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Hypomagnesemia

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Hypomagnesemia
ICD-10 E834
ICD-9 275.2
OMIM [1]
DiseasesDB 6469
MedlinePlus 000315
eMedicine med/3382 emerg/274 ped/1122
MeSH {{{MeshNumber}}}

Hypomagnesemia is an electrolyte disturbance in which there is an abnormally low level of magnesium in the blood. Usually a serum level less than 0.6 mmol/l is used as reference. It must be noted that hypomagnesemia is not equal to magnesium deficiency. Hypomagnesemia can be present without magnesium deficiency and vice versa.

Deficiency of magnesium causes among others cardiac arrhythmia and increased irritability of the nervous system with tetany. It may result from a number of conditions including inadequate intake of magnesium, chronic diarrhea, malabsorption, alcoholism, diuretic use and other disorders.

The prefix hypo- means low (contrast with hyper-, meaning high). The middle magnes refers to magnesium. The end portion of the word, -emia, means 'in the blood' (note, however, that hypomagnesemia is usually indicative of a systemic magnesium deficit).

HomeostasisEdit

The body contains 21-28 grams of magnesium (1 mmol=2mEq=24.6 mg). Of this, 53% is located in bone, 19% in non-muscular tissue, and 1% in extracellular fluid. For this reason, blood levels of magnesium are not an adequate means of establishing the total amount of available magnesium. Most of the serum magnesium is bound to chelators, (i.e. ATP, ADP, proteins and citrate). Roughly 33% is bound to proteins, and 5-10% is not bound. This "free" magnesium is essential in regulating intracellular magnesium. Normal plasma Mg is 1.7-2.3 mg/dl (0.69-0.94 mmol/l). Of this 60% is free, 33% is bound to proteins, and less than 7% is bound to citrate, bicarbonate and phosphate.

Magnesium is abundant in nature. It can be found in green vegetables, chlorophyll, coca-derivatives, nuts, wheat, seafood, and meat. It is resorbed through the small intestine, and to a lesser degree in the colon. The rectum and sigmoid colon can absorb magnesium. Hypermagnesemia has been reported after enemas containing magnesium. Forty percent of dietary magnesium is absorbed. Hypomagnesemia stimulates and hypermagnesemia inhibits this absorption.

The kidneys regulate the serum magnesium. About 2400 mg of magnesium passes through the kidneys, of which 5% (120 mg) is excreted through urine. The loop of Henle is the major site for Mg-homeostasis and 60% is resorbed.

Magnesium homeostasis comprises three systems: kidney, small intestine, and bone. In the acute phase of magnesium deficiency there is an increase in absorption in the distal small intestine and tubular resorption in the kidneys. When this condition persists serum magnesium drops and is corrected with magnesium from bone tissue. The level of intracellular magnesium is controlled through the reservoir in bone tissue.

MetabolismEdit

Magnesium is a cofactor in more than 300 enzyme regulated reactions. Most importantly forming and using ATP, i.e. kinase. There is a direct effect on sodium- (Na), potassium- (K) and calcium (Ca)channels. It has several effects:

  • Potassium channels are inhibited by magnesium. Hypomagnesemia results in increased efflux of intracellular Mg. The cell loses potassium which then is excreted by the kidneys, resulting in hypokalemia.
  • Release of calcium from the sarcoplasmic reticulum is inhibited by magnesium. Low levels of magnesium stimulate the release of calcium and thereby an intracellular level of calcium. This effect similar to calcium inhibitors makes it "nature's calcium inhibitor." Lack of magnesium inhibits the release of parathyroid hormone, which can also result in hypocalcemia. Furthermore, it makes skeletal and muscle receptors less sensitive to parathyroid hormone.
  • Through relaxation of bronchial smooth muscle it causes bronchodilation.
  • The neurological effects are:
    • reducing electrical excitation
    • blocking release of acetylcholine
    • blocking N-methyl-D-aspartate, an excitatory neurotransmitter of the central nervous system.

CausesEdit

Magnesium deficiency is not uncommon in hospitalized patients. Elevated levels of magnesium (hypermagnesemia), however, are nearly always iatrogenic. 10-20% of all hospital patients, and 60-65% of patient in the intensive care unit (ICU) have hypomagnesemia. Hypomagnesiemia is underdiagnosed, as testing for serum magnesium levels is not routine. Hypomagnesemia results in increased mortality.

Low levels of magnesium in your blood may mean either there is not enough magnesium in the diet, the intestines are not absorbing enough magnesium or the kidneys are excreting too much magnesium. Deficiencies may be due to the following conditions:

TreatmentEdit

Treatment of hypomagnesemia depends on the degree of deficiency and the clinical effects. Oral replacement is appropriate for patients with mild symptoms, while intravenous replacement is indicated for patients with severe clinical effects. Intravenous magnesium sulphate (MgSO4) can be given in the following conditions:

ArrhythmiaEdit

Magnesium is needed for the adequate function of the Na+/K+-ATPase pumps in the cells of the heart. A lack of it depolarises and results in tachyarrythmia. Magnesium inhibits release of potassium, a lack of magnesium increases loss of potassium. Intracellular levels of potassium decrease and the cells depolarise. Digoxin increases this effect. Both digoxin and hypomagnesemia inhibit the Na-K-pump resulting in decreased intracellular potassium.

