Nursing care
Why does hyponatremia cause seizures? Water, brain cells and the speed of the drop
Written and reviewed by Kevin Patel, MSN, RN, CCRN · 4 min read · Updated November 2026
Short answer
Hyponatremia causes seizures because low sodium leaves the blood more dilute than brain cells, so water moves into the cells by osmosis and the brain swells. Swollen, irritable neurons inside a rigid skull can fire abnormally. How fast the sodium fell matters as much as the number, because a quick drop gives brain cells no time to adapt.
Osmosis inside a closed box
Sodium is the main particle holding water in the blood vessels and the fluid around cells. When serum sodium falls, the blood becomes more dilute than the inside of brain cells. Water follows the concentration difference and moves into the cells. MSD Manuals describes this osmotic shift of water into brain cells as the cause of cerebral edema in hyponatremia.
Most organs can swell a little without harm. The brain cannot, because the skull does not stretch. MedlinePlus notes that brain cells are especially sensitive to swelling. As pressure rises, neurons become irritable and the client moves from headache, confusion and restlessness toward seizures, coma and, in the worst cases, brain herniation.
Why the speed of the fall matters as much as the level
Brain cells have a defense. Given time, they push solutes out, so their inside matches the more dilute blood and water stops rushing in. MSD Manuals explains that chronic hyponatremia allows this adaptation, while a rapid fall does not, leaving the brain relatively concentrated compared with the serum and still swelling.
MedlinePlus states that hyponatremia developing in less than 48 hours is more dangerous than hyponatremia that develops slowly. That is why a marathon runner who drank large amounts of water over a few hours can seize at a sodium value that an older adult with long-standing low sodium tolerates with only mild confusion. MSD Manuals places seizures and coma mostly below about 115 mEq/L, but the timeline changes the picture.
Who swells fastest
MSD Manuals singles out premenopausal women, because estrogen and progesterone inhibit the brain's sodium-potassium pump and slow solute removal from brain cells. That weakens the adaptive defense. Children also appear more susceptible. In an exam stem, a young woman with acute water intoxication after surgery or an endurance event deserves more urgency than the sodium value alone suggests.
Common causes listed by MedlinePlus include diuretics, heart failure, kidney disease, cirrhosis, SIADH, vomiting, diarrhea and heavy sweating. The cause shapes the treatment, but the seizure risk depends on how low and how fast. A client with a newly falling sodium plus headache, vomiting or new confusion is showing early brain swelling and needs reporting before a seizure happens.
Seizure care here includes fixing the sodium, carefully
Standard seizure safety still applies: protect the head, turn the client to the side, maintain the airway and time the event. The difference is that the seizure is driven by brain swelling, so correcting the cause matters. MSD Manuals describes giving hypertonic saline to severely symptomatic clients with acute hyponatremia to raise sodium by about 4 to 6 mEq/L quickly.
After that first rise, correction slows. MSD Manuals sets a limit of 8 mEq/L in 24 hours, because correcting too fast can cause osmotic demyelination syndrome, with permanent paralysis and other lasting damage. The nurse tracks sodium on the schedule ordered, watches urine output, and reports a rise faster than the prescribed target.
Hypothetical practice scenario
Hypothetical scenario, not a real exam item: a 24-year-old woman is admitted after a long-distance race with vomiting, headache and confusion. Sodium is 118 mEq/L, and she has a brief generalized seizure. Which order should the nurse anticipate: oral fluids to replace race losses, hypertonic saline under close monitoring, or a fluid restriction alone?
Hypertonic saline is the expected order, because she has acute, symptomatic hyponatremia with a seizure and the brain is swelling now. Oral fluids would dilute her sodium further. Fluid restriction has a place in slower, milder cases, but it works too slowly for a seizing client. Frequent sodium checks guard against overcorrection.
Sources and further reading
MSD Manual Professional Version: Hyponatremia. Osmotic water shift into brain cells and edema; adaptation in chronic vs rapid hyponatremia; seizures and coma mostly below about 115 mEq/L; premenopausal women and children at higher risk; hypertonic saline 4 to 6 mEq/L rise; 8 mEq/L per 24 hours limit; osmotic demyelination.
MedlinePlus: Low blood sodium. Causes of hyponatremia; brain cells especially sensitive to swelling; seizures and herniation; hyponatremia developing in under 48 hours is more dangerous.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our neurological practice questions are the closest set to what this page covers.
Common questions
At what sodium level do seizures occur?
There is no single cutoff. MSD Manuals places seizures and coma mostly below about 115 mEq/L, but speed matters. A rapid drop over less than 48 hours can cause seizures at higher values, while slow, chronic hyponatremia may cause only mild symptoms at lower ones.
Why does water move into brain cells in hyponatremia?
Low serum sodium makes the blood more dilute than the inside of brain cells. Water moves by osmosis toward the more concentrated side, so it enters the cells. In a skull that cannot expand, that swelling raises pressure and makes neurons irritable.
Why is sodium corrected slowly after a hyponatremic seizure?
Once the brain has adapted, raising sodium too fast pulls water out of brain cells and can cause osmotic demyelination syndrome. MSD Manuals recommends a quick 4 to 6 mEq/L rise for severe symptoms, then a total of no more than 8 mEq/L in 24 hours.