Hypomagnesaemia

Aetiology

• Low serum magnesium is most often caused by loss of magnesium from the gut or kidney
• Gastrointestinal causes include severe diarrhoea, malabsorption, extensive bowel resection and intestinal fistulae
• Excessive renal loss of magnesium occurs with diuretics, alcohol abuse, and with an osmotic diuresis such as glycosuria in diabetes mellitus

Clinical features

• Hypomagnesaemia increases renal excretion of potassium, inhibits secretion of parathyroid hormone and leads to parathyroid hormone resistance
• Many of the symptoms of hypomagnesaemia are therefore due to hypokalaemia and hypocalcaemia

Management

• The underlying cause must be corrected where possible and oral supplements given (magnesium chloride 5-20 mmol daily or magnesium oxide tablets 600mg four times daily)
• Symptomatic severe magnesium deficiency should be treated by intravenous infusion (40 mmol of MgCl in 100 mL of sodium chloride 0.9% or dextrose 5% over 2 hours), plus a loading dose (8 mmol over 10-15 minutes) if there are seizures or ventricular arrhythmias
• Take care when interpreting repeat serum concentrations after treatment - the extracellular values may appear to normalize quickly while the intracellular concentration requires longer to replenish (may require up to 160 mmol over 5 days to correct)