Nephrotic syndrome complicated by idiopathic central diabetes insipidus

Perspectives on edema in childhood nephrotic syndrome

There have been two major theories surrounding the development of edema in nephrotic syndrome (NS), namely, the under- and overfill hypotheses. Edema is one of the cardinal features of NS and remains one of the principal reasons for admission of children to the hospital. Recently, the discovery that proteases in the glomerular filtrate of patients with NS are activating the epithelial sodium channel (ENaC), resulting in intrarenal salt retention and thereby contributing to edema, might suggest that targeting ENaC with amiloride might be a suitable strategy to manage the edema of NS. Other potential agents, particularly urearetics and aquaretics, might also prove useful in NS. Recent evidence also suggests that there may be other areas involved in salt storage, especially the skin, and it will be intriguing to study the implications of this in NS.

Draining the edema: a new role for aquaretics?

Investigations into edema formation in nephrotic syndrome have mostly focused on the primary role of sodium. While there is controversy about whether sodium retention is an inherent aspect of nephrotic syndrome (overfill hypothesis) or a secondary consequence (underfill hypothesis), the critical role of sodium in driving fluid retention is generally accepted. Consequently, treatment of edema is based on enhancing renal sodium excretion, using saluretics to block tubular reabsorption of sodium. However, there is also evidence of renal water retention: urine in nephrotic patients is typically highly concentrated (unless urinary concentrating ability is impaired by loop diuretics), and vasopressin levels are commonly elevated. Consequently, aquaretics, i.e., drugs that inhibit renal water reabsorption, may constitute effective treatments for nephrotic edema. In fact, these drugs are already approved for the treatment of non-nephrotic edematous states, such as those encountered in congestive heart or liver failure. In this edition of Pediatric Nephrology, two case reports raise the possibility that aquaretics may also be helpful in the treatment of nephrotic edema. These case reports provide no solid evidence for such treatment, and there clearly are serious concerns about inducing critical hypovolemia with potentially catastrophically consequences, such as thrombosis and shock. Yet these concerns similarly apply to saluretics, which clinicians routinely use in the treatment of edema. In addition, the described powerful effect of aquaretics with respect to the resolution of edema, as well as our understanding of the underlying physiology, argue for a more systematic, yet careful assessment of these drugs in the treatment of nephrotic syndrome.