-has profound effects on tissues and organs (ESPECIALLY THE BRAIN) --> brain cell edema and neurological symptoms including headache, nausea, lethargy, and disorientation
*If plasma sodium potassium falls below 115-120 mmol/L, brain swelling may lead to seizures, coma, permanent brain damage, and death
-Because the skull is rigid, the brain cannot increase its volume by more than about 10 percent without it being forced down the neck (herniation), which can lead to permanent brain injury and death.
**When hyponatremia evolves more slowly over several days, the brain and other tissues respond by transporting sodium, chloride, potassium, and organic solutes, such as glutamate, from the cells into the extracellular compartment. This attenuates osmotic flow of water into the cells and swelling of the tissues
** When hypertonic solutions are added too rapidly to correct hyponatremia, this can outpace the brain's ability to recapture the solutes lost from the cells and may lead to osmotic injury of the neurons that is associated with demyelination, a loss of the myelin sheath from nerves