Renal tubular acidosis (RTA) refers to a collection of renal transport defects in the reabsorption of bicarbonate or the secretion of hydrogen ion, or both, resulting in a hyperchloremic, non-anion gap metabolic acidosis. RTA may be an isolated disorder or one feature of a more complex disorder, disease, or syndrome. Because RTA can present with nonspecific symptoms such as failure to thrive, constipation, vomiting, and lethargy, recognizing, diagnosing, and managing these disorders is essential for inpatient management.
RTA is categorized into three types, depending on the location of the tubular dysfunction. Proximal, or type II, RTA refers to disorders involving bicarbonate reabsorption at the proximal tubule. Distal, or type I, RTA encompasses syndromes of defective hydrogen ion transport in the distal convoluted tubule. Type IV RTA also involves the distal tubule but is related to defects in the aldosterone response; it is often referred to as hyperkalemic RTA. What was once called type III RTA is now believed to be a combination of types I and II.
A normal functioning kidney maintains acid–base homeostatsis by two principle processes: reabsorption of filtered bicarbonate (HCO3−) at the proximal tubule and excretion of acid through ammonium (NH4+) at the distal nephron. Under normal circumstances, the kidney excretes acid at the same rate that metabolic processes generate acid; usually about 1 to 3 mEq/kg per day in children.1 In RTA, this acid-excreting mechanism is disrupted, resulting in a net surplus of acid. Calculating the blood anion gap should be done in patients presenting with acidosis. The equation for calculating the anion gap is as follows:
Anion gap = Serum sodium (Na+) − [Serum bicarbonate (HCO3−) + Serum chloride (CI−)]
The normal range of the anion gap is 12 ± 4 mEq/L.2
In RTA, the retained acid is in the form of hydrogen ions (H+) paired with chloride; therefore the serum chloride is elevated to the same degree that the serum bicarbonate is diminished.3 Accordingly, the anion gap remains within normal limits.
Renal acidification at the proximal tubule is driven by hydrogen (H+) secretion at the luminal membrane and bicarbonate reabsorption at the basolateral membrane (Figure 116-1). About 85% of tubular reabsorption of bicarbonate occurs in the proximal tubule. Therefore if there are any defects in this process, a large amount of bicarbonate is wasted in the urine, resulting in a net retention of acid, as in proximal type II RTA.4
Renal acidification at the proximal tubule. HCO3– reabsorption aided by acid (H+) secretion mediated by NHE-3: sodium hydrogen exchanger; CA II and CAIV: intracellular and intraluminal carbonic anhydrase.