A 58-year-old man with a 10-year history of poorly controlled type 2 diabetes mellitus presents to the emergency department with confusion and lethargy. His wife reports he has been increasingly drowsy over the past 3 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 265 mOsm/kg, urine osmolality 580 mOsm/kg, urine sodium 45 mEq/L, and blood glucose 580 mg/dL. Urine dipstick shows 3+ glucose and trace ketones. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis This patient has **hyperglycemic hyperosmolar state (HHS) with dilutional hyponatremia** — a common and often overlooked electrolyte derangement in severe hyperglycemia. ## Pathophysiology of Hyponatremia in HHS **Key Point:** In severe hyperglycemia, the elevated serum glucose creates an osmotic gradient that draws water from the intracellular compartment into the extracellular space, diluting serum sodium. This is **dilutional (translocation) hyponatremia**, not true hyponatremia. 1. **Osmotic water shift**: High glucose (580 mg/dL) increases serum osmolality → water moves out of cells → ECF volume expands → serum sodium is diluted 2. **Osmolality paradox**: Serum osmolality is LOW (265 mOsm/kg), indicating the hyponatremia is *real* (not pseudohyponatremia) 3. **Urine findings**: High urine osmolality (580 mOsm/kg) and elevated urine sodium (45 mEq/L) indicate the kidneys are responding appropriately to the osmotic stress — this rules out SIADH ## Why This Is NOT SIADH | Feature | SIADH | HHS with Dilutional Hyponatremia | |---------|-------|----------------------------------| | **Serum osmolality** | Low (<280) | Low (<280) | | **Urine osmolality** | High (>200) | High (>200) ✓ | | **Urine sodium** | Variable, often >40 | >40 ✓ | | **Glucose** | Normal | Markedly elevated ✓ | | **Clinical context** | CNS disease, malignancy, drugs | Diabetes, severe hyperglycemia ✓ | | **Serum osmolality vs. urine osmolality** | Urine inappropriately concentrated for low serum osmolality | Urine appropriately concentrated for the osmotic load | **High-Yield:** The key distinguishing feature is that in HHS, the urine osmolality is HIGH *because the kidneys are responding to the osmotic load from glucose*, not because of inappropriate ADH secretion. ## Why NOT Pseudohyponatremia? Pseudohyponatremia occurs when high lipids or proteins artificially lower measured sodium. However: - Serum osmolality is LOW (265 mOsm/kg) — this confirms the hyponatremia is real, not an artifact - If pseudohyponatremia were present, osmolality would be normal or high **Clinical Pearl:** Calculate expected osmolality: $2 \times Na + \frac{Glucose}{18} + \frac{BUN}{2.8} = 2(118) + \frac{580}{18} + \frac{BUN}{2.8} \approx 268$ mOsm/kg, which matches the measured 265 — confirming real hyponatremia. ## Management 1. **Do NOT correct sodium rapidly** — risk of hyperchloremic metabolic acidosis and osmotic demyelination 2. Treat the underlying hyperglycemia with insulin and IV fluids 3. Sodium will normalize as glucose is corrected and water shifts back intracellularly 4. Target correction rate: 8–10 mEq/L per 24 hours **Warning:** Aggressive hypertonic saline is contraindicated and will worsen hyperglycemia and osmolality.