Hypomagnesaemia
The normal range of the adult serum or plasma magnesium concentration is 0.8 to 1.0 mmol/L.1
A serum magnesium concentration lower than 0.8 mmol/L usually indicates magnesium depletion; values lower than 0.4 mmol/L indicate severe deficiency. Approximately 99% of total body magnesium is intracellular, so the serum magnesium concentration is not a precise indicator of total body content. Hypomagnesaemia is common in hospitalised patients, especially those who are severely ill. It is usually due to gastrointestinal or kidney loss, often on a background of diabetes, alcoholism, diuretic drug therapy, malabsorption syndromes or poor oral intake. A mild deficiency is asymptomatic. Even moderate to severe hypomagnesaemia lacks specific features, but may cause tetany, muscle weakness, cardiac arrhythmias, neuropsychiatric changes and convulsions. The common causes of hypomagnesaemia are listed in Common causes of hypomagnesaemia.
Coexisting ion abnormalities (eg hypocalcaemia, hypokalaemia, metabolic alkalosis) are common.
Correcting the cause, when possible, is the mainstay of treatment.
Mild hypomagnesaemia can be treated with oral replacement. A suitable supplement is magnesium aspartate. Use:
magnesium aspartate 1000 to 3000 mg (elemental magnesium 74.8 to 224.4 mg) orally, daily in divided doses, with food. hypomagnesaemia: mild magnesium aspartate
Moderate to severe hypomagnesaemia (with clinically consistent signs and symptoms) may require intravenous magnesium. The rate of infusion depends on the extent of the deficit and the clinical features.
Available intravenous preparations of magnesium are:
- magnesium chloride (480 mg/5 mL). Each mL contains 1 mmol (2 mEq) of magnesium ions and 2 mmol (2 mEq) of chloride ions
- magnesium sulfate (500 mg/mL). Each mL contains 2 mmol (4 mEq) of magnesium ions and 2 mmol (4 mEq) of sulfate ions.
Use:
1 magnesium 25 to 50 mmol IV in sodium chloride 0.9% 500 mL to 1000 mL over 12 to 24 hours initially; aim to achieve and maintain serum magnesium concentration above 0.4 mmol/L hypomagnesaemia: moderate to severe
OR
1 magnesium 10 mmol IV in sodium chloride 0.9% 100 mL over 60 minutes. Repeat if needed, titrate to effect and serum magnesium concentration.
In the presence of life-threatening cardiac arrhythmia, 4 to 8 mmol magnesium can be given over 5 to 10 minutes.
During intravenous therapy, the serum magnesium concentration should be monitored every 1 to 2 hours initially. Lower doses of magnesium are required in kidney impairment, when monitoring is especially important.
|
Mechanism of low serum magnesium concentration |
Cause of low serum magnesium concentration |
|---|---|
|
severe malnutrition |
poor oral intake |
|
gastrointestinal losses |
nasogastric suction diarrhoea malabsorption extensive bowel resection primary intestinal hypomagnesaemia intestinal fistulae |
|
kidney losses |
chronic parenteral fluid therapy hypercalcaemia and hypercalciuria osmotic diuresis |
|
drugs |
diuretic drugs alcohol aminoglycoside antibiotics cisplatin amphotericin B ciclosporin foscarnet pentamidine proton pump inhibitors |
|
other |
phosphate depletion 'hungry bone' syndrome postobstructive nephropathy diuretic phase of acute kidney failure Gitelman syndrome |