A 72-year-old woman with a history of chronic kidney disease (eGFR 28 mL/min/1.73m²) and hypertension presents with nausea, vomiting, and generalized weakness for 3 days. She takes lisinopril and spironolactone for blood pressure control. On examination, she appears volume depleted: blood pressure 98/62 mmHg (baseline 140/90), heart rate 102/min, and mucous membranes are dry. Serum sodium is 122 mEq/L, serum osmolality is 264 mOsm/kg, urine osmolality is 180 mOsm/kg, and urine sodium is 12 mEq/L. What is the most likely diagnosis?

Explanation

## Diagnostic Approach to Hypovolemic Hyponatremia **Key Point:** The distinction between renal and extrarenal sodium loss in hypovolemic hyponatremia depends on **urine sodium concentration**. Urine sodium <20 mEq/L suggests extrarenal loss; >20 mEq/L suggests renal loss. ### Clinical Context Analysis **Volume Status Clues:** - Orthostatic hypotension (BP 98/62, baseline 140/90). - Tachycardia (HR 102/min). - Dry mucous membranes. - **Conclusion: Patient is hypovolemic.** ### Laboratory Interpretation | Parameter | Value | Normal | Interpretation | |-----------|-------|--------|----------------| | Serum Na | 122 | 135–145 | Hyponatremia | | Serum osmolality | 264 | 280–295 | Hypoosmolar | | Urine osmolality | 180 | Variable | Relatively dilute for hypovolemia | | Urine sodium | 12 | Variable | **<20 mEq/L = extrarenal loss** | **High-Yield:** In hypovolemia, the kidney normally responds by increasing ADH secretion to conserve water AND increasing aldosterone to conserve sodium. A urine sodium <20 mEq/L in a hypovolemic patient indicates the kidney is appropriately retaining sodium — pointing to **extrarenal (GI or skin) loss**. ### Why NOT Renal Sodium Loss? The patient is on **lisinopril and spironolactone**, which can cause renal sodium loss and hyperkalemia. However: - The urine sodium is **12 mEq/L** (very low), not high. - If renal loss were occurring, urine sodium would be >20–30 mEq/L. - The low urine sodium indicates the kidney is **conserving** sodium appropriately. **Clinical Pearl:** In a hypovolemic patient with low urine sodium, the kidneys are functioning normally and trying to retain sodium. The sodium loss must be occurring elsewhere (GI: vomiting; skin: sweating, burns). ### Why NOT SIADH? SIADH occurs in **euvolemic** patients. This patient is clearly hypovolemic (hypotension, tachycardia, dry mucous membranes). In hypovolemia, ADH secretion is stimulated by baroreceptors as a physiologic response — not inappropriate. ### Why NOT Hypervolemic Hyponatremia? Hypervolemic hyponatremia presents with edema, ascites, elevated JVP, and pulmonary crackles. This patient is volume-depleted, not overloaded. ### Pathophysiology in This Case The patient has been vomiting for 3 days, losing both sodium and water. The kidney responds by: 1. Increasing ADH → water reabsorption (urine osmolality 180 is still relatively concentrated). 2. Increasing aldosterone → sodium reabsorption (urine sodium 12 is very low). Because water is reabsorbed in excess of sodium, hyponatremia develops despite appropriate renal conservation. [cite:Harrison 21e Ch 297]