Hypomagnesaemia

Hypomagnesaemia is more common than hypermagnesemia. Defined by serum Mg levels <1.0 mmol/L

Causes of hypomagnesaemia

  1. Increased magnesium loss
    • GI losses (Pancreatitis, diarrhoea, bowel resection)
    • Renal
      • Volume expansion and osmotic diuresis
      • ATN, obstructive uropathy, Gitelman’s
    • Drugs
      • Alcohol is commonest
      • Diuretics, gentamicin, digoxin, amphotericin, cisplatin
  2. Reduced magnesium intake and absorption
    • Starvation, alcoholics, malnutrition
  3. Metabolic
  4. Endocrine
    • Hyperthyroid, Hyperparathyroidism, DKA
  5. Other
    • Burns, sepsis, lactation, hypothermia

Clinical manifestations

Non-specific and often mimic hypokalaemia and hypocalcaemia

  • Muscle weakness, paraesthesia, fasciculations and tetany
  • Decreased mental awareness, vertigo, seizures, confusion
  • Cerebellar dysfunction

Complication

Cardiac Arrhythmia and ECG changes

  • ECG changes are similar to hypocalcaemia and hypokalaemia
  • Prolonged QTc and PR
  • ST depression
  • T wave inversion and P wave inversion
  • Wide QRS…Torsades de Pointe
  • Worsening digoxin toxicity
  • Treatment resistant VF

Correction

Dependent on severity of symptoms

  • IV Magnesium Sulfate
    • 1-2 G of MgSO4 over 10 minutes if cardiac arrhythmia, torsades, seizures
    • 1 G of MgSO4 in 100mL Normal Saline over 30 minutes for slow replacement
    • NOTE: If too rapid then exceeds renal threshold with magnesium wasting
  • Calcium gluconate 1 G to replenish Calcium (normally go hand in hand)
    • Replace cautiously if renal failure to prevent hypermagnesemia
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