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Drug-Induced Syndrome of Inappropriate Antidiuretic Hormone

HYPONATREMIA, OFTEN DEFINED AS SERUM SODIUM LESS THAN 130 mmol/L, is the most common electrolyte disturbance among hospitalized patients. A complication of a variety of diseases, surgical treatments, or drugs, it may be hypertonic, hypotonic, or isotonic in nature. Antidiuretic hormone (ADH) is also known as arginine vasopressin (AVP). Produced in the posterior pituitary gland in response to an increased plasma sodium concentration, it induces water retention. It increases cellular permeability to water in the distal tubule and collecting duct of the nephron, leading to increased water resorption by the kidney. The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by either the sustained release of ADH in the absence of stimuli, or by the enhanced action of ADH on the kidneys. Increased ADH activity impairs the kidney’s ability to dilute urine, resulting in decreased excretion of ingested water and a highly concentrated and decreased volume of urine. If fluid intake is not sufficiently reduced in the setting of increased ADH activity, serum hypotonicity and hyponatremia will occur. Patients with SIADH will present with normal volume status (euvolemic) because the excess water distributes evenly throughout the body’s fluid compartments. Causes of SIADH include malignant diseases (e.g., carcinoma, lymphomas, sarcomas), pulmonary disorders (e.g., pneumonia, asthma), central nervous system disorders (e.g., meningitis, stroke, head trauma), and a number of medications.