Hyponatremia is a disorder of water homeostasis. Water balance is maintained by the collaboration of renal function and cerebral structures, which regulate thirst mechanisms and secretion of the antidiuretic hormone. Measurement of serum-osmolality, urine osmolality and urine-sodium concentration help to diagnose the different reasons for hyponatremia. Hyponatremia induces cerebral edema and might lead to severe neurological symptoms, which need acute therapy. Also, mild forms of hyponatremia should be treated causally, or at least symptomatically. An inadequate fast increase of the serum sodium level should be avoided, because it raises the risk of cerebral osmotic demyelination. Basic pathophysiological knowledge is necessary to identify the different reasons for hyponatremia which need different therapeutic procedures.
UNASSIGNED: Die Hyponatriämie ist eine Störung des Wasserhaushaltes. Die Wasserhomöostase wird durch das Zusammenspiel von Nierenfunktion und den zerebralen Strukturen des Durstempfindens und der Produktion des antidiuretischen Hormons aufrechterhalten. Durch die Messung der Serum-Osmolalität, Harn-Osmolalität und Harn- Natriumkonzentration können die meisten Ursachen der Hyponatriämie identifiziert werden. Hyponatriämien führen zu einem Hirnödem und können damit schwere neurologische Symptome verursachen, welche eine akute Therapie benötigen. Aber auch milde Formen der Hyponatriämie bedürfen einer, wenn möglich kausalen, oder zumindest symptomatischen Behandlung. Eine inadäquat rasche Korrektur der Hyponatriämie sollte vermieden werden, da diese das Risiko für ein zerebrale osmotische Demyelinisierung erhöht. Da die Art der Therapie eng mit der Ursache der Hyponatriämie zusammenhängt, ist eine Grundkenntnis der pathophysiologischen Prozesse für eine optimale Behandlung notwendig.

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Hyponatremia is a common water balance disorder that often poses a diagnostic or therapeutic challenge. Therefore, guidelines were developed by professional organizations, one from within the United States (2013) and one from within Europe (2014). This review discusses the diagnosis and treatment of hyponatremia, comparing the two guidelines and highlighting recent developments. Diagnostically, the initial step is to differentiate hypotonic from nonhypotonic hyponatremia. Hypotonic hyponatremia is further differentiated on the basis of urine osmolality, urine sodium level, and volume status. Recently identified parameters, including fractional uric acid excretion and plasma copeptin concentration, may further improve the diagnostic approach. The treatment for hyponatremia is chosen on the basis of duration and symptoms. For acute or severely symptomatic hyponatremia, both guidelines adopted the approach of giving a bolus of hypertonic saline. Although fluid restriction remains the first-line treatment for most forms of chronic hyponatremia, therapy to increase renal free water excretion is often necessary. Vasopressin receptor antagonists, urea, and loop diuretics serve this purpose, but received different recommendations in the two guidelines. Such discrepancies may relate to different interpretations of the limited evidence or differences in guideline methodology. Nevertheless, the development of guidelines has been important in advancing this evolving field.