Sodium & Water

Hyponatraemia: A Water Problem, Not a Salt Problem

It is the commonest electrolyte abnormality on the ward, and one of the few where the intuitive treatment can kill. The name says "low sodium," so the reflex is to reach for salt. But a low serum sodium almost never means the body is short of salt — it means there is too much water diluting the salt that is there. Get the reasoning backwards and you either flood a patient already drowning in water, or you correct them so fast that you destroy the brain you were trying to protect. This chapter is about thinking in the right currency: water and volume, not salt.

Hypernatraemia and Diabetes Insipidus: When Water Is Lost

Hyponatraemia is usually too much water; hypernatraemia is almost always too little. A rising sodium is the number screaming that free water has drained out of the body — through the kidney, the gut, the skin, or a failing thirst — and hasn't been replaced. The neurons pay the price: they shrink. And the treatment carries the same cruel symmetry as its mirror image — pour water back too fast and the brain, which has quietly defended itself, now swells. The disorder's most famous cause, diabetes insipidus, is where the pharmacology lives.

Osmotic Agents: Pulling Water With Mannitol and Hypertonic Saline

Most drugs work by binding a receptor, an enzyme, a channel — a molecular lock and key. Osmotic agents don't bother with any of that. They work by a law of physics: put enough of a solute in one place, and water will move toward it, all on its own. There is no receptor, no signalling, no metabolism to speak of — just a gradient and the water that follows it. From shrinking a swollen brain in a neuro-emergency, to pulling fluid off the eye in acute glaucoma, to loosening the bowel, the same single trick is doing all the work. Understand osmosis and you understand every one of these agents at once.