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
   • Hypocalcaemia
   • Hypokalaemia
   • Hypophosphataemia
   • Metabolic alkalosis
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