hypoosmolar

Hypoosmolar describes a state in which the osmolality of a body fluid is lower than that of normal plasma. Osmolality is a measure of solute concentration per kilogram of solvent, and normal plasma osmolality is about 275–295 mOsm/kg. When extracellular fluid is hypoosmolar, water tendsto move into cells by osmosis, potentially causing cellular swelling, especially in the brain where edema can be dangerous.

In physiology and medicine, hypoosmolarity often arises from an excess of free water or a deficiency of solutes relative to water. Common situations include administration of hypotonic intravenous fluids, excessive water intake, and disorders that impair water excretion such as syndrome of inappropriate antidiuretic hormone secretion (SIADH), adrenal insufficiency, or renal failure. Hyponatremia frequently accompanies hypoosmolar states and contributes to cellular swelling.

Plasma osmolality can be measured directly or estimated with a formula such as 2 × [Na+] + glucose/18 + BUN/2.8 (in mOsm/kg). A value below about 275 mOsm/kg generally indicates hypoosmolarity relative to normal plasma.

Clinical significance varies with severity and rate of onset. Hypoosmolar states can cause symptoms ranging from nausea and confusion to seizures or coma due to cerebral edema. Management depends on the cause and severity: mild cases may be managed with fluid restriction and treatment of underlying conditions, while symptomatic or severe cases may require controlled correction with hypertonic saline in a monitored setting, along with measures to address the underlying disorder and prevent rapid osmotic shifts.