lamarca et al. [94] used four different measurements for low muscle mass evaluation, with different cut-off limits and found that the prevalence varies from 3.9 to 63.3% depending on the method and cut-offs applied. The BIA method, compared to the DXA technique, is limited as it does not provide information concerning specific body compartments [23]. Both BIA and DXA assessments are affected by fluid retention, commonly seen in more advanced CKD stages and thus they should be performed after the dialysis session [95]. In summary, the lack of uniformity among definitions results in a great variability in the clinical and research approach of sarcopenia in the CKD population, including in the estimation of prevalence.
Taken into account together, the prevalence of sarcopenia in CKD individuals varies according to the operational definition used and the disease stage. The differences among studies (e.g., study duration, age, ethnicity, BMI, and gender of enrolled patients) could also contribute to the heterogeneity of the prevalence estimates of sarcopenia across studies. There is no study in literature indicating the superiority of one definition to the other.

Risk Factors for Sarcopenia and Other Observations
As evidenced in the general population, age constitutes an important risk factor for sarcopenia, with increased prevalence in advanced age [17,96].
A relationship between the stage of CKD and sarcopenia is also evident but whether there is causality between these entities cannot be determined, due to the multivariate associations and the observational character of most of the studies. In CKD without KRT, several studies have reported the association between the increased prevalence of sarcopenia and worsening kidney function as well as more advanced CKD stages [10,22,24,25]. In addition, more people in HD suffer from sarcopenia compared to CKD without KRT individuals, and longer dialysis duration was associated with higher risk of sarcopenia, probably because HD patients are longer exposed to all the pathological states (metabolic, hormonal abnormalities, and others) [39,97]. In contrast, age and dialysis duration were not significant risk factors for sarcopenia for As'habi et al. [28] but this was probably related to the fact that study population was younger (73.5% of study population < 65 years) and on PD.
Furthermore, malnutrition and inflammation status are revealed as risk factors for sarcopenia. The two conditions may appear together. Low levels of albumin and prealbumin in sarcopenic patients can be due to either poor nutritional status or chronic inflammation that affect individuals with CKD. IL-6 levels are associated with anorexia and high levels of other cytokines are related to increased lipolysis and muscle degradation contributing to the development of sarcopenia [35,39]. Similarly, diabetes was independently associated with sarcopenia because of the fact that insulin resistance leads to muscle breakdown [39,97]. Lastly, the association between diuretic drugs and sarcopenia needs further investigation.

CKD-Related Pathophysiology of Sarcopenia
A complete report on the pathophysiology of sarcopenia in CKD is beyond the scope of this review. Muscle wasting in CKD patients is multifactorial [97,98]. It is worth noting that both sarcopenia and CKD are progressive diseases with similar pathophysiological pathways and share similar risk factors. But specific factors, related or associated to the development of chronic kidney failure are responsible for muscle loss, which makes CKD a potential independent risk factor of sarcopenia [23].
Muscle homeostasis is controlled by an equilibrium among catabolic and anabolic processes [98]. The loss of muscle mass in CKD is mainly due to two mechanisms: (i) increased muscle catabolism via the activation among other things of the ubiquitin-proteasome system, caspase-3, and lysosomes pathways and (ii) impaired muscle growth, regeneration, repair, and suppression of protein synthesis with components of abnormal myogenesis. Among the factors that explain this process, we have to cite mitochondrial dysfunction, increased myostatin levels, and physical inactivity [98,99]. Τhe reduced levels of myogenesis are also in part a consequence of the impaired function of muscle precursor (satellite) cells found in CKD.
Specific factors that intervene to favor this imbalance are metabolic acidosis, endocrine disorders involving the insulin/IGF-1 signaling, low testosterone levels, alterations in the renin-angiotensin-aldosterone system, systemic inflammation, and an abnormal hypothalamic appetite regulation [100].

Sarcopenia in CKD: An Independent Predictor of Adverse Outcomes?
In the CKD population, the clinical impact of sarcopenia has not been thoroughly examined. It is very often characterized in the literature as a model for 'accelerating aging' and CKD has been associated with increased risk of falls, fractures, disability, and institutionalization [98]. The question arises whether sarcopenia can independently increase the risk of adverse outcomes in these patients.

