Monday, 8 November 2010

MRCP revision battle 43.3: Renal tubular acidosis

Renal tubular acidosis is a condition caused by the kidneys failing to correctly acidify the urine.

All types of renal tubular acidosis are associated with:
  • hyperchloraemic metabolic acidosis
  • normal anion gap

The three main types of renal tubular acidosis are:

Type 1 renal tubular acidosis: Distal

This is due to the kidney not excreting hydrogen ions in the distal tubule

  • idiopathic
  • SLE/RA
  • hypercalcaemia
  • drugs: lithium, amphotericin
  • ricketts
  • growth failure
  • nephrocalcinosis
  • renal calculi - calcium phosphate stones
  • low potassium

Diagnosis is by oral acid load with ammonium chloride - the urine should acidify, but in type 1 renal tubular acidosis urine pH will remain >5.5

Treatment is with oral bicarbonate

Type 2 renal tubular acidosis: proximal

This is  due to the kidneys failing to reabsorb bicarbonate in the proximal tubule

Causes include:
  • Wilson's syndrome
  • Fanconi syndrome
  • cystinosis
  • myeloma
  • interstitial nephritis
  • drugs - lead, acetazolamide, old tetracycline

Complications include:
  • osteomalacia
  • low potassium

Diagnosis is by IV bicarbonate loading , which will result in a high fractional excretion of bicarb.

Treatment is with oral bicarbonate.

Type 4 renal tubular acidosis

This is caused by hypoaldosteronism, resulting in hyperkalaemia.

Treatment is to treat cause and control hyperkalaemia

The astute amongst you may have noted the lack of a type 3 - this is because those clever renal physicians decided that, upon reflection, what they had named type 3 was probably just a combination of types 1 and 2.

If you had to boil the above battle down to a set of key facts, I'd go with:
  • all cause hyperchloraemic metabolic acidosis with a normal anion gap
  • types 1 and 2 both cause hypokalaemia and are both treated with oral bicarb
  • type 3 causes a hyperkalaemia and is treated by treating the cause

Now on to a symptom-based battle...