[Hyponatremia, toward a logical approach: the balance of tonicity]

Derangements in plasma sodium concentration are best analyzed by carefully determining entries and output for water, sodium and potassium, that is, by calculating a tonicity balance. Five clinical hyponatremic examples are discussed: the beer drinker has a severe deficit in total body sodium: the elderly female patient treated with a thiazide needs to be firstly repleted in potassium; the hypertensive transplanted patient with a multidrug treatment requires an increase of the urinary electrolyte-free water which has been obtained by the oral administration of urea; the post-operative hyponatremic cases (cases 4 and 5) are complex and involve a desalination phenomenon. Close observation, repeated determinations of electrolytes in plasma, urine and entries, together with measurements of water input and output, will allow the tonicity balance of the patient to be understood and thus occasional tragedies such as observed in cases 4 and 5 to be prevented.