Hyposmolality

A decreased plasma or serum osmolality (<275 mOsm/kg), reflecting an excess of free water relative to dissolved solutes (primarily sodium and its accompanying anions), leading to reduced plasma tonicity and movement of water into cells with potential cellular swelling, particularly in the brain.

Causes

• Excessive fluid intake or IV hypotonic fluids --> Hyponatremia

• Medications causing hyponatremia or impaired water regulation

  • Diuretics

  • Certain antidepressants

  • Antipsychotics

• Endocrine causes

  • Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

  • Adrenal insufficiency

  • Cortisol deficiency causing increased ADH activity

• Heart failure

• Kidney failure: Impaired renal water excretion

• Liver failure: Cirrhosis with impaired fluid regulation

• Gastrointestinal causes

  • Vomiting with electrolyte loss

  • Diarrhea causing sodium and fluid imbalance

• Substances affecting water balance or ADH secretion

  • Alcohol use

  • Ecstasy (MDMA)

• Severe illness

• Postoperative states with increased ADH release

• Exercise-associated excessive water intake