The 3 categories of hyponatremia
In general, hyponatremia can be described as an excess of body water relative to serum sodium levels, often identified as a serum sodium concentration <135 mEq/L. It is often associated with decreased serum osmolality resulting from changes in total body water, rather than from changes in body sodium content. These changes are often associated with thirst, vasopressin, and the kidney.1
Hyponatremia may be depletional, resulting from electrolyte losses in excess of water (depletional hyponatremia), or dilutional, resulting from retained water (dilutional hyponatremia). Dilutional hyponatremia is often associated with an excessive secretion of vasopressin.2
Hypotonic hyponatremia is classified into 3 main categories: hypovolemic, euvolemic, or hypervolemic based on the initial assessment of the patient's volume status, medical history, urine osmolality, and sodium concentration.1,3 This review focuses on euvolemic and hypervolemic hyponatremia.
SAMSCA is contraindicated in hypovolemic and depletional hyponatremia.
Based on assessment of volume status, medical history, and urine osmolality and sodium concentrations, hyponatremia can be classified as hypovolemic, euvolemic, or hypervolemic.1 Douglas I. Hyponatremia: why it matters, how it presents, how we can manage it. Cleve Clin J Med. 2006;73(suppl 3):S4-S12. Reprinted with permission. Copyright © 2006 Cleveland Clinic. All rights reserved.
Euvolemic hyponatremia, a dilutional form of hyponatremia, occurs when the total serum sodium is normal or near normal, but the total body water is increased without clinically evident edema. The syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of euvolemic hyponatremia. Patients with euvolemic hyponatremia have no signs of volume depletion or volume expansion.1,3
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Hypervolemic hyponatremia, a dilutional form of hyponatremia, occurs when there is an increase in total body water but a relatively smaller increase in the total serum sodium, so the available sodium is effectively diluted. Heart failure and renal disease are 2 primary causes of hypervolemic hyponatremia. The clinical signs of hypervolemic hyponatremia include signs of volume expansion, such as the presence of clinically evident edema, ascites, and pulmonary edema.3
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