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About Hyponatremia

Hyponatremia, commonly defined as serum sodium <135 mEq/L, is the most common electrolyte disorder among hospitalized patients.1,2 The disorder occurs when there is an excess of body water in relation to body solute, either due to solute depletion or excess retention of water.2

Dilutional hyponatremia occurs when a patient's sodium level is normal or even slightly elevated, but the volume of extracellular body water is increased2:

  • Euvolemic hyponatremia is characterized by normal extracellular sodium and increased body water without clinical signs of hypervolemia, such as pitting edema or ascites. It is usually associated with an elevated arginine vasopressin (AVP) level and may develop in patients with a variety of underlying diseases.2

  • Hypervolemic hyponatremia is characterized by increases in both extracellular sodium and body water, but the increase in body water exceeds the increase in sodium. Expansion of the volume of the extracellular fluid in patients with hypervolemic hyponatremia results in associated pitting edema.2

Clinical Classification of Dilutional Hyponatremia2

  EUVOLEMIA HYPERVOLEMIA
FLUID/SOLUTE
MECHANISM
Extracellular sodium normal
TBW slightly increased
Extracellular sodium increased
TBW greatly increased
POSSIBLE
UNDERLYING
CAUSES
SIADH
Chronic obstructive pulmonary
disease [COPD]
Malignancy
Congestive heart failure [CHF]
Renal impairment
Cirrhosis

SIADH = syndrome of inappropriate antidiuretic hormone secretion; TBW = total body water.

Euvolemic and hypervolemic hyponatremia are associated with numerous underlying conditions with differing pathophysiological mechanisms.2

Learn more about the pathophysiology of euvolemic hyponatremia.

Learn more about the pathophysiology of hypervolemic hyponatremia.



REFERENCES: 1. Zeltser D, Rosansky S, van Rensburg H, Verbalis JG, Smith N. Assessment of the efficacy and safety of intravenous conivaptan in euvolemic and hypervolemic hyponatremia. Am J Nephrol. 2007;27:447-457. 2. Verbalis JG, Goldsmith SR, Greenberg A, Schrier RW, Sterns RH. Hyponatremia treatment guidelines 2007: expert panel recommendations. Am J Med. 2007;120:S1-S21.