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Hypovolemic Hypoosmolar Hyponatremia

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Hypovolemic Hypoosmolar Hyponatremia

  • See Also
  1. Hyponatremia
  2. Hyperosmolar Hyponatremia
  3. Normoosmolar Hyponatremia
  4. Hypoosmolar Hyponatremia
  5. Hypervolemic Hypoosmolar Hyponatremia
  6. Isovolemic Hypoosmolar Hyponatremia
  7. Hyponatremia Management
  8. Sodium and Water Homeostasis
  • 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
  1. Gastrointestinal losses
    1. Diarrhea
    2. Vomiting
      1. Also causes Metabolic Alkalosis
      2. Triggers renal bicarbonate excretion, and concomitant renal Sodium losses
  2. Third space losses
    1. Pancreatitis
    2. Pleural Effusion
  3. Skin Losses
    1. Severe burns
  1. Diuretics (Thiazide Diuretics, Loop Diuretics)
    1. Sodium loss with overdiuresis triggers ADH release
    2. Increased ADH results in free water retention
  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. Salt Wasting Nephropathy
    1. Causative agents (esp. Chemotherapy) inhibit epithelial Sodium channels resulting in Polyuria
  7. Cerebral salt wasting (head injuries, Intracranial Hemorrhage)
    1. Rare diagnosis of exclusion
    2. Associated with loss of ADH excretion and excessive Urine Output and key Urine Sodium loss
    3. Contrast with SIADH (characterized by water retention)
  • Labs
  1. Urine Sodium < 20 meq/L
    1. Non-Renal Sodium Loss (e.g. Vomiting, Diarrhea, severe burns)
    2. Other lab findings
      1. Urine Osmolality >400 mOsm/kg
      2. Fractional Excretion of Urea <35%
        1. Use instead of Urine Sodium in patients on Diuretics
        2. Carvounis (2002) Kidney Int 62(6): 2223-9 [PubMed]
  2. Urine Sodium > 20 meq/L
    1. Renal Sodium Loss (e.g. Diuretics, RTA, Adrenal Insufficiency)
    2. Other lab findings
      1. Urine Osmolality <400 mOsm/kg
  • Differential Diagnosis
  1. Often difficult to distinguish Iso- from Hypovolemic
  2. See Isovolemic Hypoosmolar Hyponatremia
  • 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. Edwards, Yang and Mehta (2025) Crit Dec Emerg Med 39(9): 25-33
  2. Kone in Tisher (1993) Nephrology, p. 87-100
  3. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  4. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  5. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
  6. Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]