hyperosmolar

Hyperosmolar is an adjective used to describe a solution whose osmolality is higher than that of normal body fluids. Osmolality measures the concentration of osmotically active particles per kilogram of solvent; normal human plasma osmolality is about 275–295 mOsm/kg. A hyperosmolar state therefore implies plasma osmolality above the normal range.

Common causes include free water loss (dehydration), hypernatremia, and hyperglycemia, as well as the administration or ingestion of osmotically active substances such as mannitol or glycerol. Significant hyperlipidemia has a comparatively smaller effect on measured plasma osmolality. In some cases, a combination of factors raises osmolality.

Mechanistically, high osmolality draws water out of cells, especially brain cells, leading to cellular dehydration and neurologic symptoms such as confusion, lethargy, or seizures. The risk rises with rapid onset of hyperosmolarity and with severe electrolyte disturbances. Management centers on addressing the underlying cause and balancing fluids and electrolytes to avoid dangerous shifts.

A principal clinical context is the hyperosmolar hyperglycemic state (HHS), a serious complication of type 2 diabetes characterized by severe hyperglycemia, high plasma osmolality, dehydration, and altered mental status, with little or no ketoacidosis.

Diagnosis relies on measuring plasma osmolality and calculating an osmolar gap. Calculated osmolarity is approximately 2 × Na + glucose/18 + BUN/2.8 (with glucose and BUN in mg/dL). An osmolar gap greater than about 10 mOsm/kg suggests unmeasured solutes contributing to hyperosmolarity.