How to manage a patient with hyponatremia (low sodium levels) and normal serum osmolality?

Management of Hyponatremia with Normal Serum Osmolality

For patients with hyponatremia and normal serum osmolality, the most appropriate management is to identify and address pseudohyponatremia as the likely cause, rather than initiating treatments for true hypotonic hyponatremia. 1, 2

Diagnostic Approach

Step 1: Confirm Pseudohyponatremia

• Normal serum osmolality (280-295 mOsm/kg) with low sodium indicates pseudohyponatremia 3
• This discrepancy occurs due to:
  • Hyperlipidemia (check for creamy serum appearance)
  • Hyperproteinemia/paraproteinemia (check for increased serum viscosity)
  • Multiple myeloma (common cause of pseudohyponatremia) 2

Step 2: Laboratory Workup

• Measure effective serum osmolality (serum osmolality minus serum urea) 3
• Additional tests to identify underlying cause:
  • Lipid panel (triglycerides, cholesterol)
  • Serum protein electrophoresis
  • Complete blood count
  • Liver function tests
  • Thyroid-stimulating hormone 1

Management Algorithm

1. Do not treat as true hyponatremia

   • Avoid water restriction
   • Do not administer hypertonic saline
   • Do not use vaptans 1

2. Treat the underlying cause

   • For hyperlipidemia: Lipid-lowering therapy
   • For paraproteinemia: Manage the underlying condition (e.g., multiple myeloma)

3. Monitor serum sodium levels

   • No specific sodium correction needed as this is a laboratory artifact
   • Focus on managing the primary disorder 2

Important Considerations

Distinguishing from True Hyponatremia

• True hypotonic hyponatremia presents with low serum osmolality (<280 mOsm/kg) 1
• Pseudohyponatremia shows normal measured osmolality despite low calculated values 2
• Symptoms correlate with osmolality, not sodium level alone; patients with pseudohyponatremia are typically asymptomatic 2

Common Pitfalls to Avoid

• Treating pseudohyponatremia as true hyponatremia can lead to iatrogenic hypernatremia
• Failing to identify the underlying cause (especially paraproteinemia) may delay diagnosis of serious conditions
• Relying solely on serum sodium without measuring osmolality leads to misdiagnosis 3

Special Circumstances

• In patients with cirrhosis, even with pseudohyponatremia, monitor closely as they may develop true hyponatremia
• For sodium levels 126-135 mmol/L with normal creatinine in cirrhotic patients, continue diuretic therapy with close monitoring of electrolytes 4
• For patients with multiple myeloma, a normal serum sodium level may actually mask hypernatremia 2

By correctly identifying pseudohyponatremia and focusing on the underlying cause rather than the sodium level itself, unnecessary treatments and potential complications can be avoided.