Hyponatremia is a serum sodium below 135 mEq/L (normal is 135 to 145). On paper it is a lab value. In the patient it is a brain problem, twice over: low sodium swells the brain, and raising it too fast can strip the brain’s wiring for good. Both halves are about water moving across a cell membrane, which is very hard to picture from a sentence and very easy to picture once you have watched it.
Why low sodium swells the brain
When the blood is more dilute than the inside of the cells, water moves into the cells by osmosis. Brain cells sit inside a rigid skull, so their swelling shows up as headache, confusion, lethargy, and, as it worsens, seizures.
Live card · Hyponatremia lesson
Hyponatremia makes brain cells swell
Water visibly crosses into the neuron and it swells: the osmosis a sentence can only name.
The recall card that follows it
Hyponatremia, sodium 118. Which way does water move?
Where the extra water comes from
- SIADH: too much antidiuretic hormone makes the kidneys hold on to water, diluting the sodium. Fluid restriction is the usual first step.
- Thiazide diuretics make the kidneys lose sodium.
- Heart failure, vomiting and diarrhea, and drinking large amounts of plain water are other common routes.
The trap: correcting too fast
Once the brain has adapted to low sodium, raising it quickly pulls water back out of the neurons faster than they can cope, and the myelin around them can be damaged. That is osmotic demyelination syndrome (ODS), and it is permanent. The usual limit is a rise of no more than 8 to 12 mEq/L in 24 hours; the provider sets the exact target, and the nurse watches the sodium checks against it.
Live card · Hyponatremia lesson
Raising low sodium too fast causes ODS
Osmotic demyelination: the damage is permanent
The myelin peels off the neuron as water rushes back out of it, which is why a fast correction does lasting harm.
The recall card that follows it
What can happen when low sodium is raised too fast?
What the nurse does
- Frequent neuro checks: level of consciousness, confusion, headache.
- Seizure precautions when the sodium is low or falling.
- Strict intake and output, daily weights, and any ordered fluid restriction.
- Report a sodium that is rising faster than ordered, not only one that is low.
Run the whole lesson
Hyponatremia · live lesson
More on how these cards are built: visual learning without videos. The high side lives in the hypernatremia lesson.
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Common questions
Why does hyponatremia cause brain swelling?
When the blood is more dilute than the cells, water moves into the cells by osmosis. Brain cells sit inside the rigid skull, so the swelling causes headache, confusion and, as it worsens, seizures.
How fast can low sodium be corrected?
Usually no more than 8 to 12 mEq/L in 24 hours; the provider sets the exact limit. Raising it faster can cause osmotic demyelination syndrome, which is permanent.