Diagnostic approach and management of inpatient hyponatremia

Abstract

Hyponatremia is one of the most common electrolyte abnormalities encountered in clinical practice. The frequency of the disorder varies according to definition and clinical setting but has been reported to be present in 28% of patients upon hospital admission and in 7% of patients attending an ambulatory community clinic. 1 Increasing age, medications, various disease states, and administration of hypotonic fluids are among the known risk factors for the disorder.The mortality rate in hyponatremic patients is approximately 3 times that of normonatremic hospitalized patients. [2][3][4][5] Outcomes are particularly poor in those patients whose serum sodium (Na þ ) falls during a hospitalization. In 1 prospective study the mortality rate in patients with a normal serum Na þ concentration was 0.2% in comparison to a mortality rate of 11.2% and 25% in patients with a serum Na þ concentration <130 mEq/L and <120 mEq/L, respectively. 2 In a recent retrospective cohort study of 10,899 hospitalized patients, the incidence of hyponatremia (<135 mmol/L) at admission was 5.5%. 5 As compared to those with normonatremia, these patients were more likely to require intensive care and mechanical ventilation within 48 hours of hospitalization. In addition, hospital mortality, mean length of stay, and costs were significantly greater among patients with hyponatremia than those without.The association with hyponatremia and adverse outcomes could be the direct result of hyponatremia, the comormidities that lead to the electrolyte derangement, or both. Whatever the mechanism, hyponatremia should not be viewed as an innocuous condition. Rather, clinicians should view this disorder with urgency and institute measures to prevent any further decline in the serum Na þ concentration and initiate appropriate therapy for its correction. This review will first briefly summarize the pathogenesis of hyponatremia and then discuss various disease states encountered in the hospital setting in which hyponatremia is frequently present. Pathogenesis of HyponatremiaHyponatremia is generally associated with a hypoosmolar state and is a marker for a disturbance in water balance. Stated differently, all hyponatremia is dilutional. The approach to the patient with hyponatremia is outlined in Figure 1. Is the Hyponatremia Representative of a Hypoosmolar State?There are 3 causes of hyponatremia in which it is not associated with a hypoosmolar state. The first of these is pseudohyponatremia which involves an abnormal measurement of the serum Na þ . This occurs in patients with hyperglobulinemia or hypertriglyceridemia in whom plasma water relative to plasma solids is decreased in blood, leading to less Na þ in a given volume of blood. In general, this problem is becoming less prevalent as many laboratories are using Na þ electrodes without diluting the blood such that the Na þ measurement becomes independent of plasma water and nonaqueous contents.A second cause of hyponatremia in the absence of a hypoosmolar state involves true hyponatremia but with elevati...

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