PRIMARY CARE
Review Articles
Primary Care 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 HYPERNATREMIA at highest risk are patients with altered mental status, intubated patients, infants, and elderly persons.12 Hy- HORACIO J. ADROGUÉ, M.D., pernatremia in infants usually results from diarrhea, AND NICOLAOS E. MADIAS, M.D. 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 hy- HE serum sodium concentration and thus se- pernatremia because they depend on others for their rum osmolality are closely controlled by wa- water requirements.7 ter homeostasis, which is mediated by thirst, T 1 CLINICAL MANIFESTATIONS arginine vasopressin, and the kidneys. A disruption in the water balance is manifested as an abnormality Signs and symptoms of hypernatremia largely re- in the serum sodium concentration — hypernatre- flect central nervous system dysfunction and are prom- mia or hyponatremia.2,3 Hypernatremia, defined as a inent when the increase in the serum sodium con- rise in the serum sodium concentration to a value ex- centration is large or occurs rapidly (i.e., over a period ceeding 145 mmol per liter, is a common electrolyte of hours).1,6 Most outpatients with hypernatremia disorder. Because sodium is a functionally imperme- are either very young or very old.17 Common symp- able solute, it contributes to tonicity and induces the toms in infants include hyperpnea, muscle weakness, movement of water across cell membranes.4 There- restlessness, a characteristic high-pitched cry, insom- fore, hypernatremia invariably denotes hypertonic hy- nia, lethargy, and even coma.5,13 Convulsions are typ- perosmolality and always causes cellular dehydration, ically absent except in cases of inadvertent sodium at least transiently (Fig. 1). The resultant morbidity loading or aggressive rehydration.14,18,19 Unlike in- may be inconsequential, serious, or even life-threat- fants, elderly patients generally have few symptoms ening.5 Hypernatremia frequently develops in hospi- until the serum sodium concentration exceeds 160 talized patients as an iatrogenic condition, and some mmol per liter.17,20 Intense thirst may be present ini- of its most serious complications result not from the tially, but it dissipates as the disorder progresses and disorder itself but from inappropriate treatment of is absent in patients with hypodipsia.5 The level of it.6,7 In this article, we focus on the management of consciousness is correlated with the severity of the hy- hypernatremia, emphasizing a quantitative approach pernatremia.6 Muscle weakness, confusion, and coma to the correction of the fluid imbalance.8 are sometimes manifestations of coexisting disorders rather than of the hypernatremia itself. CAUSES Unlike hypernatremia in outpatients, hospital- Hypernatremia represents a deficit of water in re- acquired hypernatremia affects patients of all ages.7 lation to the body’s sodium stores, which can result The clinical manifestations are even more elusive in from a net water loss or a hypertonic sodium gain hospitalized patients because they often have preex- (Table 1). Net water loss accounts for the majority isting neurologic dysfunction. As in children, rapid of cases of hypernatremia.9-11 It can occur in the ab- sodium loading in adults can cause convulsions and sence of a sodium deficit (pure water loss) (Fig. 1B) coma.5,21 In patients of all ages, orthostatic hypoten- or in its presence (hypotonic fluid loss) (Fig. 1C and sion and tachycardia reflect marked hypovolemia. 1D). Hypertonic sodium gain usually results from Brain shrinkage induced by hypernatremia can cause vascular rupture, with cerebral bleeding, subarach- noid hemorrhage, and permanent neurologic dam- From the Department of Medicine, Baylor College of Medicine and age or death. Brain shrinkage is countered by an adap- Methodist Hospital, and the Renal Section, Department of Veterans Affairs tive response that is initiated promptly and consists Medical Center, Houston (H.J.A.); and the Department of Medicine, Tufts University School of Medicine, and the Division of Nephrology and Tup- of solute gain by the brain that tends to restore lost per Research Institute, New England Medical Center, Boston (N.E.M.). water. This response leads to the normalization of Address reprint requests to Dr. Madias at the Division of Nephrology, New brain volume and accounts for the milder symptoms England Medical Center, Box 172, 750 Washington St., Boston, MA 22-24 02111, or at [email protected]. of hypernatremia that develops slowly (Fig. 2). ©2000, Massachusetts Medical Society. However, the normalization of brain volume does
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