Renal Tubular Acidosis for the USMLE Step 2
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Renal Tubular Acidosis for the USMLE Step 2 Exam
- Definition:
- Renal tubular acidosis (RTA) is a group of disorders involving impaired acid excretion or bicarbonate reabsorption in the renal tubules, leading to non-anion gap hyperchloremic metabolic acidosis. RTA can occur at various sites in the nephron, classified into different types based on the underlying defect.
- Types of Renal Tubular Acidosis:
- Type 1 (Distal RTA):
- Impaired hydrogen ion (H⁺) secretion in the distal tubule results in the inability to acidify the urine (urine pH >5.5), leading to systemic acidosis.
- Etiology:
- Autoimmune diseases (e.g., Sjögren’s syndrome, lupus), medications (e.g., amphotericin B), hypercalciuria, or hereditary causes.
- Clinical Features:
- Hypokalemia, nephrolithiasis, nephrocalcinosis, bone demineralization (rickets/osteomalacia), failure to thrive in children.
- Type 2 (Proximal RTA):
- Defective bicarbonate reabsorption in the proximal tubule causes excessive bicarbonate loss in urine, leading to systemic acidosis. Urine pH is initially high (>5.5) but becomes acidic as bicarbonate levels fall.
- Etiology:
- Fanconi syndrome, multiple myeloma, carbonic anhydrase inhibitors, heavy metals (e.g., lead), or vitamin D deficiency.
- Clinical Features:
- Hypokalemia, bone disease, glucosuria, phosphaturia, and aminoaciduria (Fanconi syndrome).
- Type 4 (Hyperkalemic RTA):
- Impaired aldosterone secretion or resistance causes reduced potassium (K⁺) and hydrogen (H⁺) excretion in the distal tubule, leading to hyperkalemia and mild acidosis.
- Etiology:
- Diabetic nephropathy, adrenal insufficiency, medications (e.g., ACE inhibitors, ARBs, NSAIDs), or interstitial nephritis.
- Clinical Features:
- Hyperkalemia, mild acidosis, and usually no nephrolithiasis.
- Pathophysiology:
- Type 1 (Distal RTA):
- The distal nephron fails to excrete H⁺, leading to acid retention. Hypokalemia occurs as potassium is excreted to balance acid retention. Alkaline urine promotes calcium phosphate stone formation.
- Type 2 (Proximal RTA):
- The proximal nephron loses bicarbonate due to defective reabsorption, leading to systemic acidosis. Initially, urine pH is alkaline due to bicarbonaturia, but later becomes acidic.
- Type 4 (Hyperkalemic RTA):
- Reduced aldosterone activity impairs potassium excretion, causing hyperkalemia, which in turn decreases ammonium production and acid excretion.
- Diagnosis:
- Blood Tests:
- Show hyperchloremic metabolic acidosis with a normal anion gap and abnormal serum potassium (low in types 1 and 2, high in type 4).
- Urine pH:
- In type 1 RTA, urine pH remains >5.5 despite systemic acidosis. In type 2, urine pH initially exceeds 5.5 but later becomes acidic (<5.5) after bicarbonate loss.
- Urinary Anion Gap:
- A positive urinary anion gap (Na⁺ + K⁺ - Cl⁻) suggests impaired ammonium excretion, seen in RTA.
- Management:
- Type 1 RTA:
- Alkali replacement with sodium bicarbonate or potassium citrate to correct acidosis. Potassium supplementation may be required.
- Type 2 RTA:
- Large doses of bicarbonate to offset bicarbonate wasting, along with potassium supplements to address hypokalemia.
- Type 4 RTA:
- Treatment of hyperkalemia with potassium-lowering agents (e.g., diuretics) and addressing the underlying cause. Fludrocortisone may be used in cases of aldosterone deficiency.
- 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 and osteomalacia in adults.
- Chronic Kidney Disease (CKD): Prolonged acidosis and nephrocalcinosis can lead to progressive renal dysfunction.
Key Points
- Renal tubular acidosis is characterized by non-anion gap metabolic acidosis due to defects in acid excretion or bicarbonate reabsorption.
- Type 1 RTA involves defective distal H⁺ secretion, leading to alkaline urine and hypokalemia, while type 2 involves proximal bicarbonate wasting.
- Type 4 RTA is associated with hyperkalemia due to impaired aldosterone activity.
- Diagnosis is made with blood tests, urine pH measurement, and urinary anion gap calculation.
- Management involves correcting acidosis with alkali therapy and managing potassium imbalances depending on the type.