Disability/Physical Dependence
Regardless of the definition used, sarcopenic patients presented worse physical capabilities than non-sarcopenic ones. Sarcopenic subjects presented worse performances in IADL [25] and in other physical tests [32]. However, the populations studied were frequently over 65 years and further studies should be designed in younger subjects with CKD in order to evaluate whether sarcopenia could be responsible for physical disabilities or not.

Mortality
Although sarcopenia is considered an independent risk factor for all-causes of deaths in the elderly [17], data are limited in the CKD population. As proven above, two studies were found in the literature estimating the mortality risk in CKD [23] and in PD patients [34]. As it was expected, the mortality rate was higher in dialysis than in CKD without KRT, as well as in sarcopenic rather than non-sarcopenic groups. This is explained by the fact that most of the contributors of sarcopenia are present in HD states, such as inflammation, insulin resistance, and low protein intake [97]. For Kim et al., [36] both low muscle mass and low muscle strength examined separately were associated with mortality. In contrast, other studies supported that only low HGS is a risk factor of death [29,33] and reduced muscle strength is a better predictor of clinical outcomes than muscle mass [27,30,68], implying that their co-measurement underestimates the mortality risk [28,41]. This is consistent with other studies that demonstrated that low HGS, measured alone, was an independent predictor of mortality [101,102]. According to a recent meta-analysis, HD patients with low and high HGS were compared, resulting in a summary risk ratio of all-cause mortality being 1.88 (95% confidence interval 1.51-2.33; p < 0.001) [103]. These observations would probably orientate clinicians towards EWGSOP2 criteria, as the most suitable for a sarcopenia definition in CKD subjects.

Cardiovascular Risk
Only one study reported that sarcopenia is independently associated with increased risk of cardiovascular events in patients undergoing hemodialysis (HR 4.33; 95%CI, 1.51-12.43; p = 0.006) [36]. A possible explanation could be a positive association between sarcopenia and atherosclerosis as well as inflammation status. Inflammatory markers and triglycerides were significantly higher in sarcopenic patients in two studies [24,40]. Both markers have been associated with increased cardiovascular events [36,104,105]. Similar results of surrogate cardiovascular endpoints (intima-media thickness, flow-mediated dilatation, epicardial adipose tissue, BNP) have been reported in the literature of sarcopenic CKD patients [106,107]. Furthermore, in a cohort on 218 PD patients prospectively followed for 4 years, HGS in the lower tertile was an independent predictor of cardiac congestion events (p = 0.002) [108]. Further prospective studies are needed to assess the relation between sarcopenia and cardiovascular events.

Other Outcomes
No articles were found in the literature associating sarcopenia with fractures in CKD subjects. Although the prevalence of fractures in HD patients is high (52%) [109], the contributing role of sarcopenia has not been investigated.
Falls constitute an important risk factor fοr fractures, especially in older dialysis patients [110]. Up until now, it has been indicated that sarcopenic patients were prone to falls, having worse performance in the activities of daily living (ADLs) and worse functional capacity (p = 0.012) [24]. More recently, a case-control study demonstrated that the risk of falls was higher in sarcopenic than non-sarcopenic groups and that anemia, hyperparathyroidism, as well as calf circumference contribute to that. The two groups of subjects had no differences concerning sex and age but information was limited concerning CKD stage and dialysis duration of the study population [21]. In summary, whether sarcopenia constitutes an independent predictor of falls remains unclear.

