Proximal (Type 2) Renal Tubular Acidosis (pRTA)

Definition and EtiologyTop

Proximal renal tubular acidosis (pRTA), also known as type 2 renal tubular acidosis, can occur as an isolated defect in bicarbonate reabsorption or in association with other defects in proximal tubular function, leading to impaired reabsorption of other solutes, such as phosphate, glucose, uric acid, and amino acids. These patients usually have an isolated non–anion gap acidosis.

If acidosis develops in association with loss of other solutes, the patient may have hypophosphatemia, hypouricemia, renal glycosuria (with normal serum glucose concentrations), and/or aminoaciduria; this is termed Fanconi syndrome.

Isolated hereditary pRTA is rare and can be autosomal dominant or autosomal recessive. Rare sporadic cases have been reported. Acetazolamide and topiramate are two commonly reported drugs also known to cause isolated pRTA.

More commonly pRTA is found in conjunction with Fanconi syndrome, which may be caused by several genetic and acquired systemic diseases. Etiologies of pRTA and Fanconi syndrome: Table 1.

DiagnosisTop

Patients with pRTA develop an increase in their urinary pH following an alkali load. Their urine pH may rise from 5.5 at baseline to 7.5 when serum bicarbonate levels are normalized following the administration of sodium bicarbonate.

TreatmentTop

If renal tubular acidosis (RTA) persists following treatment of the underlying cause, it can be managed with supplemental doses of alkali therapy.

Treatment of patients with pRTA requires treatment of acidemia and rarely of hypokalemia and hypophosphatemia. Compared with patients with distal RTA, the requirement for alkali therapy is usually significantly higher (up to 10-15 mEq/kg/d) to exceed the urinary bicarbonate losses (although doses of about 2 mEq/kg/d may be sufficient, with 100 mEq translating into 8.4 g of oral sodium bicarbonate). This can be done with sodium bicarbonate, sodium citrate, or potassium citrate. It is not uncommon to use a combination of potassium-based and sodium-based alkali to ensure potassium replacement and compensate for the urinary potassium losses. These losses are related to increased water and salt delivery to the distal tubule and negatively charged urinary bicarbonate ions stimulating secretion of potassium into urine. Thiazide diuretics may be occasionally used, as inducing mild volume depletion leads to increased proximal sodium and bicarbonate reabsorption.

TablesTop