Why can a sodium disorder not simply be corrected as fast as possible? Treating a sodium disorder means treating its cause and also restoring the sodium concentration at a rate the brain can follow.
The picture is this. Plasma sodium is a ratio of sodium to the water that holds it, as the sodium-regulation note showed. Water crosses cell membranes freely and moves toward whichever side holds more of the solutes that cannot cross, as the sodium-regulation note also showed. The brain has already adapted to the sodium it has been living with, and over the first days brain cells adapt by unloading their own solutes, while over longer periods they make solutes of their own that hold water, as the clinical-features note showed. Those solutes are what the brain has to carry along during treatment.
The brain adapts to a changed sodium over about a day or more, and it needs a similar time to undo that adaptation. If the sodium is corrected faster than the brain can adjust, water shifts back too quickly across the cell membrane, and the cells cannot cope with it. The brain is injured. Each direction, hypernatremia and hyponatremia, has its own complication, so each is taken in turn, starting with high sodium.
High sodium: giving back the water
In hypernatremia the missing piece is water. Plasma sodium is the sodium divided by the water that holds it, so replacing the free-water deficit lowers the concentration. The underlying cause is corrected as well.
When the patient can take it, oral or enteral water is preferred. Otherwise hypotonic intravenous fluids are used.
How fast can high sodium come down?
Giving water lowers the sodium, so the question becomes how fast. The safe rate depends on how long the hypernatremia has been present.
A hypernatremia that developed within about 24 hours has not given the brain time to build its own solutes. It can be corrected relatively quickly, up to about 1 mmol/L per hour.
A hypernatremia that is chronic, or of unknown duration, must be corrected more slowly: no faster than 0.5 mmol/L per hour, and no more than 8 to 10 mmol/L in 24 hours. Why the difference? Over time the brain cells have accumulated solutes that they make themselves, and these take days to disperse. They keep water inside the cells while the sodium outside falls.
What happens if high sodium falls too fast?
Those solutes are still in the cells when the sodium falls. If the sodium is lowered too fast, water rushes back into the brain cells, and the result is cerebral edema. The cells swell inside a skull that cannot expand, as the clinical-features note showed, and that is why the swelling injures the brain.
Low sodium that is not an emergency
The choice of treatment follows the volume status and the severity of symptoms, and the volume status sorts the disorder into groups that point to different causes, as the diagnosis note showed.
In hypovolemic hyponatremia, treatment restores the volume with isotonic (0.9%) saline. After a loss, a fall in blood pressure triggers ADH, which makes the kidney keep water, as the disorders note showed. Restoring volume removes that stimulus, so the kidney can excrete the water and the sodium rises.
In euvolemic hyponatremia, most often SIADH, treatment usually begins with fluid restriction, often to about 500 mL less than the daily urine volume. The kidney keeps water whatever comes in, because ADH is high, so cutting the intake is the first measure. The cause is treated together with this.
In hypervolemic hyponatremia, the underlying heart failure, cirrhosis or kidney injury is treated. In cirrhosis and in heart failure, extracellular fluid volume rises while tissue perfusion falls, as the body-water note showed, so the kidney holds water as it does when effective intravascular volume falls. Treating the disease is aimed at that trigger. It is done alongside sodium and fluid restriction and alongside diuretics that remove the excess water. When the body retains sodium, water follows it, so restricting sodium limits the water that follows, and restricting fluid limits the extra intake.
When low sodium is an emergency
Some patients cannot wait for the cause to be treated, because the brain is affected. Severe symptomatic hyponatremia means a reduced level of consciousness, seizures or coma. It is a medical emergency. The brain sits in a skull that cannot expand, so swelling compresses it, and the swelling can progress to herniation and death.
These patients are treated with hypertonic 3% saline. Water leaves the swollen cells, because the saline holds more of the solutes that cannot cross than the cells do. It is given as small boluses, commonly 100 to 150 mL, and the boluses are repeated until symptoms resolve. Each bolus is small and the patient is reassessed before a repeat, which might help prevent overcorrection; this is proposed rather than established. The symptoms come from swollen brain cells, and a rise of about 4 to 6 mmol/L is usually enough to relieve them.
How far may low sodium rise?
Once the symptoms ease, the rise must slow. The brain has adapted to the low sodium, and a faster rise is the danger the limits guard against. After the first rise, sodium should rise by no more than 6 to 12 mmol/L in the first 24 hours, and by no more than 18 mmol/L in 48 hours.
Correcting chronic hyponatremia too quickly can cause osmotic demyelination syndrome (ODS). In chronic hyponatremia the brain cells have lost solutes. A proposed explanation is that a fast rise draws water out of the cells before their solutes can be rebuilt, so the cells shrink and their myelin is injured. ODS is a rare demyelinating brain injury. Its risk is greater when the hyponatremia is chronic, and greater in malnourished patients, in patients with alcoholism and in patients with liver disease.
Where saline fits
Saline has appeared in these treatments: isotonic in volume loss, hypertonic in the emergency. In SIADH, where the urine is concentrated, a saline must be judged against the urine rather than the plasma. In SIADH, is a saline judged against the blood or against the urine?