Snapshot A 79-year-old woman is brought to the emergency department from her nursing home due to altered mental status. Her nurse states that the patient is generally conversational and witty; however, the patient appears confused and reports feeling nauseous. On physical exam, the patient is alert but not oriented to place or time. Moist mucous membranes and an absence of edema are appreciated on exam. Pulmonary auscultation reveals localized crackles. Laboratory tests are significant for a serum Na+ of 124 mEq/L, plasma osmolality of 273 mOsm/kg, and urine osmolality of 205 mOsm/kg. Introduction Inappropriate release of antidiuretic hormone (ADH) results in impaired water excretion which leads to dilutional hyponatremia Causes of inappropriate ADH release includes malignancy e.g., small cell (oat cell) carcinoma of the lung medications e.g., carbamazepine, cyclophosphamide, nonsteroidal anti-inflammatory drugs (NSAIDs), sulfonylureas, and selective serotonin reuptake inhibitors (SSRIs) central nervous system (CNS) disturbances infection e.g., encaphalitis and meningitis malignancy stroke e.g., hemorrhagic and ischemic traumatic brain injury pulmonary disturbances pneumoniae e.g., bacterial and viral tuberculosis Epidemiology patients at greater risk for SIADH include those with malignant, CNS, and pulmonary disease in nursing homes on medications that enhance ADH activity hospitalized patients particularly on mechanical ventilation Presentation Symptoms symptoms of hyponatremia asymptomatic (in those with mild hyponatremia) headache lethargy disorientation hallucinations seizures Physical exam euvolemic absence of edema, ascites, and normal skin turgor Evaluation Laboratory testing blood tests ↓ serum sodium (< 135 mmol/L) ↓ plasma osmolality (< 275 mOsm/kg) ↓ serum uric acid (< 4 mg/dL) adrenal and thyroid function is normal urine tests ↑ urine osmolality (> 100 mOsm/kg) ↑ urine sodium (> 40 mmol/L) in a normal salt diet Differential Other causes of euvolemic hyponatremia hypothyroidism psychogenic polydypsia thiazide-diuretic use certain diets e.g., "tea and toast" (low solute diet) and beer potomania Treatment Fluid restriction initial therapy Sodium management is dependent on symptom severity and duration of hyponatremia as a rule of thumb, try to correct the underlying cause of hyponatremia if possible in chronic cases or unknown duration that are symptomatic severe symptoms hypertonic (3%) saline mild-to-moderate symptoms fluid restriction isotonic (0.9%) saline with furosemide in acute cases that are symptomatic severe symptoms hypertonic (3%) saline mild-to-moderate symptoms hypertonic (3%) saline in those with low risk of herniation frequently check serum sodium to prevent overcorrection in asymptomatic cases fluid restriction can consider salt tablets Medications vaptans (ADH receptor antagonist) e.g., tolvaptan and conivaptan demeclocycline (ADH receptor antagonist) lithium (decreases responsiveness to ADH) Prognosis, Prevention, and Complications Complications rapid correction of chronic hyponatremia may lead to osmotic demyelination syndrome (central pontine myelinolysis) severe hyponatremia may lead to seizure respiratory arrest coma