Hyponatremia is dangerous because the brain lives inside a fixed skull.
When serum sodium falls, plasma osmolality falls. Water shifts into brain cells. Cerebral edema can develop.
The speed of that change matters enormously.
Acute hyponatremia is poorly tolerated because the brain has not had time to adapt. Water movement into brain cells can rapidly increase intracranial pressure and cause headache, vomiting, confusion, seizures, coma, or herniation.
Chronic hyponatremia is different.
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