Disorders of water metabolism in children: hyponatremia and hypernatremia.

1. Michael L. Moritz, MD* 2. Juan Carlos Ayus, MD† 1. *Assistant Professor of Pediatric Nephrology, Children’s Hospital of Pittsburgh, Pittsburgh, PA 2. †Professor of Medicine, University of Texas Health Science Center at San Antonio, San Antonio, TX After completing this article, readers should be able to: 1. Describe the clinical manifestations of hyponatremic encephalopathy. 2. Identify the risk factors for developing hyponatremic encephalopathy. 3. List the risk factors for developing cerebral demyelination following the correction of hyponatremia. 4. Characterize the clinical manifestations of hypernatremia. 5. Identify patients at greatest risk for developing hypernatremia. In conjunction with the tremendous medical advances of the past century, an increasing number of hospitalized patients are dependent on parenteral fluids. Caring for children who have complex medical conditions has resulted in new challenges for prescribing parenteral therapy to maintain sodium and water homeostasis; most electrolyte disturbances occur in the hospital. Although the kidneys play an important role in the development of disorders in water handling, most of the morbidity and mortality results from central nervous system dysfunction (Table 1). This review discusses common disorders of water metabolism, emphasizing the neurologic sequelae. View this table: Table 1. Abnormalities of Water Metabolism Leading to Brain Damage Hyponatremia is defined as a serum sodium level less than 135 mEq/L (135 mmol/L). It is one of the most common electrolyte disorders encountered in hospitals, occurring in approximately 3% of hospitalized children. The cause usually is identified easily, and the condition rarely is fatal, but sometimes the cause can be elusive and mortality can result from inappropriate therapy. ### Pathogenesis Under normal circumstances, the human body can maintain plasma sodium levels within the normal range (135 to 145 mEq/L [135 to 145 mmol/L]), despite wide fluctuations in fluid intake. The body’s primary defense against developing hyponatremia is the kidney’s ability to generate dilute urine and excrete free water. The primary reasons that children develop hyponatremia encompass underlying conditions that impair the kidney’s ability to excrete free water (Table 2). Hyponatremia usually occurs in the setting of excess water intake, with or without sodium losses, in the presence of impaired …