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 pneumonia tuberculosis etiology Ectopic production small cell carcinoma of the lung Central nervous system disorders infection e.g., encephalitis and meningitis malignancy stroke trauma Drugs cyclophosphamide nonsteroidal anti-inflammatory drugs carbamazepine selective serotonin reuptake inhibitors (SSRIs) citalopram Pathogenesis impaired water excretion due to ↑ ADH may be due to increased production of ADH may be due to enhanced effects of ADH due to medications may be due to overdose of desmopressin Presentation Symptoms depends on level of hyponatremia mild symptoms (sodium usually 125-130 mEq/L) nausea and malaise moderate symptoms (115-125 mEq/L) headaches lethargy weakness severe symptoms (< 120 mEq/L) seizures coma respiratory arrest Physical exam euvolemic fluid status absence of edema normal skin turgor Studies Diagnostic testing studies serum studies ↓ serum osmolality (< 275 mOsm/kg) ↓ serum sodium (< 135 mEq/L) ↓ serum uric acid (< 4 mg/dL) adrenal and thyroid function are normal urine studies urine osmolality > serum osmolality ↑ urine osmolality (> 100 mOsm/kg) ↑ urine sodium (40 mEq/L) Differential Other causes of euvolemic hyponatremia hypothyroidism psychogenic polydipsia distinguishing factor ↑ urine osmolality on water deprivation test thiazide-diuretic use certain diets e.g., "tea and toast" (low solute diet) and beer potomania Treatment Management approach sodium management is dependent on symptom severity duration of hyponatremia (acute vs chronic) chronic hyponatremia must be corrected slowly to prevent osmotic demyelination syndrome (central pontine myelinolysis) correct underlying cause when possible First-line fluid restriction indication mild or moderate symptoms asymptomatic cases intravenous hypertonic (3%) saline indication chronic hyponatremia severe symptoms acute hyponatremia all cases Second-line salt tablets indication adjuvant therapy in patients who are asymptomatic Other medications demeclocycline (ADH receptor antagonist) vaptans (ADH receptor antagonist) Complications Osmotic demyelination syndrome (central pontine myelinolysis) caused by rapid correction of chronic hyponatremia