Edema is one of the principal symptom of nephrotic syndrome (NS) and remains one of the main reason for admission to the hospital. There have been two theories explaining the development of edema in nephrotic syndrome (NS), namely the under- and overfill hypotheses.1,2 The pathogenetic mechanism of edema in the NS was traditionally considered to be the result of a series of events, postulates that a decrement in oncotic pressure leads to excess filtration of fluid into the interstitial compartment causing hypovolemia, secondary hyperaldosteronism and secondary renal sodium retention. However, this hypovolemia concept or underfill hypothesis, cannot explain all features of volume regulation and sodium reabsorption in the nephrotic syndrome.3–5 Children with edema associated with idiopathic NS (INS) do not commonly have intravascular volume depletion; actually several studies suggest that children with INS may have normal or even increased intravascular pressure.6,7 The overfill hypothesis now suggests that edema is a result of a primary renal defect in sodium excretion that is independent of low oncotic pressure and is responsible for primary extracellular volume expansion, which leads to secondary leakage of fluid toward the interstitium.8,9

While the mechanism of edema is a theoretical topic of discussion, understanding the intravascular volume status of the patients accurately is a crucial issue while planning the treatment of edema. However, a significant issue in the consideration of this problem is the lack of a gold standard for the assessment of intravascular volume.10 Several noninvasive techniques for the assessment of hydration status exist. Monitoring of relative blood volume changes, inferior vena cava diameter (IVCD) measurements and biochemical markers such as atrial atriuretic peptide (ANP), brain natriuretic peptide (BNP) are indirect methods, which provide information about the intravascular filling. Although dilution methods are accepted as the most appropriate standards for fluid volume determination, they cannot be applied in routine clinical practice due to the patient availability for many hours and time-consuming assays of blood samples.10–12 In recent years, Bioelectrical impedance analysis (BIA) has found a place in the literature as a practical method for measuring body fluids particularly for patients on hemodialysis due to its advantages such as ease of application, non-invasiveness, and reproducibility.12–17

The present study aimed to determine the volume status using multifrequency BIA, NT-proBNP levels, IVC, LAD measurements to examine the relationship between various volume indicators and to investigate the utility of BIA in evaluating the volume status in children with INS.

Nineteen children (11 males, 8 females) with NS at debut or relapse admitted to our nephrology unit were prospectively studied from September 2012 to October 2013. The age range was 1–11 years (mean±SD, 5.13±2.53 years). Nephrotic syndrome was defined as severe proteinuria over 40mg/m2/h, hypoalbuminemia (<2.5g/dL), and edema. Relapse was defined as three consecutive days with plus two or more on urinary albumin dipstick. Remission was achieved when urinary dipstick was negative for protein for three consecutive days. Patients were classified as steroid resistant when remission was not achieved with prednisone therapy at 60mg/m2/day for 4 weeks. All patients were treated with prednisone, per International Study of Kidney Disease in Children regimen18 (started after routine workup for new NS patients and immediately upon admission in relapsed NS patients). Patients were excluded from the study if they met any of the following criteria: established cardiovascular disease, evidence of secondary NS and specific glomerulonephritis, usage of steroids or immunosuppressive drugs during the previous six months.

Twenty-five age and sex matched healthy volunteers served as the control group. These subjects had normal blood pressure (BP), no clinical electrocardiographic (ECG) or echocardiographic evidence of cardiovascular disease, and no evidence of pulmonary disease.

In this study, we observed that neither debut nor relapse nephrotic patients had symptoms of hypovolemia. Also, we found that plasma renin activities, aldosterone levels were not different between groups indicating that edema did not develop as a result of activation of renin–angiotensin–aldosterone (RAA) with a hypovolemic contributing factor. Moreover, our study showed that VCI index, left atrial diameter measured in children with debut or relapse NS were not significantly different than the controls whereas NT-proBNP values were higher than the remission and control groups. Lastly, by using BIA we demostrated that TBW values were increased in association with high ECW but without any change in ICW in patients with NS during attack.

Several methods including clinical signs and symptoms, neurohumoral hormone assays, blood volume measurement with radioactive labeling techniques and IVCD measurements have been utilized in NS patients to assess the hydration status.6,7,22–33 Consistent to several recent studies, during the debute or relapse period, the symptoms of hypovolemia including tachycardia, pallor or abdominal pain were not observed in our study.23,25,28 Plasma renin and aldosterone levels of patients with NS have been investigated widely in previous studies. Elevated renin or aldosterone levels have only been confirmed in about half of studied nephrotic subjects10,22,27,30,31 while some other studies have noted normal or lower values.5,23,31 In our study, the sodium reabsorption was significantly decreased in Group I patients compared to Group II and III. Lower FENa+ values in Group I patients than other two groups, despite the unsignificant difference in terms of plasma renin and aldosterone levels between groups may indicate the role of intrarenal mechanisms in sodium reabsorption in children with INS. Besides renin and aldosterone, we investigated the role of NT-proBNP in determining the volume status of children with NS. NT-proBNP was high in the debut/relapse group during disease. Higher concentration of NT-proBNP in the debut/relapse group suggests the overfill blood volume in these patients.

Previous studies investigating the central volume status by measurements of IVC and left atrial diameter by echocardiography yielded conflicting results in children with INS.23–25,28,34 Recently, Gurgoze et al.,23 reported left atrial diameter and IVCCI values were similar between patients with and without edema and concluded that patients with NS tended to be normovolemic instead of being hypovolemic. Similarly our data show no significant difference between the groups in terms of IVC index and left atrial diameter but a tendency toward an increased IVC inspiratory diameter/BSA ratio was seen in Group I compared with the group III. Our data showed no significant correlation between IVC diameter and NT-proBNP and fluid volume by BIA.

