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Renal Tubular Acidosis

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Overview

Renal tubular acidosis (RTA) describes a group of disorders leading to hyperchloraemic metabolic acidosis with a normal ion gap. There are 4 types:

  • Type I (classic distal) RTA
  • Type II (proximal) RTA
  • Type III (combined proximal and distal) RTA – rare
  • Type IV (hyperkalaemic distal) RTA

RTA can also occur due to CKD progressing.

Epidemiology

  • All forms of RTA are generally uncommon, but RTA is likely under-reported

Type 1 (Distal) RTA

Overview

Type 1 RTA occurs due to a failure of H+ secretion into the nephron lumen by cells in the distal nephron. An inability of H+ secretion also means K+ cannot be reabsorbed and is lost, therefore H+ is retained (leading to acidosis) and hypokalaemia may occur.

Causes and associations

Some causes and associations include:

Features

Some features include:

  • Features of hypokalaemia (e.g. arrhythmia)
  • Nephrocalcinosis – deposition of calcium in the kidneys
  • Renal stones – due to higher urinary pH (calcium tends to deposit at higher pHs)
  • Rickets/osteomalacia – chronic acidosis leeches calcium out of bones

Investigations

Investigations may show:

  • Hypokalaemia
  • Hyperchloraemic metabolic acidosis with a normal ion gap
  • Elevated urinary pH
  • Hypercalciuria

Type 2 (Proximal) RTA

Overview

Proximal RTA occurs due to cells in the proximal nephron tubule failing to reabsorb bicarbonate ions from the urine. This leads to a loss of bicarbonate and acidosis as bicarbonate normally acts as a buffer in the blood.

Potassium ions bind to bicarbonate and are lost in the urine along with phosphate.

Causes and associations

Some causes and associations include:

  • Idiopathic
  • Fanconi’s syndrome

Features

  • Hypokalaemia
  • Rickets/osteomalacia – due to acidosis and phosphate loss

Investigations

Investigations may show:

  • Hypokalaemia
  • Hyperchloraemic metabolic acidosis with a normal ion gap

Type 3 (Combined Proximal and Distal) RTA

Overview

Type 3 RTA is extremely rare and occurs due to a deficiency in carbonic anhydrase II, which is essential for converting CO2 and H2O into bicarbonate (HCO3–).

Causes and associations

  • Some causes and associations include:
  • Inherited
  • Iatrogenic (e.g. acetazolamide, a carbonic anhydrase inhibitor)

Features

  • Osteopetrosis – abnormally overly dense and brittle bones
  • Cerebral calcification

Investigations

Investigations may show:

  • Hypokalaemia
  • Hyperchloraemic metabolic acidosis with a normal ion gap

Type 4 (Hyperkalaemic) RTA

Overview

Type 4 RTA is technically not an intrinsic tubular disorder. It occurs when hypoaldosteronism is present, leading to reduced proximal tubular ammonium excretion. Aldosterone normally leads to water and Na+ retention and excretes sodium. Therefore, in this type of RTA, patients may have hyperkalaemia instead of hypokalemia.

Causes and associations

Some causes and associations include:

  • Hypoaldosteronism (e.g. Addison’s disease)
  • Drugs (e.g. ACE inhibitors, aldosterone antagonists such as spironolactone, NSAIDs)
  • Diabetes mellitus

Investigations

  • Hyperkalaemia
  • Normal urinary pH – acidification of urine is unaffected

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