Normoosmolar Hyponatremia

Aka: Normoosmolar Hyponatremia, Isoosmolar Hyponatremia, Isotonic Hyponatremia, Pseudohyponatremia

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II. Definition

  1. Hyponatremia with measured Serum Osmolality = 280-300

III. Pathophysiology

  1. Water normally comprises 92-94% of serum
  2. Lipids or Proteins decrease water fraction

IV. Causes

  1. Pseudohyponatremia (Isotonic)
    1. Hyperlipidemia (Serum Triglycerides >1500 mg/dl)
    2. Hyperproteinemia (Serum Protein > 10 g/dl)
  2. Isotonic (Sodium-free) infusion
    1. Glucose
    2. Mannitol
    3. Glycine (used in some urologic procedures)
    4. Post-Transurethral prostatic resection syndrome
      1. Large volume intraoperative hypotonic irrigation
      2. Neurologic and cardiopulmonary signs

V. Labs

  1. Pseudohyponatremia is not an issue when Serum Sodium is measured by Sodium electrode
    1. In this case, measured Sodium is the true Sodium concentration
    2. Many labs in U.S. (as of 2015) use Sodium electrode to measure Serum Sodium

VI. Management

  1. Pseudohyponatremia
    1. In Pseudohyponatremia, Sodium does not need to be managed specifically
    2. Evaluate and treat underlying cause
      1. Hyperlipidemia
      2. Hyperproteinemia (e.g. Multiple Myeloma - consider SPEP, UPEP)
  2. Isotonic Infusion
    1. Reflects true Hyponatremia and requires correction

VII. References

  1. Kone in Tisher (1993) Nephrology, p. 87-100
  2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  3. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  4. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
  5. Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]

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