Magnesium Disorders

Chapter 57 Magnesium Disorders



Linda G. Martin, DVM, MS, DACVECC



KEY POINTS



• Hypomagnesemia is a common electrolyte disorder in critically ill veterinary patients.

• Given that less than 1% of total body magnesium is located in the serum, serum magnesium concentrations do not always reflect total body magnesium stores. Consequently, a normal serum magnesium concentration can occur when there is a total body magnesium deficiency.

• Magnesium homeostasis is primarily a function of intestinal absorption and urinary excretion; therefore hypomagnesemia is almost always caused by disturbances in one or both of these organ systems.

• Most cases of hypermagnesemia involve a component of renal insufficiency.

• Severe hypomagnesemia and hypermagnesemia cause clinical signs associated with the cardiovascular and neuromuscular systems.


INTRODUCTION


For more than a decade, there has been a growing interest in the clinical effects and therapeutic role of magnesium in veterinary medicine. Magnesium disorders are common in both feline and canine critically ill patients. Increased morbidity, mortality, and prevalence of concurrent electrolyte disorders occur in critically ill animals with altered total serum magnesium concentrations when compared with normomagnesemic critically ill animals.1-3


Magnesium is the second most abundant intracellular cation, exceeded only by potassium. The vast majority of magnesium is found in bone and muscle. Sixty percent of the total body magnesium content is present in bone, incorporated into the crystal mineral lattice or in the surface-limited exchangeable pool. This pool consists of magnesium that is in equilibrium with the magnesium ions in the extracellular fluid and serves as a reservoir for maintenance of the extracellular magnesium concentration. Twenty percent is located in skeletal muscle and the remainder is located in other tissues, primarily the heart and liver. Less than 1% of total body magnesium is present in the serum.4,5 In the serum, magnesium exists in three distinct forms: an ionized fraction, an anion-complexed fraction, and a protein-bound fraction. The ionized fraction is thought to be the physiologically active component and accounts for approximately 66% and 63% of the total serum magnesium concentration in cats and dogs, respectively. Approximately 4% and 6% are complexed to compounds such as phosphate, bicarbonate, sulfate, citrate, and lactate in cats and dogs, respectively. The remaining 30% and 31% of total serum magnesium are bound to protein (primarily albumin) in cats and dogs, respectively.6,7


Magnesium is required for many metabolic functions, most notably those involved in the production and use of adenosine triphosphate (ATP). This electrolyte is a coenzyme for the membrane-bound sodium-potassium ATPase pump and functions to maintain the sodium-potassium gradient across all membranes. Calcium ATPase and proton pumps also require magnesium. Magnesium is also essential for protein and nucleic acid synthesis, regulation of vascular smooth muscle tone, cellular second messenger systems, and signal transduction. In addition, data suggest that magnesium exerts an important influence on lymphocyte activation, cytokine production, and systemic inflammation.8-10


Magnesium homeostasis is achieved through intestinal absorption and renal excretion. Absorption occurs primarily in the small intestine (jejunum and ileum) with little or none occurring in the large intestine. The loop of Henle is the main site of magnesium absorption in the kidney. The kidney appears to be the main regulator of serum magnesium concentration and total body magnesium content.8 This is achieved by both glomerular filtration and tubular reabsorption.5 Renal magnesium excretion will increase in proportion to the load presented to the kidney; conversely, the kidney conserves magnesium in response to a deficiency.8


Lactation appears to play a role in gut and renal handling of magnesium. Increased levels of parathyroid hormone, in addition to calcium concentration, most likely participate in magnesium conservation during lactation to supply the mammary glands with a sufficient amount.11 No primary regulatory hormone has been identified for magnesium homeostasis, although the parathyroid, thyroid, and adrenal glands are likely involved.12



HYPOMAGNESEMIA


Most magnesium-related disorders are caused by conditions that lead to the depletion of total body stores. Hypomagnesemia is a common electrolyte abnormality in both canine and feline intensive care unit patients.1-3 However, this electrolyte disorder appears to be less common in the general canine hospital population.13 Ionized hypomagnesemia has been documented in perioperative feline renal transplant recipients, as well as cats with diabetes mellitus and diabetic ketoacidosis.14,15 Other evidence suggests that animals on peritoneal dialysis, dogs with congestive heart failure being treated with furosemide, dogs with protein-losing enteropathy, and lactating dogs are also at risk for hypomagnesemia.16-19



Causes


Causes of magnesium deficiency are both numerous and complex. Three general categories are involved: decreased intake, increased losses, and alterations in distribution. Potential causes are listed in Box 57-1. Decreased dietary intake, if sustained for several weeks, can lead to significant magnesium depletion. In addition, catabolic illness and prolonged intravenous fluid therapy or parenteral nutrition without sufficient magnesium supplementation can contribute to depletion.4,8,16



Box 57-1 Causes of Hypomagnesemia



I Decreased Intake
A Inadequate nutritional intake

B Prolonged intravenous fluid therapy or parenteral nutrition without magnesium replacement

II Increased Losses
A Gastrointestinal
1 Malabsorption syndromes

2 Extensive small bowel resection

3 Chronic diarrhea

4 Inflammatory bowel disease

5 Cholestatic liver disease

B Renal
1 Intrinsic tubular disorders
a Glomerulonephritis

b Acute tubular necrosis

c Postobstructive diuresis

d Drug-induced tubular injury
(1) Aminoglycosides

(2) Amphotericin B

(3) Cisplatin

(4) Cyclosporine

2 Extrarenal factors influencing renal magnesium handling
a Diuretic-induced states
(1) Furosemide

(2) Thiazides

(3) Mannitol

b Digitalis administration

c Diabetic ketoacidosis

d Hyperthyroidism

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Magnesium Disorders

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