Hyponatremia pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Pathophysiology
The etiology of hyponatremia can be categorized pathophysiologically in three primary ways, based on the patient's plasma osmolality.
- Hypertonic hyponatremia, caused by resorption of water drawn by osmols such as glucose (hyperglycemia or diabetes) or mannitol (hypertonic infusion).
- Isotonic hyponatremia, more commonly called "pseudohyponatremia," is caused by lab error due to hypertriglyceridemia (most common) or hyperparaproteinemia.
- Hypotonic hyponatremia is by far the most common type, and is often used interchangeably with "hyponatremia." Hypotonic hyponatremia is categorized in 3 ways based on the patient's blood volume status. Each category represents a different underlying reason for the increase in ADH that led to the water retention and thence hyponatremia:
- Hypervolemic hyponatremia, wherein there is decreased effective circulating volume even though total body volume is increased (by the presence of edema). The decreased effective circulating volume stimulates the release of ADH, which in turn leads to water retention. Hypervolemic hyponatremia is most commonly the result of congestive heart failure, liver failure (cirrhosis), or kidney disease (nephrotic syndrome).
- Euvolemic hyponatremia, wherein the increase in ADH is secondary to either physiologic but excessive ADH release (as occurs with nausea or severe pain) or inappropriate and non-physiologic secretion of ADH, i.e. syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). Often categorized under euvolemic is hyponatremia due to inadequate urine solute as occurs in beer potomania or "tea and toast" hyponatremia, hyponatremia due to hypothyroidism or adrenal insufficiency, and those rare instances of hyponatremia that are truly secondary to excess water intake (i.e., extreme psychogenic polydipsia)
- Hypovolemic hyponatremia, wherein ADH secretion is stimulated by volume depletion.
The volemic classification fails to include spurious and/or artifactual hyponatremia, which is addressed in the osmolar classification. This includes hyponatremia that occurs in the presence of massive hypertriglyceridemia, severe hyperglycemia, and extreme elevation of immunoglobulin levels.
In chronic hyponatremia, sodium levels drop gradually over several days or weeks and symptoms and complications are typically moderate. Chronic hyponatremia is often called asymptomatic hyponatremia in clinical settings because it is thought to have no symptoms; however, emerging data suggests that "asymptomatic" hyponatremia is not actually asymptomatic.
In acute hyponatremia sodium levels drop rapidly, resulting in potentially dangerous effects, such as rapid brain swelling, which can result in coma and death.
| Term | Definitions[1][2][3] |
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| Hyponatremia | Hyponatremia is defined as a serum sodium concentration < 135 mEq/L. |
| Hypotonic hyponatremia | Hyponatremia with low osmolality (hypotonic hyponatremia) is defined as hyponatremia with a serum osmolality below 280 mOsm/kg. |
| Hypertonic hyponatremia | Hyponatremia with high osmolality (hypertonic hyponatremia) is defined as hyponatremia with a serum osmolality greater than 295 mOsm/kg. |
| Isotonic hyponatremia | Hyponatremia with normal osmolality (Isotonic hyponatremia) is defined as hyponatremia with a serum osmolality ranging between 280-295 mOsm/kg. |
| Hyponatremia based on ECF volume | |
| Hypovolemic hyponatremia | Hyponatremia plus decreased ECF volume. Usually diagnosed by HPE showing water depletion plus spot urine sodium <20 to 30 mmol/L, unless kidney is the source of sodium loss. |
| Euvolemic hyponatremia | Hyponatremia plus normal ECF volume. Majority of cases are of this type. Usually diagnosed by spot urine sodium ≥ 20 to 30 mmol/L, unless secondarily sodium depleted. |
| Hypervolemia hyponatremia | Hyponatremia plus increased ECF volume. Usually diagnosed by HPE showing water retention plus spot urine sodium <20 to 30 mmol/L |
References
- ↑ Laczi, F. (2008). "[Etiology, diagnostics and therapy of hyponatremias]". Orv Hetil. 149 (29): 1347–54. doi:10.1556/OH.2008.28409. PMID 18617466. Unknown parameter
|month=ignored (help) - ↑ Douglas, I. (2006). "Hyponatremia: why it matters, how it presents, how we can manage it". Cleve Clin J Med. 73 Suppl 3: S4–12. PMID 16970147. Unknown parameter
|month=ignored (help) - ↑ Verbalis, JG.; Goldsmith, SR.; Greenberg, A.; Korzelius, C.; Schrier, RW.; Sterns, RH.; Thompson, CJ. (2013). "Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations". Am J Med. 126 (10 Suppl 1): S1–42. doi:10.1016/j.amjmed.2013.07.006. PMID 24074529. Unknown parameter
|month=ignored (help)