A 58-year-old man with a 10-year history of type 2 diabetes mellitus presents to the emergency department with confusion and lethargy. His wife reports he has been increasingly drowsy over the past 2 days. On examination, he is afebrile, blood pressure is 128/82 mmHg, and there are no focal neurological deficits. Laboratory investigations reveal: serum sodium 118 mEq/L, serum osmolality 248 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 45 mEq/L. Chest X-ray is normal. What is the most likely diagnosis?

Explanation

## Diagnostic Approach to Hyponatremia **Key Point:** The diagnosis of hyponatremia requires assessment of plasma osmolality, urine osmolality, and urine sodium to determine the underlying mechanism. ### Step-by-Step Analysis 1. **Confirm hyponatremia**: Serum sodium 118 mEq/L (normal 135–145) — confirmed. 2. **Assess plasma osmolality**: 248 mOsm/kg (normal 280–295) — hypoosmolar hyponatremia. 3. **Evaluate urine osmolality**: 420 mOsm/kg (inappropriately high for low plasma osmolality) — kidney is concentrating urine despite hypoosmolality. 4. **Check urine sodium**: 45 mEq/L (>30 mEq/L) — indicates adequate intravascular volume. ### SIADH Diagnostic Criteria | Feature | Finding | Status | |---------|---------|--------| | Hyponatremia | Na 118 | ✓ | | Hypoosmolality | Osm 248 | ✓ | | Urine osmolality | >100 mOsm/kg (420) | ✓ | | Urine sodium | >30 mEq/L (45) | ✓ | | Normal renal/thyroid/adrenal function | Implied | ✓ | | Euvolemia | BP normal, no edema | ✓ | **High-Yield:** SIADH is the most common cause of hyponatremia in hospitalized patients. The pathognomonic finding is **inappropriately concentrated urine in the face of hypoosmolality**. **Clinical Pearl:** In diabetes mellitus, SIADH can occur secondary to medications (e.g., carbamazepine, SSRIs, vincristine), infection, or CNS disease. The patient's normal chest X-ray makes pulmonary malignancy less likely but does not exclude SIADH. **Mnemonic: SIADH causes** — **SIADH** = Small cell lung cancer, Infections (pneumonia, TB, meningitis), Antipsychotics/Antidepressants, Drugs (carbamazepine, vincristine), Head trauma/CNS disease ### Why This Patient Has SIADH - Euvolemic (normal BP, no edema, normal JVP implied). - Hypoosmolar hyponatremia with inappropriately high urine osmolality. - Urine sodium >30 mEq/L excludes volume depletion. - Normal renal function (implied by clinical context). [cite:Harrison 21e Ch 297]