# Clinical Cases: Renal Acid-Base Regulation ## 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. --- ## Case 2: Type 2 (Proximal) Renal Tubular Acidosis in Multiple Myeloma ### Patient Presentation A 67-year-old male recently diagnosed with multiple myeloma presents with bone pain, fatigue, and laboratory abnormalities detected during oncology workup. ### History of Present Illness - Diagnosed with IgG kappa multiple myeloma 2 weeks ago - Progressive fatigue and lower back pain - 12-pound weight loss over 3 months - No vomiting or diarrhea - No diuretic or acetazolamide use ### Physical Examination - Blood pressure: 132/78 mmHg - Pallor - Point tenderness over lumbar spine - No hepatosplenomegaly ### Workup **Laboratory Studies:** - Serum potassium: 3.1 mEq/L (low) - Serum bicarbonate: 16 mEq/L (low) - Anion gap: 11 (normal) - Serum phosphorus: 2.0 mg/dL (low) - Serum uric acid: 2.2 mg/dL (low) - Serum glucose: 98 mg/dL (normal) - Urine pH: 5.2 (acidic - can acidify once threshold reached) - Urine glucose: 2+ (despite normal serum glucose) - Urine amino acids: Elevated (generalized aminoaciduria) - Fractional excretion of bicarbonate (after bicarbonate load): 18% - Serum free light chains: Kappa markedly elevated ### Diagnosis **Type 2 (Proximal) Renal Tubular Acidosis with Fanconi Syndrome Secondary to Light Chain Deposition** ### Discussion This case illustrates proximal RTA and Fanconi syndrome: - **Reduced Bicarbonate Threshold**: The lecture describes how proximal RTA results from impaired bicarbonate reabsorption. Plasma bicarbonate falls to a new, lower threshold (typically 15-18 mEq/L) where the reduced filtered load can finally be reclaimed. - **Acidic Urine pH**: Unlike distal RTA, the urine can be appropriately acidified once the bicarbonate threshold is reached. This patient's urine pH of 5.2 demonstrates intact distal acidification. - **Fanconi Syndrome Components**: Light chain deposition damages the proximal tubule globally, causing glucosuria at normal glucose levels, aminoaciduria, phosphaturia (hypophosphatemia), and uricosuria (hypouricemia) in addition to bicarbonaturia. - **Fractional Excretion of Bicarbonate**: The elevated FE-HCO3 >15% (after raising serum bicarbonate above the threshold) confirms proximal tubule bicarbonate wasting. - **Hypokalemia Mechanism**: Bicarbonaturia creates a non-reabsorbable anion in the collecting duct that enhances potassium secretion. ### Treatment - Chemotherapy for underlying myeloma (bortezomib-based regimen) - High-dose bicarbonate replacement (10-15 mEq/kg/day needed because most is lost in urine) - Potassium supplementation - Phosphate supplementation for hypophosphatemia - Thiazide diuretics may help by inducing mild volume contraction that enhances proximal reabsorption ### Clinical Pearl Proximal RTA requires much higher alkali doses than distal RTA because administered bicarbonate is lost in the urine until levels exceed the lowered threshold. The condition is self-limiting - bicarbonate stabilizes at 15-18 mEq/L without treatment. Associated Fanconi syndrome features (glucosuria with normal glucose, hypophosphatemia, hypouricemia) help identify the proximal tubule as the problem site. --- ## Case 3: Metabolic Alkalosis from Vomiting ### Patient Presentation A 28-year-old female presents to the emergency department with weakness, dizziness, and muscle cramps. Her roommate reports she has been vomiting frequently for several weeks and seems preoccupied with her weight. ### History of Present Illness - Self-induced vomiting after meals for 3 months - Significant weight loss (15 pounds) - Progressive weakness and lightheadedness - Muscle cramps and tingling in hands - Constipation ### Physical Examination - Blood pressure: 96/58 mmHg supine, 78/50 standing - Heart rate: 102 bpm supine, 124 