Magnesium intravenously helps in refractory arrhythmia, most notably torsade de pointes. Others are ventricular tachycardia, supraventricular tachycardia and atrial fibrillation.

The effect is based upon decreased excitability by depolarisation and the slowing down of electric signals in the AV-node. Magnesium is a negative inotrope as a result of decrease calcium influx and calcium release from intracellular storage. It is just as effective as verapamil. In myocardial infarction there is a functional lack of magnesium, suppletion will decrease mortality.

ObstetricEdit

Most importantly pre-eclampsia. It has an indirect antithrombotic effect upon thrombocytes and the endothelial functions (increase in prostaglandin, decrease in thromboxane, decrease in angiotensin II), microvascular leakage and vasospasm through its function similar to calcium channel blockers.

Convulsions are the result of cerebral vasospasm. The vasodilatatory effect of magnesium seems to be the major mechanism.

Electrolyte disturbancesEdit

  • Hypokalemia: 42% of patients with hypokalemia also have hypomagnesemia, not responding to potassium supplementation. Magnesium is needed for the ATPase, Na-K-pump.
  • Hypocalcemia is present in 33% of patients in the intensive care unit, not responding to calcium supplementation. This is because of decreased function of the calcium pump, but also because of a decreased release of calcium by inhibition of parathyroid hormone release.

PulmonaryEdit

Acute asthma, here there is a bronchodilatatory effect, probably by antagonizing a calcium-mediated constriction. Also, adrenergic stimulation, i.e. sympatheticomimetics used for treatment of asthma, might lower serum levels of magnesium, which must therefore be supplemented.

Sedation and anxiolytics may help in decreasing bronchoconstriction.

ReferencesEdit

  1. Cecil Textbook of Medicine
  2. Harrison's Principles of Internal Medicine
  3. Intensive Care Medicine by Irwin and Rippe
  4. The ICU Book by Marino
  5. The Oxford Textbook of Medicine
  6. Saeed M.G. Al-Ghamdi, Eugene C. Cameron, MD and Roger A.L. Sutton, "Magnesium Deficiency: Pathophysiology And Clinical Overview", American Journal Of Kidney Diseases, 1994; 24 (5), 737-752.
  7. Delhumeau, J.C. Granry, J.P. Monrigal, F. Costerousse, "Indications Du Magnésium En Anesthésie-Réanimation", Annales Francaises D'Anesthésie Et De Réanimation, 1995; 14, 406-416.
  8. J. Durlach, V. Durlach, P. Bac, M. Bara and A. Gulet-Bara, "Magnesium And Therapeutics", Magnesium Research 1994; 7 (3-4), 313-328.
  9. Mark D. Faber, Warren L. Kupin, Charles W. Heilig and Robert G. Narins, "Common Fluid-Electrolyte and Acid-Base Problems In The Intensive Care Unit: Selected Issues", Seminars In Nephrology 1994; 14 (1), 8-22.
  10. Lee Goldman, J. Claude Bennett, Cecil's Textbook of Medicine, 21st Edition, 2000, 1137-1139.
  11. Paul L. Marino, The ICU Book, Second Edition 1998, Chapter 42, 660-672.
  12. A.E. Meinders, Professor of Internal Medicine at Leids Universitair Medisch Centrum, "Magnesium", Bij Intensive Care Patiënten
  13. R. Mills, M. Leadbeater and A. Ravalia, "Case Report: Intravenous Magnesium Sulphate In The Management Of Refractory Bronchospasm In A Ventilated Asthmatic", Anaesthesia, 1997; 52, 782-785.
  14. Michael A. Olerich, MD; Robert K. Rude, MD, "Should We Supplement Magnesium In Critically Ill Patients?", New Horizons, 1994; 2 (2),186-192.
  15. James G. Ramsay, MD, "Cardiac Management In The ICU", Chest, 1999; 115: 138S-144S.
  16. Richard A. Reinhart, MD, "Magnesium Deficiency: Recognition And Treatment In The Emergency Medicine Setting", American Journal Of Emergency Medicine, 1992; 10 (1), 76-83.
  17. Richard A. Reinhart, MD; Norman A. Desbiens, MD, "Hypomagnesemia In Patients Entering The ICU", Critical Care Medicine, 1985; 13 (6), 506-507.
  18. Elisabeth Ryzen, MD; Park W. Wagers, MD; Frederick R. Singer, MD; Robert K. Rude, MD, "Magnesium Deficiency In A Medical ICU Population", Critical Care Medicine, 1985; 13 (1), 19-21.
  19. Elisabeth Ryzen, MD, "Magnesium Homeostasis In Critically Ill Patients", Magnesium, 1998; 8, 201-212.
  20. Robert Whang, Edward M. Hampton and David D. Whang, "Magnesium Homeostasis And Clinical Disorders Of Magnesium Deficiency", The Annals Of Pharmacotherapy, 1994; 28, 220-226.

See alsoEdit

External linksEdit

de:Magnesiummangel
pl:Hipomagnezemia
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