syndrome of inappropriate ADH
Summary
SIADH is characterized by excessive ADH secretion leading to water retention, hyponatremia, and concentrated urine despite normal/low plasma osmolality. Classic presentation includes euvolemic hyponatremia with inappropriately concentrated urine (>100 mOsm/kg) and normal kidney/adrenal function.
Detail
Syndrome of Inappropriate ADH (SIADH) occurs when ADH is released despite normal or low plasma osmolality, leading to excessive water reabsorption in the collecting duct. This results in dilutional hyponatremia while maintaining normal extracellular fluid volume (euvolemia). Key pathophysiology involves continued water retention without sodium retention, causing plasma osmolality to fall below the normal threshold for ADH release (280-295 mOsm/kg). Common causes include CNS disorders (stroke, trauma, infections), pulmonary diseases (pneumonia, lung cancer), medications (SSRIs, carbamazepine, cyclophosphamide), and malignancies (especially small cell lung cancer via ectopic ADH production). Diagnosis requires: serum sodium <135 mEq/L, plasma osmolality <280 mOsm/kg, urine osmolality >100 mOsm/kg, euvolemia, and normal thyroid/adrenal function. Treatment involves fluid restriction (primary), hypertonic saline for severe symptoms, and ADH receptor antagonists (tolvaptan) in refractory cases. Rapid correction risks osmotic demyelination syndrome.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- Robbins Basic Pathology
- UpToDate Clinical Decision Support
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