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Hypovolemic Hypoosmolar Hyponatremia

See Also
1. Hypoosmolar Hyponatremia
2. Isovolemic 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
Labs
1. Non-Renal Sodium Loss
  1. Urine Sodium < 20 meq/L
  2. Urine Osmolality >400
2. Renal Sodium Loss
  1. Urine Sodium > 20 meq/L
  2. Urine Osmolality <400
Causes
1. 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
2. Renal Losses (Renal inappropriate sodium losses)
  1. Diuretics (e.g. chronic Thiazide Diuretic use)
  2. 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
  3. Mineralocorticoid and Glucocorticoid deficiency
    1. Example: Adrenal Insufficiency
  4. Osmotic Diuresis (Bicarbonate, Glucose, Ketones)
    1. Excess osmotically active solutes in urine
    2. Draws increased sodium and water renal losses
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

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This page was written by Scott Moses, MD. This page was last revised before 5/10/08, prior to consistent tracking of revision date. This page was last published on 6/1/2008. Please send comments and questions to the editor. Contact information may be found on the page About the Family Practice Notebook. These pages are best viewed with Microsoft Internet Explorer 6.0.