Type I-Distal RTA

• Classical form of RTA

• Failure of acid secretion by the alpha intercalated cells of the cortical collecting duct.

• Failure to acidify the urine to a pH of less than 5.5 even if the blood is too acidic

• Clinical features of dRTA:

  • Normal anion gap metabolic acidosis/acidemia

  • Hypokalemia (may be severe)

  • Urinary stone formation, nephrocalcinosis

  • Bone demineralisation (causing rickets in children and osteomalacia in adults)

• Diagnosis:

  • Urinary pH >5.3, systemic acidemia, serum bicarbonate < 20 mmol/l.

  • Ammonium chloride test: failure to acidify the urine following an oral acid loading challenge

  • More recently, an alternative test using furosemide and fludrocortisone has been described.

• Treatment:

  • Oral sodium bicarbonate or sodium citrate: correction of the acidemia.

  • Potassium citrate: correct hypokalemia and urinary stone formation and nephrocalcinosis by inhibiting calcium excretion.

Type II-Proximal RTA

• Failure of the proximal tubular cells to reabsorb filtered bicarbonate.

• Distal intercalated cells function normally, so the acidemia is less severe than type I. Urine can acidify to < 5.3.

• Hypokalemia, less severe than type I.

• Highly associated with Fanconi's syndrome (glycosuria and normal blood glucose).

• Treatment: oral bicarbonate, but may exacerbate urinary potassium losses and precipitate hypokalemia

Type III RTA

• Genetic defect in type 2 carbonic anhydrase (CA2) in both the proximal and distal tubular cells.

• Clinical features:

  • Proximal renal tubular acidosis

  • Distal renal tubular acidosis

  • Osteopetrosis

  • Cerebral calcification and subsequent mental impairment.

Type IV RTA (Hypoaldosteronism)

• Causes: hypoaldosteronism or aldosterone resistant.

• Mild (normal anion gap) metabolic acidosis, reduction in K+ secretion and distal tubular ammonium excretion.

• Cardinal feature: hyperkalemia, normal urinary acidification.

• Treatment: mineralocorticoid ( fludrocortisone), as well as possibly a glucocorticoid for cortisol deficiency, if present.

REFERENCES

• http://en.wikipedia.org/wiki/Renal_tubular_acidosis