Equis ISSN 2398-2977

Hyponatremia

Contributor(s): Sarah Binns, Graham Munroe

Introduction

  • Definition: serum sodium concentration <132 mEq/l (<132 mmol/l).
  • Sodium is the major cation of extracellular fluid (ECF). 
  • It does not freely move into the intracellular space (ICF) but is dependent on the action of the Na/K ATP-ase and is therefore the major determinant of plasma osmolality. 
  • Water moves freely between the ECF and ICF following an osmotic gradient. 
  • Measured sodium concentration does not reflect total body sodium content but indicates the amount of sodium relative to the amount of the ECF water, therefore knowledge of the hydration of an animal is important for accurate interpretation of serum sodium concentrations. 
  • Hyponatremia usually implies hypo-osmolality, ie a relative water excess.
  • Cause: loss of sodium, often into intestines; also urine, sweat, saliva; may be iatrogenic.
  • Signs: non-specific; vary considerably.
  • Diagnosis: history, serum biochemistry, urinalysis, ACTH test, TSH, T4, ultrasonography, CT, MRI.
  • Treatment: oral or intravenous fluid/electrolyte replacement.
  • Prognosis: depends on cause.

Pathogenesis

Etiology

  • Increased sodium loss and dehydration:
    • Enteritis, especially colitis.
    • Renal failure; rupture of urinary bladder.
    • Increased sweating.
    • Increased salivation.
  • Overhydration:
    • Congestive heart failure.
    • Chronic hepatic failure   Liver disease: overview  .
    • Nephrotic syndrome.
    • Iatrogenic.
    • Excessive anti-diuretic hormone.
  • Fluid redistribution:
    • Hyperglycemia.
    • Hyperproteinemia.

Pathophysiology

Timecourse

  • If severe hyponatremia and the onset is acute  then clinical signs develop quickly. 
  • If hyponatremia chronic (over at least 2-3 days) potentially none or only minor clinical signs. 
  • If correction of hyponatremia is too fast (>1 mEq/kg/h (>1 mmol/kg/h)) neurologic signs of myelinolysis develop up to 3-4 days later.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Schaer M (1999)Disorders of serum potassium, sodium, magnesium and chloride.JVEEC9(4), 209-217 VetMedResource.
  • Ecke P, Hodgson D R, and Rose R J (1998)Induced diarrhea in horses part 1 - Fluid and electrolyte balance.Vet J155(2), 149-159 PubMed.
  • Johnson P J (1998)Physiology of body fluids in the horse.Vet Clin North Am - Equine Pract14(1), 1 PubMed.
  • McCutcheon L J and Geor R J (1998)Sweating - fluid and ion losses and replacement.Vet Clin North Am - Equine Pract14(1), 75 PubMed.
  • Lakritz J, Madigan J, and Carlson G P (1992)Hypovolemic hyponatremia and signs of neurologic disease associated with diarrhea in a foal.JAVMA200(8), 1114-1116 PubMed.

Other sources of information

  • Corley K T T (2004)Correction of acid-base and electrolyte disturbances.In:Equine Clinical Pharmacology. Eds: Bertone J J & Horspool L J I. Saunders, NY. pp 352-353.
  • Wingfield W E & Raffee M R (2002) EdsThe Veterinary ICU Book.Teton New Media, Jackson Hole, WY.
  • Schmall L M (1997)Fluid and Electrolyte Therapy.In:Current Therapy in Equine Medicine 4. Ed: Robinson N E. W B Saunders, Philadelphia.


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