Snapshot A 79-year-old woman is brought to the emergency department from home for altered mental status. Her family reports that she is normally alert and oriented to person, place, and time, and very conversational. On exam, she is confused, alert only to person, and appears anxious. She has moist mucous membranes and normal skin turgor. Pulmonary auscultation reveals localized crackles. Laboratory tests are significant for serum sodium of 126 mEq/L. Introduction Clinical definition syndrome of inappropriate antidiuretic hormone (SIADH) is characterized by excessive free water retention and impaired water excretion, leading to dilutional hyponatremia Epidemiology Demographics common in hospitalized patients, particularly those on mechanical ventilation Risk factors older age malignancy pulmonary disease login to view 2 more bullets etiology Ectopic production small cell carcinoma of the lung Central nervous system disorders infection login to view 1 more bullet malignancy stroke trauma Drugs cyclophosphamide nonsteroidal anti-inflammatory drugs carbamazepine selective serotonin reuptake inhibitors (SSRIs) login to view 1 more bullet Pathogenesis impaired water excretion due to ↑ ADH login to view 3 more bullets Presentation Symptoms depends on level of hyponatremia mild symptoms (sodium usually 125-130 mEq/L) login to view 1 more bullet moderate symptoms (115-125 mEq/L) login to view 3 more bullets severe symptoms (< 120 mEq/L) login to view 3 more bullets Physical exam euvolemic fluid status login to view 2 more bullets Studies Diagnostic testing studies login to view 9 more bullets Differential Other causes of euvolemic hyponatremia hypothyroidism psychogenic polydipsia login to view 2 more bullets thiazide-diuretic use certain diets login to view 1 more bullet Treatment Management approach sodium management is dependent on login to view 2 more bullets chronic hyponatremia must be corrected slowly to prevent osmotic demyelination syndrome (central pontine myelinolysis) correct underlying cause when possible First-line fluid restriction login to view 3 more bullets intravenous hypertonic (3%) saline login to view 5 more bullets Second-line salt tablets login to view 2 more bullets Other medications demeclocycline (ADH receptor antagonist) vaptans (ADH receptor antagonist) Complications Osmotic demyelination syndrome (central pontine myelinolysis) caused by rapid correction of chronic hyponatremia