Etiology Vasopressin is an antidiuretic hormone normally produced by the posterior pituitary gland. Ectopic vasopressin production by tumors is a common cause of the syndrome of inappropriate antidiuretic hormone (SIADH), occurring in at least half of patients with SCLC. Compensatory mechanisms, such as decreased thirst, suppression of aldosterone, and production of atrial natriuretic peptide (ANP), may mitigate the development of hyponatremia in patients who produce excessive vasopressin. Tumors with neuroendocrine features, such as SCLC and carcinoids, are the most common sources of ectopic vasopressin production, but it also occurs in other forms of lung cancer and with central nervous system. | Chapter 096. Paraneoplastic Syndromes Endocrinologic Hematologic Part 4 Etiology Vasopressin is an antidiuretic hormone normally produced by the posterior pituitary gland. Ectopic vasopressin production by tumors is a common cause of the syndrome of inappropriate antidiuretic hormone SIADH occurring in at least half of patients with SCLC. Compensatory mechanisms such as decreased thirst suppression of aldosterone and production of atrial natriuretic peptide ANP may mitigate the development of hyponatremia in patients who produce excessive vasopressin. Tumors with neuroendocrine features such as SCLC and carcinoids are the most common sources of ectopic vasopressin production but it also occurs in other forms of lung cancer and with central nervous system CNS lesions head and neck cancer and genitourinary gastrointestinal and ovarian cancers. The mechanism of activation of the vasopressin gene in these tumors is unknown but often involves concomitant expression of the adjacent oxytocin gene suggesting derepression of this locus. Clinical Manifestations Most patients with ectopic vasopressin secretion are asymptomatic and are identified because of the presence of hyponatremia on routine chemistry testing. Symptoms may include weakness lethargy nausea confusion depressed mental status and seizures. The severity of symptoms reflects the rapidity of onset as well as the extent of hyponatremia. Hyponatremia usually develops slowly but may be exacerbated by the administration of IV fluids or the institution of new medications. Thirst is typically suppressed. Diagnosis The diagnostic features of ectopic vasopressin production are the same as those of other causes of SIADH Chaps. 46 and 334 . Hyponatremia and reduced serum osmolality occur in the setting of an inappropriately normal or increased urine osmolality. Urine sodium excretion is normal or increased unless volume depletion is present. Other causes of hyponatremia should be excluded including renal adrenal or .
