Hyponatremia

Electrolyte Disordersmedium

A 60-year-old man with primary biliary cholangitis and obstructive jaundice is found on routine labs to have a serum sodium of 128 mEq/L. He is asymptomatic and euvolemic, with normal blood pressure and no neurologic symptoms. Serum glucose is 90 mg/dL, BUN is 14 mg/dL, and total cholesterol is markedly elevated at 620 mg/dL. Measured serum osmolality is 290 mOsm/kg. Which of the following diagnoses BEST fits these findings?

  1. A.PseudohyponatremiaCorrect
  2. B.Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
  3. C.Hyponatremia from liver failure
  4. D.Hyponatremia from cerebral salt wasting

Explanation

Calculate the osmolality: calculated osmolality = 2 x Na + glucose/18 + BUN/2.8 = 2(128) + 90/18 + 14/2.8 ≈ 266 mOsm/kg, which is well below the measured 290 — an osmolar gap of ~24. A normal measured osmolality despite apparent hyponatremia is the signature of pseudohyponatremia: the true sodium in plasma water is normal (a measured osmolality of ~290 corresponds to a real sodium near 140), and the reported 128 is a lab artifact. In cholestatic/obstructive jaundice, markedly elevated lipids — particularly lipoprotein X — and hypercholesterolemia raise the non-aqueous fraction of plasma, so sodium measured by indirect (diluted) ion-selective electrode or flame photometry reads falsely low. True hypotonic hyponatremia (SIADH, liver failure, cerebral salt wasting) would show a low measured osmolality. Toxic alcohols also cause an osmolar gap but not hyponatremia. Recognize the artifact (confirm with direct ISE); no sodium correction is needed.

Why each option

A.
Correct. Pseudohyponatremia: severe hyperlipidemia (lipoprotein X in cholestasis) or hyperproteinemia increases the non-aqueous plasma fraction, so indirect ISE/flame photometry reports a falsely low sodium while measured osmolality is normal.
B.
Incorrect. SIADH causes true hypotonic hyponatremia with a low measured osmolality; here measured osmolality is normal (290).
C.
Incorrect. Hyponatremia of liver failure is also true hypotonic hyponatremia (low measured osmolality).
D.
Incorrect. Cerebral salt wasting is a true hypotonic, hypovolemic hyponatremia; measured osmolality would be low.

Reference: Hoorn EJ & Zietse R, JASN 2017 (diagnosis and treatment of hyponatremia)

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