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Review Articles

Primary Care




HE serum sodium concentration and thus serum osmolality are closely controlled by water homeostasis, which is mediated by thirst, arginine vasopressin, and the kidneys.1 A disruption in the water balance is manifested as an abnormality in the serum sodiumconcentration — hypernatremia or hyponatremia.2,3 Hypernatremia, defined as a rise in the serum sodium concentration to a value exceeding 145 mmol per liter, is a common electrolyte disorder. Because sodium is a functionally impermeable solute, it contributes to tonicity and induces the movement of water across cell membranes.4 Therefore, hypernatremia invariably denotes hypertonic hyperosmolality andalways causes cellular dehydration, at least transiently (Fig. 1). The resultant morbidity may be inconsequential, serious, or even life-threatening.5 Hypernatremia frequently develops in hospitalized patients as an iatrogenic condition, and some of its most serious complications result not from the disorder itself but from inappropriate treatment of it.6,7 In this article, we focus on the managementof hypernatremia, emphasizing a quantitative approach to the correction of the fluid imbalance.8

clinical interventions or accidental sodium loading (Table 1 and Fig. 1E). Because sustained hypernatremia can occur only when thirst or access to water is impaired, the groups at highest risk are patients with altered mental status, intubated patients, infants, and elderly persons.12Hypernatremia in infants usually results from diarrhea, whereas in elderly persons it is usually associated with infirmity or febrile illness.6,13,14 Thirst impairment also occurs in elderly patients.15,16 Frail nursing home residents and hospitalized patients are prone to hypernatremia because they depend on others for their water requirements.7

Hypernatremia represents adeficit of water in relation to the body’s sodium stores, which can result from a net water loss or a hypertonic sodium gain (Table 1). Net water loss accounts for the majority of cases of hypernatremia.9-11 It can occur in the absence of a sodium deficit (pure water loss) (Fig. 1B) or in its presence (hypotonic fluid loss) (Fig. 1C and 1D). Hypertonic sodium gain usually results from

From theDepartment of Medicine, Baylor College of Medicine and Methodist Hospital, and the Renal Section, Department of Veterans Affairs Medical Center, Houston (H.J.A.); and the Department of Medicine, Tufts University School of Medicine, and the Division of Nephrology and Tupper Research Institute, New England Medical Center, Boston (N.E.M.). Address reprint requests to Dr. Madias at the Division ofNephrology, New England Medical Center, Box 172, 750 Washington St., Boston, MA 02111, or at nmadias@infonet.tufts.edu. ©2000, Massachusetts Medical Society.

Signs and symptoms of hypernatremia largely reflect central nervous system dysfunction and are prominent when the increase in the serum sodium concentration is large or occurs rapidly (i.e., over a period of hours).1,6 Most outpatients withhypernatremia are either very young or very old.17 Common symptoms in infants include hyperpnea, muscle weakness, restlessness, a characteristic high-pitched cry, insomnia, lethargy, and even coma.5,13 Convulsions are typically absent except in cases of inadvertent sodium loading or aggressive rehydration.14,18,19 Unlike infants, elderly patients generally have few symptoms until the serum sodiumconcentration exceeds 160 mmol per liter.17,20 Intense thirst may be present initially, but it dissipates as the disorder progresses and is absent in patients with hypodipsia.5 The level of consciousness is correlated with the severity of the hypernatremia.6 Muscle weakness, confusion, and coma are sometimes manifestations of coexisting disorders rather than of the hypernatremia itself. Unlike...
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