[Causes, diagnosis and differential diagnosis of hyponatremia].

Hyponatremia is a common electrolyte disorder that arises from disturbances in water metabolism. In cases of acute advanced hyponatremia, serious symptoms are predominant, while chronic mild hyponatremia causes minor symptoms such as slowness, depression or unsteadiness of gait. Any therapy of hyponatremia depends on the severity of its symptoms and on its specific etiology and diagnosis. Concerning the differential diagnosis of the type of hyponatremia, it is initially helpful to distinguish between euvolemic, hypervolemic and hypovolemic forms of hyponatremia. In order to distinguish between these 3 types of hyponatremia, it is best to assess the spontaneous urinary sodium concentration and to consider evidence from the medical history and the physical examination. Once the type of hyponatremia has been diagnosed, the next step is to decide which of the known etiologies of hyponatremia applies. Diagnostic problems may arise in mixed hyponatremia, a condition in which different types and etiologies of hyponatremia occur at the same time. In such cases it may be best to determine what appears to be the leading diagnosis. Another kind of diagnostic difficulty often occurs in simultaneous diuretic use. It may help to distinguish to which extracellular volume the types of hyponatremia relate on the basis of the fractional excretion of uric acid rather than on the urinary sodium concentration.