Hyperosmolarity is characterized by an increase in the solute concentration of body fluids brought about by loss of water, inadequate water intake, or administration of large amounts of hypertonic solutions. When the osmolar concentration of the extracellular fluid is increased, transfer of water from cells to extracellular fluid exceeds that in the opposite direction. Net water movement is thus out of the cells, with resultant dilution of extracellular fluid and concentration of the fluid within cells themselves. If the solute which increases the extracellular fluid osmolarity is one which is in the main excluded from intracellular fluid, such as sodium and chloride ions, this disturbance in water distribution cannot be mitigated by movement of solute into cells. Hence, the ultimate effect of hypernatremia is dehydration of cells. This physicochemical disorder is most frequently encountered in infants and children with severe diarrhea and vomiting, but it also occurs in adults.
LUTTRELL CN, FINBERG L. Hemorrhagic Encephalopathy Induced by HypernatremiaI. Clinical, Laboratory, and Pathological Observations. AMA Arch NeurPsych. 1959;81(4):424-432. doi:10.1001/archneurpsyc.1959.02340160022004