Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Sodium
Question 6#

A 68-year-old male with long-standing smoking and alcohol use history presents to his primary care physician. His wife noticed progressively worsening jaundice and poor appetite with an associated weight loss of 35 lbs over the past 2 months. He also reports early satiety and vague abdominal pain. 

His temperature is 37.5°C, blood pressure is 110/75 mm Hg, pulse rate is 84 beats/min, and respiratory rate is 12 breaths/min. An ultrasound of his abdomen confirms a mass in the head of the pancreas. He is admitted for surgical intervention, and laboratory data obtained show the following:

• Sodium 131 mEq/L (mmol/L)
• Potassium 3.8 mEq/L (mmol/L)
• BUN 7 mg/dL
• Creatinine 1.0 mg/dL
• Total bilirubin 8.7 mg/dL
• Amylase 90 U/L
• Lipase 67 U/L
• Total cholestrol 485 mg/dL
• LDL cholesterol 157 mg/dL
• HDL cholesterol 42 mg/dL
• Triglycerides 349 mg/dL
• Plasma osmolality 295 mOsm/kg
• rine osmolality 420 mOsm/kg
• TSH 2.1 mIU/L
• Glucose 93 mg/dL

What is the MOST likely cause for the patient’s hyponatremia?

A. Beer potomania
B. Psychogenic polydipsia
C. SIADH
D. Adrenal insufficiency
E. Pseudohyponatremia

Correct Answer is E

Comment:

Correct Answer: E

While the patient’s calculated plasma osmolality is 270 mOsm/kg H₂O, his measured plasma osmolality is normal at 295 mOsm/kg H₂O, making the diagnosis of pseudohyponatremia most likely.

Plasma osmolality (mOsm/kg H₂O) = 2 × serum sodium (mEq/L) + plasma glucose (mg/dL)/18 + blood urea nitrogen (mg/dL)/2.8.

Plasma consists of 93% water and 7% lipids and proteins. However, in the presence of other substances, such as elevated lipids or paraproteins, the aqueous fraction of plasma will be diluted and thereby falsely lower the serum sodium concentration while the actual serum sodium concentration remains normal. Pseudohyponatremia is due to a laboratory error in the measurement of the serum sodium that leads to low reported sodium with normal plasma osmolality in the presence of total serum cholesterol and lipoprotein X as in patients with obstructive jaundice. Other causes for pseudohyponatremia include severe hypertriglyceridemia, diabetic ketoacidosis, plasma cell dyscrasia, such as in patients with multiple myeloma, and obstructive jaundice as in this patient.

References:

1. Hussain I, Ahmad Z, Garg A. Extreme hypercholesterolemia presenting with pseudohyponatremia – a case report and review of the literature. J Clin Lipidol. 2015;9(2):260-264.
2. Vo I, Gosmanov AR, Garcia-Rosell M, Wall BM. Pseudohyponatremia in acute liver disease. Am J Med Sci. 2013;345(1):62-64.