[Hyponatremia in cancer patients]

Hyponatremia is one of the most common electrolyte disturbances in cancer patients. Patients with extremely severe symptomatic hyponatremia need treatment with the administration of hypertonic saline. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a significant cause of cancer-related hyponatremia. A prospective study at a dedicated cancer hospital in Belgium demonstrated that SIADH was the most common cause of hyponatremia (30.4%). Ectopic ADH production by malignant cells (especially in small-cell lung cancer), several anticancer drugs (cyclophosphamide, ifosfamide, vincristine, cisplatin, et al.), stress from surgery, pain, and nausea, may cause SIADH in cancer patients. The second most common cause of hyponatremia in the Belgian investigation was sodium depletion (28.7%). In addition to gastrointestinal losses of sodium (vomiting, diarrhea), salt wasting syndrome (SWS) must be considerd as a cause of sodium depletion. Cerebral salt wasting syndrome (CSWS) with severe central nervous system diseases and renal salt wasting syndrome (RSWS) with cisplatin administration are especially important. Although identifying SWS or SIADH as the cause of hyponatremia is difficult, the treatment strategy for SWS is basically different from that for SIADH. Fluid restriction is generally prescribed for the hyponatremia associated with SIADH, and fluid replacement is indicated for the volume depletion associated with SWS. Furthermore, central nervous system disease and cisplatin administration, may cause both SWS and SIADH. This fact complicates the differential diagnosis, and careful management is necessary.