Sodium

Hypovolemic Hypoosmolar Hyponatremia

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Hypovolemic Hypoosmolar Hyponatremia

  • Pathophysiology
  1. Total Body Sodium Deficit exceeds water losses
  2. Decreased Extracellular fluid volume
    1. Increased proximal tubule fluid reabsorption
    2. Decreased distal segment flow where dilution occurs
  3. Hypovolemia stimulates non-osmotic fluid conservation
    1. Thirst
    2. ADH secretion
  • Differential Diagnosis
  1. Often difficult to distinguish Iso- from Hypovolemic
  2. See Isovolemic Hypoosmolar Hyponatremia
  • Causes
  • Non-Renal Losses (Sodium appropriately conserved)
  1. Gastrointestinal losses
    1. Vomiting
    2. Diarrhea
  2. Third space losses
    1. Pancreatitis
    2. Pleural Effusion
  3. Skin Losses
    1. Severe burns
  • Causes
  • Renal Losses (Renal inappropriate Sodium losses)
  1. Diuretics (e.g. chronic Thiazide Diuretic use)
  2. Renal Tubular Acidosis
    1. Hyperchloremic Metabolic Acidosis
    2. Increased urinary pH
    3. Fractional Excretion of Bicarbonate >15-20%
  3. Salt-losing Glomerulonephritis
    1. Chronic Renal Insufficiency on low Sodium diet
    2. Severe interstitial Kidney disease
      1. Polycystic Kidney Disease
      2. Medullary cystic disease
      3. Chronic Pyelonephritis
  4. Mineralocorticoid and Glucocorticoid deficiency
    1. Adrenal Insufficiency (Addison's Disease)
  5. Osmotic Diuresis (Bicarbonate, Glucose, Ketones)
    1. Excess osmotically active solutes in urine
    2. Draws increased Sodium and water renal losses
  6. Cerebral salt wasting (head injuries, Intracranial Hemorrhage)
    1. Diagnosis of exclusion
  • Management
  1. See Hyponatremia Management
  2. Stop all Diuretics
  3. Correct non-renal fluid losses
  4. Replace Sodium deficit
    1. Calculate Total Body Sodium Deficit
    2. Use Normal Saline (0.9% = 150 meq/L)
    3. Replace one third Sodium deficit over first 6-8 hours
    4. Replace remaining Sodium deficit in next 24-48 hours
  • References
  1. Kone in Tisher (1993) Nephrology, p. 87-100
  2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  3. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  4. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]