Renal tubular acidosis

• This is a metabolic acidosis due to impaired acid secretion by the kidney
• There is a hyperchoraemic metabolic acidosis with normal anion gap

Type I

• Distal RTA due to inability the excrete H+ ions and generate acidic urine in the distal tubule, even in states of severe acidosis
• It may complicate many renal disorders
• Features include;
  • Rickets or osteomalacia due to buffering of H+ with calcium in bone
  • Nephrocalcinosis with renal calculi
  • This is due to a combination of hypercalciuria from bone, reduced urinary citrate (which inhibits calcium phosphate precipitation) which is reabsopbed to act as a buffer for H+ and an alkaline urine
  • It is also associated with hypokalaemia as there isn’t H+ to exchange for Na so K+ is exchanged instead
• Causes;
  • Idiopathic
  • Genetic;
    • Ehlers-Danlos
    • Marfan’s
  • Autoimmune disease;
    • SLE
    • Sjogren’s
    • Autoimmune hepatitis
  • Nephrocalcinosis
    • Hypercalcaemia
    • Medullary sponge kidney
  • Tubulointerstitial disease
    • Chronic pyelonephritis
    • Chronic interstitial nephritis
    • Obstructive uropathy
    • Renal transplant rejection
  • Drugs;
    • Lithium
    • Amphotericin
• Diagnosis
  • Acid load – oral ammonium chloride – there is a failure to acidify the urine beyond pH 5.5
• Treatment is with oral sodium bicarbonate or citrate

Type II

• Proximal RTA due to a defect in bicarbonate reabsorption in the proximal tubule resulting in excess bicarbonate in the urine
• The tubules are able to reabsorb some bicarbonate so can acidify urine during systemic acidosis
• It is rarer than type I
• May be an isolated thing or may be associated with Fanconi’s anaemia
• Hypokalaemia is common – the bicarbonate of the urine causes and osmotic diuresis causing increased flow rate to the distal tubule and therefore increased K+ excretion
• Causes;
  • Idiopathic
  • Fanconi’s syndrome
  • Tubulointerstitial disease
    • Myeloma
    • Interstitial nephritis
  • Drugs
    • Lead and other heavy metals
    • Acetazolamine
    • Out of date tetracycline
• Diagnosis
  • Give an IV sodium bicarbonate load – high excretion in the urine
• Treatment is with high doses of bicarbonate

Type III

• Very rare – combination of Type I and Type II

Type IV

• Due to hyporeninaemic hypoaldosteronism
• Hypoaldosteronism causes hyperkalaemia and acidosis (reduced K+ and H+ excretion)
• Causes;
  • Mild renal impairment
  • Causes of hypoaldosteronism
  • Drugs;
    • K+ sparing diuretics
    • NSAIDs
    • ACEI/ARB

Fanconi’s syndrome

• May be genetic or acquired – heavy metal poisoning, drugs, renal diseases
• Disturbance of the proximal tubule which does all the reabsorbing so there is defective reabsorption of;
  • Amino acids
  • Potassium
  • Phosphate (leading to hypophosphataemic rickets and osteomalacia)
  • Glucose (glucosuria)
  • Bicarbonate (Type II RTA)