standing - Dry mucous membranes, poor skin turgor - Dental enamel erosions - Positive Chvostek sign - Calluses on knuckles (Russell sign) ### Workup **Laboratory Studies:** - Serum potassium: 2.4 mEq/L (severely low) - Serum bicarbonate: 38 mEq/L (elevated) - Serum chloride: 84 mEq/L (low) - BUN: 32 mg/dL (elevated) - Creatinine: 1.3 mg/dL - Calcium: 8.2 mg/dL - Urine chloride: 8 mEq/L (low - chloride responsive) - Urine pH: 5.5 (paradoxically acidic) ### Diagnosis **Chloride-Responsive Metabolic Alkalosis with Severe Hypokalemia from Purging Behavior** ### Discussion This case demonstrates the generation and maintenance of metabolic alkalosis: - **Generation vs. Maintenance**: The lecture emphasizes this distinction. Vomiting generates alkalosis through HCl loss, but the kidney should excrete excess bicarbonate. Maintenance factors prevent this correction. - **Maintenance Factors**: Volume depletion activates angiotensin II, stimulating proximal bicarbonate reabsorption. Chloride depletion limits the chloride-bicarbonate exchange needed to excrete bicarbonate. Hypokalemia increases hydrogen ion secretion and ammoniagenesis. - **Paradoxical Aciduria**: Despite systemic alkalosis, urine pH is acidic because the kidney continues to secrete hydrogen ions due to the maintaining factors. This paradoxical aciduria indicates chloride-responsive alkalosis. - **Low Urine Chloride**: The lecture notes that urine chloride <20 mEq/L identifies chloride-responsive alkalosis (vomiting, NG suction, prior diuretics). Chloride-resistant forms (hyperaldosteronism) have urine chloride >20 mEq/L. - **Hypokalemia Mechanism**: Vomiting causes minimal direct potassium loss. The hypokalemia results from renal potassium wasting driven by the alkalosis and secondary hyperaldosteronism. ### Treatment - IV normal saline for volume and chloride repletion - IV potassium chloride replacement (severe hypokalemia) - Monitor electrolytes closely - Psychiatry consultation for eating disorder - Once stable, oral potassium chloride supplementation - Nutritional rehabilitation ### Clinical Pearl In metabolic alkalosis from vomiting, urine chloride is more reliable than urine sodium for assessing volume status. Obligate bicarbonaturia carries sodium with it, making urine sodium elevated despite volume depletion. Paradoxical aciduria (acidic urine during alkalosis) signals that maintaining factors are present and the patient needs saline and chloride repletion. --- ## Image Reference For visual reference of renal acid-base concepts, see: - Radiopaedia: [Renal tubular acidosis](https://radiopaedia.org/articles/renal-tubular-acidosis) - Clinical information and classification - Wikimedia Commons: [Acid-base balance](https://commons.wikimedia.org/wiki/Category:Acid-base_balance) - Diagrams of renal acid handling - Radiopaedia: [Nephrocalcinosis](https://radiopaedia.org/articles/nephrocalcinosis) - Imaging findings in distal RTA --- ## Learning Points 1. **Urine pH Distinguishes RTA Types**: Distal RTA cannot acidify urine below pH 5.5 during acidosis; proximal RTA can acidify once the threshold is reached. 2. **Urine Anion Gap Assesses NH4+ Excretion**: Negative gap indicates appropriate ammonium excretion (extrarenal acidosis); positive gap indicates impaired renal acid excretion (RTA). 3. **Fanconi Syndrome Components**: Glucosuria with normal glucose, aminoaciduria, phosphaturia, uricosuria, and type 2 RTA indicate generalized proximal tubule dysfunction. 4. **Alkalosis Maintenance Requires Treatment**: Addressing the maintaining factors (volume, chloride, potassium) is essential - the alkalosis will not correct with observation alone. 5. **Type 4 RTA is Hyperkalemic**: Unlike types 1 and 2, type 4 RTA (hypoaldosteronism) causes hyperkalemia and mild acidosis from impaired ammonium production.