This is the first tutorial in a short series on hyponatremia. About 15% of our critical care patients has a problem with dysnatremia — high or low sodium levels in plasma. Hyponatremia, with symptoms, is a medical emergency as it can result in cerebral edema and irreversible brain injury.

In this tutorial I first present two case of hyponatremia – one that needs to be treated emergently and one that does not, despite both having the same plasma sodium levels. I then proceed to discuss the physiology of sodium and why it is a key component of body osmolality. The main part of the tutorial is developing a decision tree for working the hyponatremic problem. The key question is whether this is hypotonic or non hypotonic hyponatremia. If it is non hypotonic you need to look for other sources of unmeasured osmoles (usually alcohols). Hypotonic hyponatremia may be associated with myriad problems – but your main concern is whether or not this is being caused by kidney injury or blockade or normal renal pathways (e.g. diuretics). Ultimately I provide an algorithm for how to make a firm diagnosis of the cause of hyponatremia.  @ccmtutorials