The use of IVC as a marker of central blood volume filling should be researched in a larger patient group. By the way, this method can have some limitations. That means for example, the toddlers are less likely to lie still during the ultrasound tests. It causes most probably undesired biases for measurements. Since the toddlers cry, intra-thoracic and intra-abdominal pressure will obviously increase. Therefore it will show some critical deviances in the IVC diameter measurements. These limitations may underlie the discordance between the current results and the previously reported of echocardiographic IVC measurements24,25,28,34. Point-of-care ultrasound of IVC was not reliable enough to be used as an independent screening method for volume overload to assess younger children.

Although fluid management is based primarily on subjective clinical assessment, clinicians need noninvasive bedside tools to provide a more objective assessment of fluid status. Multifrequency bioimpedance measurement has been claimed to be a reliable noninvasive technology for estimating body water compartments in adults.16,35 Its usage in pediatric patients’ has also been advocated.23,34,36–40 The use of relative overhydration (Rel OH) allows for hydration status comparison between all patient groups including children. Two pediatric abstracts showed the utility of BIA in children older than 2 years of age. One of them validated and established fluid and body composition reference values in 430 healthy children; the other one showed a very similar total body water content estimate by BIA and by the Morgenstern equation, which is the only equation for TBW assessment validated in children on dialysis.38,40

As in the latter study, we observed increased TBW in patients with debut or relapse NS compared to healthy controls in association with increased ECW but without any change in ICW. Previously, Gurgoze et al.23 calculated TBW by measuring resistance and reactance values using a 50kHz BIA device and TBW was found to be markedly higher in patients with edema. Despite the methodological differences between the two studies, patients with NS tended to be hypervolemic instead of being hypovolemic and the increase in TBW is likely associated with increased ECW without any change in ICW. Although BIA was found to be reliable and practical method in pediatric patients on hemodialysis, there is scarce data regarding the usefulness of BIA to evaluate volume status in children with nephrotic syndrome. More recently, Özdemir et al.34 evaluated the clinical findings, echocardiographic measurements (IVC index and IVC collapsibility index) and BIA to determine the volume status of 34 children with NS and controls. The authours concluded that BIS may be a superior technique to echocardiography since the sensitivity and specificity of BIA is relatively higher.

Natriuretic peptides play a key role in regulation of blood pressure and volume homeostasis due to their natriuretic/diuretic and vasodilatory actions.41 Several previous studies have shown the strong relationship between natriuretic peptides and left ventricular hypertrophy, systolic dysfunction but most of the patients included in these studies were prevalent dialysis patients.42,43 We preferred the use of NT-proBNP due to its in vitro stability appropriate for routine clinical use and longer half-life of BNP compared to ANP. We found that NT-proBNP level in Group I was significantly higher compared to Group II and Group III. NT-proBNP levels waned during remission, supporting the volume expansion with NS. In the study by Andersen et al.24 higher ANP and NT-proBNP levels were found in children with aldosterone-suppressed group compared to remission groups suggesting possible overfill blood volume.

To date, however there has been no study evaluating the relationship among NT-proBNP, echocardiography measurements and BIA parameters in children with NS. We did not find any correlation between the parameters of BIA (OH, Rel OH, TBW/BSA, ECW/BSA, ECW/ICW, ECW/TBW) and NT-proBNP in Group I and Group II but positive correlations was observed between ECW/BSA, ECW/ICW and ECW/TBW and NT-proBNP in Group III. These findings may be explained by the renal unresponsiveness to the natriuretic peptides which was previously proposed in NS patients.44,45 Despite their increased levels in NS and other edematous conditions, the underlying renal unresponsiveness to natriuretic peptides include; decreased renal perfusion pressure, RAAS, increased levels of antagonist hormones such as endothelin and AVP, down-regulation in natriuretic peptide receptors, increased breakdown by endopeptidases and release of less active forms.45 The presence of correlation between BIA values and NT-proBNP only in the control group and not in the patients groups may indicate the alteration of the relationship between NT-proBNP and hydration status. The present study has not been specifically designed to explain this observation; however, down-regulation of natriuretic peptide receptors or increased breakdown by neutral endopeptidases may play a role in renal unresponsiveness.

One of this study's weakness is the fact that the subgroup of patients with NS and remission is very small and the lack of long-term, longitudinal-serial measurements. The other limitation was that the sodium intake could not be standardized in these patients studied just at debut or relapse. This may contribute the substantial variation on urinary sodium excretion. It is also important to note that our data represent a single center report.

In conclusion, our results indicate that renin, aldosterone, VCI index, left atrial diameter measured in children with debut or relapse NS were not significantly different than the controls whereas NT-proBNP values were higher than the remission and control groups. Increased TBW values, in association with high ECW but without any change in ICW, indicates the lack of hypovolemia in children with NS. Since BIA provides results immediately, it might be an additional tool for everyday care of difficult patients with NS, whereas natriuretic peptides results are not available in a fast manner most of the time. Further studies are needed to evaluate the utility of BIA at the follow-up children with NS.

This study was supported by Ondokuz Mayis University Department of Scientific Research Projects. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee at which the studies were conducted (PYO. TIP.1901.12.019) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

The authors have declared that no conflict of interest exists.