Renal Tubular Acidosis (RTA)

Key Features

• Three subtypes of RTA are recognized

  • Type I or distal RTA: the classic form

  • Type II or proximal RTA: the bicarbonate-wasting form

  • Type III, or hyperkalemic RTA (rare in children): associated with hyporeninemic hypoaldosteronism or inherited in an autosomal manner

• Types I and II and their variants are encountered most frequently in children

• Type III is described historically as a combination of types I and II

Clinical Findings

• Type 1 (distal RTA)

  • Failure to thrive, anorexia, vomiting, and dehydration

  • Hyperchloremic metabolic acidosis, hypokalemia, and a urinary pH exceeding 6.5 are found

  • Concomitant hypercalciuria may lead to nephrocalcinosis, nephrolithiasis, and renal failure

  • Some forms are associated with hearing loss

  • Distal RTA is often permanent

• Type II (proximal RTA)

  • Characterized by failure to reabsorb bicarbonate appropriately in the proximal tubule with associated reduced serum bicarbonate concentration and normal anion gap hyperchloremic metabolic acidosis

  • Once a steady state is reached, the intact distal nephron appropriately excretes hydrogen ion, leading to a low urine pH

  • In the newborn can be considered an aspect of renal immaturity that improves with increasing gestational age

  • In infants is accompanied by failure to thrive and sometimes hypokalemia

  • Secondary forms result from reflux or obstructive uropathy or occur in association with other tubular disorders

Diagnosis

• Diagnostic findings include

  • Normal anion gap

  • Hyperchloremic metabolic acidosis in the absence of diarrhea

  • Intravascular volume depletion

• A concomitant urine pH is helpful in many cases; it is elevated in distal RTA despite the metabolic acidosis

• The finding of hypophosphatemia or glycosuria should lead to further investigation of proximal tubular function (eg, Fanconi syndrome that is also associated with aminoaciduria)

• A renal ultrasound should be obtained to exclude

  • Urinary tract obstruction (which can be seen with either proximal or distal RTA)

  • Nephrocalcinosis (seen in distal RTA)

Treatment

• Citrate or bicarbonate supplementation is provided to target a serum bicarbonate level of 20–24 mEq/L, as an index of normal serum pH

• Citrate solutions are more effective and often better tolerated than sodium bicarbonate

• Due to bicarbonate wasting, children with proximal RTA typically require 5–20 mEq/kg/d citrate to achieve normal serum pH and bicarbonate concentration