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Snap Shot
  • 25-year-old male complains of an unabated thirst that began three weeks ago. He is constantly drinking and goes to the bathroom around five times a night. He has lost five pounds over the last few weeks, and is on lithium for a bipolar disorder. His BP is 115/70.
Introduction
  • A condition characterized by the inability of the body to reabsorb free water in the collecting duct secondary to
    • dysfunctional production of ADH
    • dysfunctional response to ADH
  • Pathophysiology
    • classified as central or nephrogenic
      • central
        • characterized by decreased ADH secretion 
        • caused by
          • idiopathic
          • trauma 
          • pituitary infection (fungal, TB)
          • pituitary surgery  
          • histiocytosis X
      • nephrogenic
        • characterized by ADH resistance 
        • caused by
          • drugs (lithium, amphotericin B) 
          • severe hypokalemia (makes tubules resistant to ADH)
Presentation
  • Symptoms 
    • polyuria
    • polydipsia
    • nocturia
Evaluation
  • Labs
    • hypernatremia
      • loss of more water than salt
  • Water deprivation test
    • hold all water 
    • positive DI results include
      • urine specific gravity < 1.010
      • urine osmolarity < 500
      • serum osmolarity > 300
    • represent an inability to concentrate urine despite increasing serum osmolarity
  • Desmopressin (DDAVP) test  
    • response to desmopressin distinguishes between central and nephrogenic
      • if urine osmolarity rises >50% over the pre-administration osmolarity after administration the DI is central
      • if urine osmolarity rises <50% over the pre-administration osmolarity the DI is nephrogenic
Treatment
  • Central
    • pharmacologic
      • DDAVP/desmopressin 
        • ADH analogue
  • Nephrogenic
    • lifestyle
      • increased water intake
      • sodium restriction
    • pharmacologic
      • thiazides
        • volume depletion => decreased GFR => increased PCT reabsorption of solutes and decreased delivery of solutes and water to the collecting duct
 

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