Common Electrolyte Disorders: Hyponatremia

Hospital Physician January 2009 23 Hyponatremia represents an abnormal ratio of total body sodium to water and is commonly defined as a plasma sodium concentration less than 135 mEq/L (1 mEq/L = 1 mmol/L).1–7 Hyponatremia is the most common electrolyte abnormality in hospitalized patients, with mild hyponatremia (plasma sodium < 135 mEq/L) present in approximately 15% to 22% of patients, moderate hyponatremia (< 130 mEq/L) present in 1% to 7% of patients, and severe hyponatremia (< 120 mEq/L) present in less than 1% of patients.8,9 Several populations are at increased risk of developing hyponatremia, including intensive care unit, postoperative, psychiatric, elderly, and nursing home patients.10–14 This article reviews the diagnostic and management approach to hyponatremia.