Hypomagnesaemia

• Signs and symptoms:

  • Tremor
  • Tetany
  • Seizures
  • Weakness
  • Delirium
  • Coma
• Hypomagnesaemia causes resistance to PTH and causes hypocalcaemia
• ECG changes = QT prolongation
• The normal range of serum magnesium is 0.7-1.1 mmol/L
• Magnesium is principally absorbed from the intestines and excreted by the kidneys.
• Bone is the main reservoir of magnesium in the body, but exchange with serum concentrations is not freely accessible.
• As magnesium is reliant on urinary excretion for clearance, a positive magnesium balance (e.g. intravenous infusion) in the context of renal impairment can lead to hypermagnesaemia.
• Magnesium is predominantly lost from the gastrointestinal tract and kidneys.
• The predominant cause of hypomagnesaemia is reduced dietary intake leading to a negative magnesium balance. This is commonly seen in alcoholic or severely malnourished patients.
• Hypomagnesaemia may be seen in both vomiting and diarrhoea. Diarrhoeal losses account for more cases as the lower gastrointestinal content contains a higher concentration of magnesium.
• Hypomagnesaemia has been described in the chronic use of proton pump inhibitors (PPI) such as omeprazole. Hypomagnesaemia in the context of PPI use is much more common with concurrent diuretic use. PPIs are suspected to inhibit the magnesium transport receptor TRPM6.
• Renal loss:

  • Medications: diuretics (loop and thiazide), aminoglycosides, digoxin
  • Alcohol
  • Hypercalcaemia: typically seen in hypercalcaemia secondary to hyperparathyroidism
  • Renal recovery following injury: may be seen during recovery of acute tubular necrosis or following post-obstructive diuresis
• Features of hypocalcaemia may occur because it impairs the action of PTH. Hypomagnesaemia causes resistance to PTH and in severe cases reduces secretion.

  • Paraesthesia (numbness and tingling sensation)
  • Tetany
  • CNS disturbance (seizures, irritability, confusion)
  • Cardiovascular disturbance (chest pain, palpitations)
  • Trousseau's sign and Chvostek's sign
• Hypokalaemia is seen in 40-60% of patients with hypomagnesaemia
• Common side-effects of oral magnesium replacement are abdominal discomfort and diarrhoea. Oral replacement may be poorly tolerated.
• Intravenous doses of magnesium are needed for severe or symptomatic hypomagnesaemia (e.g. seizures, tetany, arrhythmias). Intravenous magnesium replacement requires cardiac monitoring, the duration of administration depends on hospital policy and the indication.