Author: Martin Crook
Magnesium is a predominately intracellular divalent cation, which is an essential cofactor to many enzyme systems. It is also important in cell membrane function and can antagonize calcium in cellular responses.
Classification of plasma magnesium concentration is given in Box 88.1.
Causes of hypermagnesaemia are given in Table 88.1.
Hypercalcaemia and hyperkalaemia are associated with hypermagnesaemia, and plasma levels of these ions should be checked, as well as renal function, including eGFR and phosphate.
In resistant hypermagnesaemia, exclude hypothyroidism and adrenal insufficiency.
Symptomatic severe hypermagnesaemia
Treat the underlying cause.
Give 5–10 mL of 10% calcium gluconate slowly IV over 30s with ECG monitoring.
Insulin and glucose infusion can be used in severe hypermagnesaemia, as for severe hyperkalaemia.
If renal function is normal, urinary magnesium loss can be increased by forced saline diuresis.
If there is impaired renal function, seek advice from a nephrologist: haemodialysis may be indicated, particularly if plasma magnesium concentration is >4 mmol/L.
Causes of hypomagnesaemia are given in Table 88.2. The manifestations of hypomagnesaemia are very similar to those of hypocalcaemia (Chapter 87).
Hypocalcaemia, hypophosphataemia and hypokalaemia are associated with hypomagnesaemia, and plasma levels of these ions should be checked. Severe hypomagnesaemia can cause hypocalcaemia due to decreased PTH release and activity. There is an association with diabetes mellitus, which should be considered. Measurement of 24-h urinary magnesium excretion can be useful in assessing the response to treatment.
Mild to moderate hypomagnesaemia
Treat the underlying cause.
Give oral replacement therapy with a magnesium salt.
Symptomatic severe hypomagnesaemia
Treat the underlying cause.
Give magnesium sulphate, 2–4g of 50% solution (8.3–16.6 mmol) IV, diluted in saline or glucose intravenously (IV) over 30–60 min (for adults: 0.5 mmol/kg IV), but use caution if there is renal impairment.