Sodium Disorders

A serum sodium result is primarily a measure of water balance, not a direct inventory of total-body sodium. Sodium is the principal determinant of extracellular tonicity, so changes in its concentration alter the movement of water across cell membranes. The brain is especially vulnerable: water shifts into brain cells when plasma tonicity falls and out of them when tonicity rises. Hyponatraemia is a serum sodium concentration below 135 mmol/L. Canadian laboratories report sodium in mmol/L; the numerical value is identical to the mEq/L convention often used in the United States for this monovalent ion. Most hyponatraemia reflects too much electrolyte-free water relative to body solute. Non-osmotic release of antidiuretic hormone, also called vasopressin, makes the collecting ducts retain water even when plasma tonicity is already low. Sodium may be lost at the same time, but sodium loss alone does not explain most cases. This is why giving salt without correcting the water-retention mechanism may fail. Hypernatraemia, defined as sodium above 145 mmol/L, is almost always a water-deficit state. The patient may be unable to obtain water, unable to communicate thirst,...