Renal Tubular Acidosis for the USMLE Step 3
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Renal Tubular Acidosis for the USMLE Step 3
- Definition:
- Renal tubular acidosis (RTA) encompasses a group of disorders that impair the kidneys’ ability to regulate acid-base balance, leading to non-anion gap hyperchloremic metabolic acidosis. This results from defects in acid excretion or bicarbonate reabsorption in the renal tubules.
- Types of Renal Tubular Acidosis:
- Type 1 (Distal RTA):
- Impaired hydrogen ion (H⁺) secretion in the distal tubule leads to failure to acidify urine (urine pH >5.5), causing systemic acidosis.
- Etiology:
- Autoimmune diseases (e.g., Sjögren’s syndrome, lupus), hereditary defects, medications (e.g., amphotericin B), and hypercalciuria.
- Clinical Features:
- Hypokalemia, nephrolithiasis, nephrocalcinosis, bone demineralization (rickets/osteomalacia), and failure to thrive in children.
- Type 2 (Proximal RTA):
- Defective bicarbonate reabsorption in the proximal tubule causes bicarbonate loss, leading to acidosis. Urine pH is initially high (>5.5) but later becomes acidic as filtered bicarbonate is depleted.
- Etiology:
- Fanconi syndrome, multiple myeloma, carbonic anhydrase inhibitors (e.g., acetazolamide), heavy metals (e.g., lead), or vitamin D deficiency.
- Clinical Features:
- Hypokalemia, bone demineralization, glucosuria, phosphaturia, and aminoaciduria (Fanconi syndrome).
- Type 4 (Hyperkalemic RTA):
- Impaired aldosterone secretion or resistance leads to decreased potassium (K⁺) and hydrogen (H⁺) excretion in the distal tubule, resulting in hyperkalemia and mild acidosis.
- Etiology:
- Diabetic nephropathy, adrenal insufficiency, medications (e.g., ACE inhibitors, ARBs, NSAIDs), or interstitial nephritis.
- Clinical Features:
- Hyperkalemia, mild metabolic acidosis, and no nephrolithiasis.
- Pathophysiology:
- Type 1 (Distal RTA):
- The distal nephron fails to secrete H⁺, leading to acid retention and hypokalemia as potassium is excreted to balance acid retention. Chronic acidosis leads to bone demineralization and calcium phosphate stone formation.
- Type 2 (Proximal RTA):
- Proximal bicarbonate reabsorption is defective, leading to bicarbonaturia and systemic acidosis. Initially, urine pH is alkaline but becomes acidic once filtered bicarbonate decreases.
- Type 4 (Hyperkalemic RTA):
- Aldosterone deficiency or resistance impairs potassium excretion, causing hyperkalemia and decreasing ammonium production, which impairs acid excretion.
- Diagnosis:
- Blood Tests:
- Hyperchloremic metabolic acidosis with a normal anion gap, low bicarbonate (HCO₃⁻), and abnormal potassium levels (low in types 1 and 2, high in type 4).
- Urine pH:
- In type 1 RTA, urine pH remains >5.5 despite acidosis. In type 2 RTA, urine pH is initially high but becomes <5.5 as bicarbonate is depleted.
- Urinary Anion Gap:
- A positive urinary anion gap (Na⁺ + K⁺ - Cl⁻) indicates reduced ammonium excretion, supporting the diagnosis of RTA.
- Management:
- Type 1 RTA:
- Alkali therapy (sodium bicarbonate or potassium citrate) to correct acidosis. Potassium supplements are often needed.
- Type 2 RTA:
- High doses of bicarbonate to counteract bicarbonate wasting, with potassium supplements to address hypokalemia.
- Type 4 RTA:
- Treat hyperkalemia with potassium-lowering agents (e.g., diuretics) and fludrocortisone if aldosterone deficiency is present. Correct underlying causes such as diabetic nephropathy or adrenal insufficiency.
- Complications:
- Nephrolithiasis: Common in type 1 RTA due to alkaline urine and hypercalciuria.
- Bone Disease: Chronic acidosis leads to bone demineralization, causing rickets in children or osteomalacia in adults.
- Chronic Kidney Disease (CKD): Prolonged acidosis and nephrocalcinosis may lead to CKD.
Key Points
- Renal tubular acidosis is a disorder of renal acid-base regulation, leading to non-anion gap hyperchloremic metabolic acidosis.
- Type 1 (distal) RTA involves impaired H⁺ secretion, resulting in alkaline urine, hypokalemia, and nephrolithiasis. Type 2 (proximal) RTA leads to bicarbonate wasting and bone disease.
- Type 4 (hyperkalemic) RTA is associated with aldosterone deficiency or resistance, leading to hyperkalemia and mild acidosis.
- Diagnosis involves blood tests, urine pH measurement, and urinary anion gap analysis.
- Treatment includes correcting acidosis with alkali therapy and managing potassium imbalances.