Renal Tubular Acidosis for the USMLE Step 1
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Renal Tubular Acidosis for the USMLE Step 1 Exam
- Definition:
- Renal tubular acidosis (RTA) is a group of disorders characterized by a defect in renal acid handling, resulting in non-anion gap hyperchloremic metabolic acidosis. The kidneys fail to properly excrete hydrogen ions (H⁺) or reabsorb bicarbonate (HCO₃⁻), leading to acidosis.
- Types of Renal Tubular Acidosis:
- Type 1 (Distal RTA):
- Inability to secrete H⁺ in the distal nephron, leading to failure to acidify urine (urine pH >5.5) despite metabolic acidosis.
- Etiology:
- Autoimmune diseases (e.g., Sjögren’s syndrome, systemic lupus erythematosus), genetic causes, medications (e.g., amphotericin B), or hypercalciuria.
- Clinical Features:
- Hypokalemia, hypercalciuria, nephrolithiasis, nephrocalcinosis, and bone demineralization (rickets or osteomalacia).
- Type 2 (Proximal RTA):
- Defective bicarbonate reabsorption in the proximal tubule, leading to excessive bicarbonate loss.
- Etiology:
- Fanconi syndrome, multiple myeloma, carbonic anhydrase inhibitors, heavy metal exposure (e.g., lead), vitamin D deficiency.
- Clinical Features:
- Hypokalemia, bone disease (rickets or osteomalacia), glucosuria, phosphaturia, aminoaciduria (Fanconi syndrome).
- Type 4 (Hyperkalemic RTA):
- Impaired aldosterone secretion or resistance leads to reduced potassium (K⁺) and hydrogen (H⁺) excretion in the distal nephron, causing hyperkalemia and mild acidosis.
- Etiology:
- Hypoaldosteronism (e.g., diabetic nephropathy, Addison’s disease), medications (e.g., ACE inhibitors, ARBs, NSAIDs), or interstitial nephritis.
- Clinical Features:
- Hyperkalemia, mild metabolic acidosis, usually no nephrolithiasis.
- Pathophysiology:
- In RTA, defects in renal acid excretion or bicarbonate reabsorption lead to systemic acidosis.
- Hypokalemia: Common in types 1 and 2 due to increased renal potassium excretion.
- Hyperkalemia: Seen in type 4 due to aldosterone deficiency or resistance, impairing K⁺ excretion.
- Bone Disease: Chronic acidosis causes bone demineralization, as the body buffers excess H⁺ by releasing calcium from bones, leading to rickets in children or osteomalacia in adults.
- Diagnosis:
- Blood Tests:
- Hyperchloremic metabolic acidosis (normal anion gap), decreased bicarbonate (HCO₃⁻), low potassium in types 1 and 2, high potassium in type 4.
- Urine pH:
- In distal RTA (type 1), urine pH remains >5.5 despite acidosis. In proximal RTA (type 2), urine pH is initially high, but becomes acidic (<5.5) as the filtered bicarbonate decreases.
- Urinary Anion Gap:
- A positive urinary anion gap (Na⁺ + K⁺ - Cl⁻) suggests reduced ammonium excretion, as seen in RTA.
- Management:
- Type 1 RTA:
- Sodium bicarbonate or potassium citrate to correct acidosis and prevent stone formation. Potassium supplements may be needed.
- Type 2 RTA:
- High-dose bicarbonate to counteract bicarbonate wasting, along with potassium supplements to address hypokalemia.
- Type 4 RTA:
- Treatment of hyperkalemia with potassium binders or diuretics. Fludrocortisone may be given in hypoaldosteronism to increase K⁺ excretion.
- Complications:
- Nephrolithiasis: Common in type 1 RTA due to hypercalciuria and alkaline urine.
- Bone Disease: Chronic acidosis leads to bone demineralization and increases the risk of rickets and osteomalacia.
- Chronic Kidney Disease (CKD): Prolonged acidosis and nephrocalcinosis can cause progressive renal dysfunction.
Key Points
- Renal tubular acidosis leads to hyperchloremic metabolic acidosis with a normal anion gap due to defective acid excretion or bicarbonate reabsorption.
- Type 1 (distal) RTA presents with alkaline urine, hypokalemia, and nephrolithiasis, while type 2 (proximal) RTA presents with bicarbonate loss and bone disease.
- Type 4 RTA involves hyperkalemia due to aldosterone deficiency or resistance.
- Diagnosis is based on metabolic acidosis, urine pH, and serum potassium levels.
- Treatment involves correcting acidosis with bicarbonate or citrate and managing potassium imbalances.