Makindo Medical Notes

Related Subjects: |Metabolic acidosis |Aspirin or Salicylates toxicity |Ethylene glycol toxicity |Renal Tubular Acidosis |Lactic acidosis

🧪 Renal Tubular Acidosis (RTA) is characterized by the inability of the kidneys to excrete acidic urine, leading to a persistently high urinary pH >5.5 even under an acid load. It results in a normal anion gap (hyperchloremic) metabolic acidosis due to failure of renal acid excretion.

ℹ️ About

• The body is a net acid producer from metabolism (≈1 mmol/kg/day H⁺ ions).
• Normal plasma is slightly alkaline (pH ~7.40), while urine is normally acidic (pH 5–6).
• RTA arises from failure of the tubules to handle H⁺ or HCO₃⁻ appropriately.

🧬 Aetiology

• Defects in tubular transport mechanisms.
• Failure in:
  • Distal tubule H⁺ secretion (distal RTA).
  • Proximal tubule HCO₃⁻ reabsorption (proximal RTA).
• Leads to normal anion gap (hyperchloremic) metabolic acidosis.

📑 Types of RTA

• Type 1 (Distal RTA): Failure to excrete H⁺ ions → urine cannot be acidified.
• Type 2 (Proximal RTA): Failure to reabsorb HCO₃⁻.
• Type 3: Rare, mixed features of Types 1 & 2.
• Type 4: Hypoaldosteronism/hyporeninaemia → impaired H⁺ & K⁺ excretion → hyperkalemic RTA.

🩺 Clinical Features

• Type 1: Growth delay, nephrolithiasis, nephrocalcinosis, polydipsia, muscle weakness.
• Type 2: Growth delay, rickets/osteomalacia, Fanconi syndrome (glycosuria, aminoaciduria, phosphate wasting).
• Type 4: Often asymptomatic; may present with hyperkalemia in diabetics or interstitial nephritis.

🔎 Investigations

• U&E: Hypokalaemia (Types 1 & 2), Hyperkalaemia (Type 4).
• VBG: Normal anion gap metabolic acidosis (hyperchloremic).
• Urine pH: >5.5 in Type 1 (inability to acidify), variable in Type 2, normal/low in Type 4.

🧪 Diagnostic Tests

• Type 1: NaHCO₃ infusion → failure to generate urine–blood pCO₂ gradient.
• Type 2: NaHCO₃ loading test; check for Fanconi features.
• Type 4: NH₄⁺ loading test; confirms impaired distal acidification.

💊 Management

• Alkali therapy (oral bicarbonate or citrate) for Types 1 & 2.
• Potassium supplementation for hypokalaemic patients (Types 1 & 2).
• Vitamin D and phosphate for osteomalacia/rickets (Type 2).
• For Type 4: fludrocortisone (if hypoaldosteronism), loop/thiazide diuretics, potassium binders, alkali therapy.

💡 Exam Pearls:
• RTA = normal anion gap metabolic acidosis (hyperchloremic).
• Type 1 → renal stones/nephrocalcinosis, urine pH >5.5.
• Type 2 → Fanconi features, phosphate wasting.
• Type 4 → hyperkalaemia, common in diabetics on ACEi/ARB.

📚 References

• Renal Tubular Acidosis and Management Strategies – Narrative Review
• NIDDK – Renal Tubular Acidosis