Sarcopenia in CKD: Treatment Strategies
The main interventions for prevention and treatment of sarcopenia in CKD individuals are aerobic and resistance exercises along with nutritional interventions. Furthermore, optimizing dialysis, vitamin D status, acidosis as well as management of comorbidities such as infections and depression are mandatory elements for patient care [111].
Aerobic and resistance training, alone or in combination, have shown some degree of improvement in muscle mass, strength, and physical function in CKD but to our knowledge only one study [45] has focused on a baseline well-defined sarcopenic population that was established by consensual definitions. Furthermore, the majority of these studies were performed on a limited number of patients and for a short period of time (≤ 12 weeks). In a systematic review (16 clinical trials), progressive resistance training caused muscle hypertrophy (increased muscle size and quality) and improved lower body strength as well as certain dimensions of health-related quality of life [112]. A systematic review of RCT in 2015 established that the strongest evidence in dialysis patients were the effects of aerobic exercise on physical fitness, muscular strength, and quality of life [113]. More recently, in 2018, a meta-analysis of 11 RCT (362 patients) in CKD stages 3-4 reported an increased exercise tolerance with an average of a 35-week aerobic training program as compared to standard care [114]. The prescription of exercise in CKD patients is still the most corroborated intervention and could be effective in preventing and reversing sarcopenia, thus it should be a considered for all patients [58,115,116].
Further, early and individualized nutritional intervention is essential to prevent protein energy wasting [117] and as seen in the elderly, could probably improve sarcopenia in CKD patients. Nutrition status has been associated with lean mass index and handgrip strength in CKD patients [118]. Some RCTs have shown the beneficial impact of essential amino acids on lean body mass and handgrip strength. However, these study groups were restricted to few patients (20 patients) without sarcopenia and hypoalbuminemic at baseline. It remains unclear which concrete interventions are optimal to improve sarcopenia in this population.
Testosterone therapy for older men in the general population is associated with an increased lean body mass and improvement of handgrip strength [119]. In CKD, nandrolone decanoate and oxymetholone are the two main anabolic steroids studied in RCT. They were used for short periods (< 24 weeks) on a limited number of dialysis patients and have shown benefits on certain muscle parameters including functional performance. No data are available on the role of anabolic steroids in CKD without KRT and further studies are required to evaluate their benefits. In the same vane, conflicting data are reported concerning the improvement of lean body mass with growth hormone therapy in hypoalbuminemic CKD subjects. Overall, before common use of these hormonal therapies, more RCTs are needed to confirm the beneficial effects in CKD population in addition to the optimal dosage, duration of therapy, and therapeutic targets.
In summary, whether these therapeutic interventions are effective in a long-term manner, to what extent, at what cost, and their impact on specifically CKD-associated sarcopenia should be determined by further prospective studies.

Strengths and Limitations
One of the strengths of this review is the use of a recommended and rigorous methodology, entailing a systematic method, to provide an overview of the type and extent of research available, as well as identifying research gaps and making recommendations for the future research. All articles were assessed in a replicable and transparent way. To our knowledge, this is the first scoping review which provides a detailed overview of sarcopenia prevalence in the CKD population based on recent working groups' recommendations.
This scoping review also has certain limitations. Firstly, the literature was extremely vast and heterogeneous regarding sarcopenia in CKD individuals. Especially, the nomenclature used with the term "sarcopenia" and the absence of a worldwide accepted definition may have restricted the search process and led to exclude meaningful explorations of sarcopenia in CKD. Secondly, CKD population presents great heterogeneity and studies mostly failed to present detailed data for CKD without KRT (including kidney function and kidney injury markers). Further analysis on CKD subpopulations and CKD stages should be conducted in future. Thirdly, we only included papers published in the English language; it is possible that other potentially relevant studies may have been missed. Lastly, due to the scoping nature of the review, a quality assessment of the studies included was not undertaken.

Conclusion
This scoping review highlights that sarcopenia is prevalent in a significant proportion of individuals with CKD and increases with the severity of the disease. Prevalence estimates vary widely depending on the sarcopenia definition used and further research is needed to determine which is most effective in this population.
Considering the negative impact of sarcopenia upon important health outcomes in CKD population, there is a pressing need for the identification and development of preventative and therapeutic strategies. Especially, it remains to be fully determined whether specific exercise and nutritional interventions are effective to counteract sarcopenia in CKD.
Author Contributions VC and AT had the idea for the review. VC performed the literature search. MJB and MH verified data. AT resolved all discrepancies. VC and MJB performed the data analysis. VC, MJB, MH, and AT drafted and/or critically revised the manuscript, and approved the final version. AT had the primary responsibility for the manuscript's final content.