Equis ISSN 2398-2977

Hypomagnesemia

Synonym(s): Transport tetany

Contributor(s): Graham Munroe, Vetstream Ltd

Introduction

  • Definition: serum ionized magnesium <1.1 mg/dl (<0.45 mmol/l).
  • Magnesium is an essential intracellular cation involved in many intracellular enzymatic reactions including regulating the calcium channel function, neurotransmission, vasomotor tone, muscular contraction and cardiac excitability. 
  • Because of its importance, homeostatic mechanisms normally maintain intracellular and extracellular magnesium concentrations within narrow limits, and overt hypomagnesemia is rare in horses. 
  • Magnesium is primarily an intracellular cation, with only 1% being found in the serum. 
  • Extracellular magnesium exists in three forms - protein-bound, complexed with anions (such as phosphate) and ionized. 
  • Ionized magnesium is the physiologically active form. 
  • Total serum levels therefore may not reflect either the active moiety or the total body concentration. Intracellular magnesium is primarily in the bone (67%), muscle (2%) and other soft tissues (11%).
  • Magnesium is antagonistic to the actions of calcium.

Pathogenesis

Etiology

  • Decreased intake due to anorexia. 
  • Gastrointestinal loss from profuse diarrhea, malabsorption, maldigestion. 
  • Urinary loss due to renal tubular acidosis, primary renal disease, diabetes mellitus   Pancreas: chronic pancreatitis - diabetes mellitus  /diabetic ketoacidosis, hyperthyroidism   Endocrine: hyperthyroidism  , hypokalemia, hypocalcemia   Hypocalcemia  , hypophosphatemia Link: Hyperphosphatemia (NEW).
  • Parathyroid hormone increases magnesium reabsorption via the renal tubules. 
  • Iatrogenic due to parathyroidectomy, drug administration (insulin, loop diuretics, cisplatin, aminoglycosides) or inadequate supply in intravenous fluids.

Predisposing factors

General
  • Ingested magnesium is absorbed into the body from the proximal (25%) and distal (35%) small intestine, and the colon (5%). Prolonged malnutrition, anorexia or significant intestinal disease may lead to hypomagnesemia.
  • High levels of dietary potassium or nitrogen reduce magnesium absorption. 
  • Renal homeostasis is vital for regulation of magnesium levels therefore any condition causing excessive renal loss can also result in hypomagnesemia.
  • Calcium-containing compounds lower serum magnesium concentrations.
  • Exhaustion, hyperaldosteronism and hyperthyroidism   Endocrine: hyperthyroidism  can all lead to hypomagnesemia.

Specific

  • Magnesium is secreted into mares milk and may be associated with hypocalcemia   Hypocalcemia   in early lactation.

Pathophysiology

  • Hypomagnesemia is a common finding in the critically ill patient but whether it is contributory to mortality, or a result of, the severe disease is unknown.
  • Magnesium homeostasis is mainly determined by renal elimination and reabsorption. 80% of total serum magnesium is filtered by the glomerulus. 10-15% is reabsorbed in the proximal tubule, 60-70% in the cortical thick ascending loop of Henle, 10-15% in the distal convoluted tubule. 
  • Magnesium deficit is most commonly associated with conditions leading to increased glomerular filtration rate or other reasons for decreased renal function. 
  • Digestive secretions contain considerable magnesium (0.5-1.0 mg/kg/day). Magnesium levels will decrease if resorption by the distal small intestine does not keep pace with this loss.
  • Magnesium is secreted into the milk of lactating mares, especially in early lactation.
  • Substantial amounts of magnesium can be lost in sweat.
  • A significant role for intracellular magnesium is as a co-factor for the calcium-ATP pump that moves intracellular calcium into the sacroplasmic reticulum. It performs a similar role with the sodium-potassium-ATP pump that moves these electrolytes across cell membranes. Reduction of the activity of these ion pumps is likely to be the major cause of the clinical signs of hypomagnesemia.

Diagnosis

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Treatment

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Prevention

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Further Reading

Publications

Refereed papers

Other sources of information

  • Marino P (2007)Renal and Electrolyte Disorders: Magnesium. In:The ICU Book. Lippincott Williams & Wilkins. pp 625638.
  • Stewart A J (2004)Magnesium Disorders. In:Equine Internal Medicine. Eds: Reed S M, Bayly W M & Sellon D C. 2nd edn. Saunders, Missouri, USA. pp 1365-1379.


ADDED