Sodium Retention and Intravascular Volume Status in Childhood Nephrotic Syndrome

Introduction: Overfill and underfill hypotheses have been posited to explain the development of sodium retention in children with nephrotic syndrome (NS). The clinical assessment of intravascular volume status during the oedematous phase of NS in children is challenging. We aimed to study the intravascular volume status in nephrotic children using urinary electrolyte indices and echocardiographic (echo) measurements of inferior vena cava (IVC) collapsibility and Aortic (Ao) diameter.

Methods: Prospective observational study. Twenty nephrotic children with oedema and ascites and not on any medications were enrolled. The intravascular volume status was assessed using urinary electrolyte indices [Fractional excretion of sodium (FeNa) and Urinary potassium index (UKI)] and echo IVC collapsibility index (IVCI) and ratio of IVC and Ao diameters (IVC/Ao). FeNa ≤1% with UKI <60% indicated primary sodium retention and with UKI >60% suggested secondary sodium retention due to intravascular hypovolemia.

Results: Out of 20 nephrotic children, 16 showed urinary sodium retention (FeNa ≤1%). Two out of these 16 children also had high UKI (>60%) indicative of secondary sodium retention. In the remaining 14 children out of 16, UKI was <60% indicative of primary sodium retention. None of the subjects had IVCI in hypovolemic range. Three subjects had IVC/Ao ratio in hypovolemic range and two of these had urinary indices indicative of secondary sodium retention due to hypovolemia.

Conclusion: Echocardiographic measurement of IVC/Ao ratio is useful for assessment of intravscaular volume status in children with nephrotic syndrome with oedema and ascites. Sodium retention during oedematous phase of NS in children is mainly due to primary sodium retention and is not associated with intravascular hypovolemia.