Hyponatremia in the cancer patient.

Hyponatremia is a common metabolic complication in cancer patients. It can be caused by the primary tumor itself, metastasis, by diagnostic or therapeutic interventions, or by a secondary complication. A clear understanding of water and sodium homeostasis is required for evaluation. The initial diagnostic step is to define a hypoosmolar state. The history and physical examination should be tailored to determine the volume status of the patient--volume depleted, euvolemic, or edematous. In general, hyponatremia associated with volume depletion is treated with saline, while fluid restriction is the primary therapy in patients with normal or increased extracellular fluid volume. In symptomatic patients, the duration of the hyponatremia and its causes should be determined. Both overzealous and inadequate treatment put the patient at risk of serious neurologic sequelae.