Differentials
Hypertonic hyponatremia
SIGNS / SYMPTOMS
Due to hyperglycemia or administration of an active osmolyte such as mannitol.
Signs and symptoms are variable depending on the underlying condition.
In hyperglycemia, patients may be asymptomatic, have polyuria due to uncontrolled diabetes mellitus, or present with hyperosmolar hyperglycemia and/or diabetic ketoacidosis.
INVESTIGATIONS
Serum blood glucose level: elevated.
Serum osmolality: high.
If the patient is hyperglycemic, a sodium correction formula should be used. If the glucose level is >100 mg/dL, the most accurate correction formula is: corrected serum sodium (mEq/L) = measured serum sodium (mEq/L) + 2.4 x {[serum glucose (mg/dL) - 100]/100}. This formula should be used to determine if true hyponatremia is present.[42]
Pseudohyponatremia
SIGNS / SYMPTOMS
Also known as isotonic hyponatremia.
An artifact of incorrect measurement of serum sodium concentrations due to high lipid and/or protein in the plasma.
Most common cause of high protein levels is multiple myeloma.
Signs and symptoms are variable depending on the underlying condition.
INVESTIGATIONS
Serum lipids and proteins: elevated.
Serum osmolality: normal.
Use of ion-specific electrodes to measure sodium directly may help reduce the incidence of pseudohyponatremia.
Evaluation of hyponatremia
SIGNS / SYMPTOMS
See our Evaluation of hyponatremia topic for a full review of the differential diagnosis.
INVESTIGATIONS
Laboratory investigations such as serum osmolality, urine osmolality, and urine sodium concentration help to distinguish the various causes.
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