Hypoosmolar Hyponatremia


Hypoosmolar Hyponatremia, Hypotonic Hyponatremia, Hypoosmolar Hyponatremia Evaluation

  • Definition
  1. Hyponatremia with Measured Serum Osmolality <280 mOsm
  2. Hyponatremia is a water excess state
  • Pathophysiology
  1. Impaired renal water excretion with continued water intake
  2. Identify the cause of Hyponatremia by identifying why the Kidney can not excrete excess water
  • Causes
  • Hypoosmolar Hyponatremia (impaired water excretion)
  1. Glomerulus: Reduced Glomerular Filtration Rate
    1. Renal Failure (GFR <20% of normal)
  2. Proximal Tubule: Increased water reabsorption
    1. Gastrointestinal losses (especially Vomiting) with free water replacement
      1. Extracellular fluid volume depletion results in Antidiuretic Hormone release, and fluid retention
    2. Edematous State (e.g. CHF, Cirrhosis, Nephrosis)
      1. Sodium retention and continued free water intake
  3. Distal Convoluted Tubule: Impaired water excretion
    1. Thiazide Diuretics
  4. Medullary collecting tubule: ADH-mediated water retention
    1. Syndrome of Inappropriate ADH or SIADH (e.g. CNS disease, lung disease, cancer, medications, postoperative)
  5. Miscellaneous mechanisms
    1. Endocrine cause (Hypothyroidism, Adrenal Insufficiency)
    2. Excessive free water intake (or excessive or prolonged hypotonic infusion)
    3. "Tea and Toast" Diet (elderly) or excessive beer (Alcoholism)
      1. Low solute diet of Carbohydrates with too little Protein and Sodium
      2. Kidney requires solute to effectively excrete water
      3. Carbohydrates are metabolized to carbon dioxide and exhaled
  • Evaluation
  1. Is Renal Failure present (with GFR <20% of normal)?
    1. Check Serum Creatinine and calculate Glomerular Filtration Rate (GFR)
  2. Identify source of free water intake
    1. Excessive oral free water
    2. Hypotonic fluids (Hypotonic Saline, medication IV solutions)
  3. Identify related medications
    1. Thiazide Diuretics (e.g. Hydrochlorothiazide, Chlorthalidone)
    2. Trimethoprim (causes high renin and high Aldosterone levels)
    3. Consider Medication Causes of SIADH
  4. Consider endocrine causes
    1. Hypothyroidism
    2. Adrenal Insufficiency
  5. Assess for Extracellular Fluid Overload (Edematous State)
    1. Peripheral Edema
    2. Jugular Venous Distention
    3. Pulmonary rales
    4. IVC Ultrasound for Volume Status
    5. Third spacing of fluid (Pleural Effusion, Ascites)
  6. Assess for Extracellular Fluid Depletion
    1. Orthostatic Blood Pressure and Pulse
    2. Decreased Skin Turgor
    3. Dry mucous membranes
    4. Serum markers increased with Dehydration (Uric Acid, BUN, Serum Creatinine)
  7. What is Patient's Volume Status?
    1. Hypervolemic Hypoosmolar Hyponatremia (Edematous State)
      1. Congestive Heart Failure, Cirrhosis, Nephrotic Syndrome, Renal Failure
    2. Non-Hypervolemic (Iso- or Hypovolemic)
      1. May be difficult to distinguish Isovolemic from Hypovolemic state
      2. Hypovolemic Hypoosmolar Hyponatremia (volume depletion, esp. Gastrointestinal losses)
      3. Isovolemic Hypoosmolar Hyponatremia (e.g. SIADH)
  8. Obtain Urine Sodium
    1. Differentiate renal causes (e.g. Renal Failure) in which Urine Sodium >20 mEq/L
    2. Urine Sodium <20 mEq/L suggests Sodium retention
      1. Volume depletion (Hypovolemic Hypoosmolar Hyponatremia)
      2. Volume overload or Edematous State (Hypervolemic Hypoosmolar Hyponatremia)
      3. Water Intoxication or tea and toast diet (Isovolemic Hypoosmolar Hyponatremia)
  9. Consider Urine Osmolality
    1. Urine Osmolality (normally 300-900 mOsm/L) is decreased with impaired renal dilutional function
      1. Do not use specific gravity in place of Urine Osmolality (does not accurately correlate)
      2. Voinescu (2002) Am J Med Sci 323(1):39-42 +PMID:11814141 [PubMed]
  10. Consider 24 hour urine for total solute excretion
    1. Total solute excretion is consistent with Tea and Toast Diet (high Carbohydrate, low Protein)
  • Management
  1. See Hyponatremia Management
  2. See specific protocols based on fluid status above
  • References
  1. Kone in Tisher (1993) Nephrology, p. 87-100
  2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  3. Preston (2011) Acid-Base, Fluids and Electrolytes, Medmaster, Miami
  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]