The treatment of hyponatremia is critically dependent on its rate of development, severity, presence of symptoms and underlying cause. If hyponatremia has developed rapidly (< 48 hours) and there are signs of cerebral oedema, such as obtundation or convulsions, sodium levels should be restored rapidly to normal by infusion of hypertonic (3%) sodium chloride. A common approach is to give an initial bolus of 150 mL over 20 minutes, which may be repeated once or twice over the initial hours of observation, depending on the neurological response and rise in plasma sodium.
Rapid correction of hyponatremia that has developed more slowly (> 48 hours) can be hazardous, since brain cells adapt to slowly developing hypo-osmolality by reducing the intracellular osmolality, thus maintaining normal cell volume. Under these conditions, an abrupt increase in extracellular osmolality can lead to water shifting out of neurons,
abruptly reducing their volume and causing them to detach from their myelin sheaths. The resulting ‘myelinolysis’ can produce permanent structural and functional damage to mid-brain structures and is generally fatal. The rate of correction of the plasma Na concentration in chronic asymptomatic hyponatremia should not exceed 10 mmol/L/24 hrs, and an even slower rate is generally safer.
