Hypomagnesemia

A hypomagnesemia is defined by a magnesium level < 0.7 mmol/l, subject to the reference laboratory's own norms. It is frequently encountered in hospitalized patients (affects 10% of patients, up to 70% in the ICU).

Clinical Presentation

General weakness, irritability, depression, lethargy, paresthesias, tremors, hyperreflexia, cramps, tetany, arrhythmias, Chvostek and Trousseau signs, fasciculations, seizures, nystagmus, delirium, coma... Several cases of Wernicke syndromes have also been reported (magnesium is an essential co-factor for thiamine pyrophosphate).

A hypokalemia (in 50% of cases) and/or a hypocalcemia are often associated and should always be investigated!

Etiologies

• Renal losses (magnesuria > 2 mmol/24 hours):
  • Iatrogenic: furosemide, thiazides, cisplatin, amphotericin B, aminoglycosides, cyclosporin A, pentamidine...
  • Osmotic diuresis: diabetes and other causes of polyuria
  • Ethanol intoxication
  • Metabolic acidosis
  • Hereditary tubulopathies
• Extrarenal causes (magnesuria < 1 mmol/24 hours):
  • Gastrointestinal losses: steatorrhea, diarrhea, malabsorption
  • Redistribution: alkalosis, Hungry bone syndrome, pancreatitis (third-space sequestration), insulin therapy, adrenaline
  • Insufficient intake, chronic alcoholism
  • Increased demand: pregnancy

Additional Investigations

• Electrocardiogram (ECG): prolonged QT interval, taller T waves
• Biochemistry: ionogram, renal function
• 24-hour urine collection: [Mg++] → if > 1-2 mmol/24 hours: renal losses

Therapeutic Management - Treatments

NB: 1 mg of MgSO4 = 8.12 mEq (4.06 mmol) of magnesium

• Etiological treatment if applicable
• Magnesium-rich diet: fruits, vegetables, nuts
• Oral magnesium: 3 to 5 x 4 mmol/day
• If severe hypomagnesemia (< 0.5 mmol/l): MgSO4 IV 2-3 g (8-12 mmol) in 10-15 ml of a 20% solution over 1 to 2 minutes, then 40 mmol over 5 hours
• Treat any associated hypocalcemia and hypokalemia! They may be refractory to supplementation if hypomagnesemia is not corrected at the same time!

Bibliography

EMC, Traité de néphrologie, Elsevier, 2018

Longo DL et al., Harrison - Principes de médecine interne, 18e éd., Lavoisier, 2013