Hyponatremia - Pathophysiology

Pathophysiology

The hyponatremia can be spurious and/or artifactual hyponatremia in which case there is no hypotonicity. 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.

True hyponatremia is Hypotonic hyponatremia and is by far the most common type, and is often simply alluded to as "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 central 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 or asociated with 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.