Extreme hypercholesterolemia presenting with pseudohyponatremia - a case report and review of the literature

- Extremely high cholesterol levels may present as pseudohyponatremia.
- Cholestasis may result in accumulation of lipoprotein-X.
- Total cholesterol levels and routine sodium levels are inversely proportional.
- Presence of lipoprotein-X is shown by lipoprotein electrophoresis.
- Serum osmolality should be measured when pseudohyponatremia is suspected.

Pseudohyponatremia has been reported in association with severe hypertriglyceridemia and hyperparaproteinemia, but its association with severe hypercholesterolemia is not well-known. We report a 43-year-old woman with refractory primary biliary cirrhosis who presented with asymptomatic hyponatremia (121 mmol/L; normal range: 135-145 mmol/L). She was ultimately found to have a total serum cholesterol level of 2415 mg/dL (normal range: 120-199 mg/dL) - secondary to accumulation of lipoprotein-X-causing pseudohyponatremia. The diagnosis was confirmed by measurement of serum osmolality (296 mOsm/kg H2O; normal range: 270-300 mOsm/kg H2O) and serum sodium by direct potentiometry (141 mmol/L). Furthermore, following 16 sessions of plasmapheresis over a 4-month period, there was marked lowering of serum cholesterol to 200 mg/dL and normalization of serum sodium (139 mmol/L) as measured by indirect potentiometry. This case shows that extreme hypercholesterolemia from elevation of lipoprotein-X particles in cholestasis can be a rare cause of pseudohyponatremia. It highlights the need to measure serum sodium with direct potentiometry in the setting of extreme hypercholesterolemia and consider this possibility before initiating treatment of hyponatremia.