In hyponatremia, the serum osmolality is used to classify the disorder into hypotonic, isotonic, and hypertonic categories. Which of the following conditions causes hypertonic hyponatremia?

Explanation

## Classification of Hyponatremia by Osmolality **Key Point:** Hypertonic hyponatremia (serum osmolality > 295 mOsm/kg) occurs when solutes other than sodium draw water into the intracellular space, diluting serum sodium despite elevated total osmolality. ### Osmolality-Based Classification | Type | Serum Osmolality | Mechanism | Example | |------|------------------|-----------|----------| | **Hypotonic** | < 275 mOsm/kg | Free water excess relative to solute | SIADH, primary polydipsia | | **Isotonic** | 275–295 mOsm/kg | Pseudohyponatremia or hypertriglyceridemia | Severe lipemia, hyperproteinemia | | **Hypertonic** | > 295 mOsm/kg | Non-sodium solute draws water out, diluting Na⁺ | Hyperglycemia, mannitol infusion | ### Hyperglycemia as the Cause In hyperglycemia (especially in uncontrolled diabetes): 1. High blood glucose raises serum osmolality 2. Water shifts from intracellular to extracellular space (osmotic diuresis) 3. This dilutes serum sodium concentration despite high total osmolality 4. Serum sodium may drop 1.6–2.4 mEq/L for every 100 mg/dL rise in glucose above 100 mg/dL **High-Yield:** The formula for **corrected sodium** in hyperglycemia is: $$\text{Corrected Na}^+ = \text{Measured Na}^+ + 0.016 \times (\text{Glucose} - 100)$$ This accounts for the osmotic effect and is essential for interpreting hyponatremia in diabetic patients. **Clinical Pearl:** Hypertonic hyponatremia is rare and often overlooked. It occurs only when a non-sodium osmotically active substance accumulates (glucose, mannitol, contrast agents). The key distinguishing feature is **elevated serum osmolality with low serum sodium**.