Renal · Year 2 · from Renal
Case 1: Type 1 (Distal) Renal Tubular Acidosis
Patient Presentation
A 35-year-old female with Sjogren syndrome presents for evaluation of recurrent kidney stones and chronic fatigue. She has had three calcium phosphate stones in the past 5 years.
History of Present Illness
- Recurrent nephrolithiasis since age 30
- Chronic fatigue and muscle weakness
- Dry eyes and dry mouth (on pilocarpine)
- No diarrhea or laxative use
- No carbonic anhydrase inhibitor use
Physical Examination
- Blood pressure: 118/74 mmHg
- Heart rate: 78 bpm
- Dry mucous membranes
- No peripheral edema
- Mild proximal muscle weakness
Workup
Laboratory Studies:
- Serum potassium: 2.9 mEq/L (low)
- Serum bicarbonate: 14 mEq/L (low)
- Serum chloride: 114 mEq/L (elevated)
- Anion gap: 10 (normal - hyperchloremic acidosis)
- Creatinine: 1.1 mg/dL
- Urine pH: 6.8 (inappropriately alkaline despite systemic acidosis)
- Urine anion gap: +15 mEq/L (positive - impaired NH4+ excretion)
- 24-hour urine citrate: 95 mg/day (low)
Imaging:
- CT abdomen: Bilateral nephrocalcinosis, 4 mm right renal pelvis stone
Diagnosis
Type 1 (Distal) Renal Tubular Acidosis Secondary to Sjogren Syndrome
Discussion
This case demonstrates distal RTA pathophysiology:
- Inability to Acidify Urine: The lecture explains that the hallmark of distal RTA is urine pH >5.5 despite systemic acidosis. The collecting duct H+-ATPase cannot secrete hydrogen ions effectively.
- Hypokalemia Mechanism: When H+ secretion is impaired, sodium that would normally be reabsorbed in exchange for H+ is instead exchanged for K+, causing potassium wasting.
- Nephrolithiasis and Nephrocalcinosis: Alkaline urine reduces citrate excretion (citrate is reabsorbed more in acidosis, but here the proximal tubule senses systemic acidosis while urine remains alkaline) and increases calcium phosphate precipitation. The lecture notes these as characteristic complications.
- Positive Urine Anion Gap: The formula (UNa + UK - UCl) is positive because ammonium excretion is impaired; normally NH4+ is excreted with Cl-, making the gap negative in acidosis.
- Autoimmune Etiology: Sjogren syndrome causes lymphocytic infiltration of the collecting duct, damaging intercalated cells.
Treatment
- Sodium bicarbonate or sodium citrate 1-2 mEq/kg/day in divided doses
- Potassium citrate (provides both K+ and alkali)
- Monitor for resolution of hypokalemia with alkali therapy
- Continue treatment for underlying Sjogren syndrome
- Increase fluid intake for stone prevention
Clinical Pearl
In distal RTA, the alkali requirement is relatively modest (1-3 mEq/kg/day) because the problem is impaired acid secretion, not bicarbonate loss. The administered alkali is retained. Always check urine pH - inability to acidify below 5.5 during metabolic acidosis is the diagnostic hallmark.