hyperosmolarity

Hyperosmolarity refers to an increased osmolality of body fluids, particularly serum, caused by a higher concentration of solutes relative to water. Normal serum osmolality is about 275–295 mOsm/kg; values above this indicate hyperosmolarity. Osmolality is determined by solutes such as sodium, glucose, and urea, with glucose and sodium often contributing most in clinical settings.

Causes and pathophysiology: Hyperosmolarity can result from hyperglycemia (notably in the diabetic hyperosmolar state), hypernatremia due to dehydration or fluid loss, and ingestion or administration of osmotically active substances such as mannitol, glycerol, sorbitol, or certain alcohols. Urea contributes to osmolality but diffuses readily, so it has a smaller effect on cellular water balance. In hyperosmolar states, extracellular solute concentration draws water out of cells, causing cellular dehydration, especially in brain tissue, which can lead to confusion, seizures, or coma if severe. Rapid shifts in osmolality are particularly dangerous because brain cells require time to adapt.

Clinical features and diagnosis: Symptoms often reflect dehydration and CNS involvement, including thirst, dry mucous membranes, tachycardia, hypotension, and altered mental status. Serum osmolality can be measured or estimated with calculations such as 2[Na+] + glucose/18 + BUN/2.8; values above 295 mOsm/kg indicate hyperosmolarity, with severe cases typically exceeding 320 mOsm/kg. In diabetes, the diabetic hyperosmolar state is a common, life-threatening cause.

Treatment and prognosis: Management focuses on identifying and treating the underlying cause, careful fluid replacement (often starting with isotonic saline), and gradual correction of hypernatremia and hyperglycemia to avoid cerebral edema. Ongoing monitoring of electrolytes, kidney function, and neurologic status is essential. Prognosis depends on the cause, severity, and the speed of treatment.