[The pitfall "dehydration" and hyponatremia].

Hyponatremia represents a very common disturbance in hospitalized eldery. It correlates with delirium, falls, and intrahospital mortality. Often hyponatremia is ignored or missclassified. The represented algorithm is based on only a few parameters: serum and urine osmolality, and sodium. Treatment of the underlying causes of hyponatremia, e.g., cessation of medications and polypharmacy, is essential. The syndrome of inadequate secretion of antidiuretic hormone (SIADH; hypotonic isovolemic hyponatremia) is also considered.