Hyponatremia: how to approach this confusing abnormality.

The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is not so frequent but it is a representative entity of hyponatremia. Although the clinical features were established by Bartter and Schwartz more than 30 years ago (1), a prototype of their diagnostic criteria has been applied so far. Hyponatremia is not a rare condition, especially in elderly persons. Nearly 1 to 5% of hospital patients showed hyponatremia, defined as a serum sodium level of less than 1 30 mEq// (2, 3), therefore, the appropriate differential diagnosis is essential. Hyponatremia is generally divided into two different pathophysiological states, hypervolemic and hypovolemic hyponatremia. SIADHis usually classified as normovolemic hyponatremia according to standard clinical assessment of extracellular fluid (ECF) volume. However, strictly speaking, a mild but apparent increase in ECFvolume is present which then induces dilutional hyponatremia. In clinical practice, the differentiation of these two states is important because the treatments are quite varied. Generally, water restriction is the first therapeutic choice in hypervolemic hyponatremia including SIADH,on the other hand, water deprivation aggravates the symptoms of patients with hypovolemic hyponatremia. The concept of SIADH is very familiar to physicians, thus the