In a patient with serum osmolality of 315 mOsm/kg, serum sodium of 128 mmol/L, and urine sodium of 10 mmol/L, how should the hyponatraemia be managed?

Hypertonic Hyponatremia: Treat with Isotonic Saline

This patient has hypertonic hyponatremia (serum osmolality 315 mOsm/kg with hyponatremia), indicating a non-hypotonic cause—most likely hyperglycemia or another osmotically active substance—and should be managed with isotonic (0.9%) saline to restore volume, NOT hypertonic saline or fluid restriction.

Diagnostic Framework

The elevated serum osmolality (315 mOsm/kg) with hyponatremia (128 mmol/L) immediately excludes hypotonic hyponatremia and points to:

• Hyperglycemia (most common)
• Mannitol administration
• Rarely: severe uremia or toxic alcohol ingestion

The low urine sodium (10 mmol/L) indicates hypovolemia with appropriate renal sodium conservation 1, 2.

Key Physiological Point

Normal serum osmolality is 275-295 mOsm/kg. When osmolality is elevated (>295) in the presence of hyponatremia, this represents hypertonic hyponatremia—water has shifted from the intracellular to extracellular space due to an osmotically active solute (typically glucose), diluting serum sodium 1, 3.

Management Algorithm

Step 1: Identify and Correct the Underlying Cause

• Check serum glucose immediately: For every 100 mg/dL glucose elevation above 100 mg/dL, sodium decreases by approximately 1.6-2.4 mmol/L
• If hyperglycemic: Treat with insulin and fluids
• If mannitol-related: Discontinue mannitol

Step 2: Volume Repletion

Administer isotonic (0.9%) saline at 1 mL/kg/h initially 4:

• The low urine sodium (10 mmol/L) confirms hypovolemia requiring volume replacement
• As the osmotic agent (glucose) is corrected, water will shift back intracellularly and serum sodium will rise spontaneously
• Do NOT use hypertonic (3%) saline—this is contraindicated in hypertonic hyponatremia 1
• Do NOT restrict fluids—this patient is volume depleted

Step 3: Monitor Closely

• Serum sodium every 2-4 hours initially 1
• Serum glucose hourly if hyperglycemic
• Urine output to assess volume repletion
• Recalculate corrected sodium as glucose normalizes

Step 4: Adjust Rate of Correction

Critical caveat: As the osmotic agent is removed, serum sodium can rise rapidly and unpredictably:

• Target correction: ≤8-10 mmol/L in first 24 hours 5, 6
• If correction exceeds 8 mmol/L in 24 hours, consider 5% dextrose in water or desmopressin to prevent overcorrection and osmotic demyelination syndrome 4, 6
• The risk of overcorrection is particularly high once hyperglycemia resolves and water shifts back intracellularly

Common Pitfalls to Avoid

1. Misclassifying as hypotonic hyponatremia: The elevated osmolality excludes SIADH, cerebral salt wasting, or other hypotonic causes that would require different management 1, 3

2. Using hypertonic saline: This would worsen hyperosmolality and is only indicated for hypotonic hyponatremia with severe symptoms 1, 5

3. Fluid restriction: Inappropriate here given hypovolemia (urine sodium 10 mmol/L) 2, 7

4. Ignoring the risk of overcorrection: Once the osmotic agent is removed, spontaneous rapid correction can occur, risking osmotic demyelination syndrome 4, 6

5. Treating the sodium number in isolation: The corrected sodium (accounting for hyperglycemia) may actually be near-normal, making aggressive sodium correction unnecessary and dangerous 3

Symptom-Based Modifications

• If severely symptomatic (seizures, altered mental status): This likely represents cerebral edema from the underlying metabolic derangement rather than hyponatremia itself—focus on correcting the primary cause (e.g., diabetic ketoacidosis, hyperosmolar hyperglycemic state)
• If asymptomatic: Continue isotonic saline and monitor; symptoms are unlikely with hypertonic hyponatremia 2, 5