Hypermagnesaemia

Hypermagnesaemia is an elevated serum magnesium, and is comparatively uncommon because normal kidneys excrete magnesium efficiently. It therefore occurs almost exclusively in the setting of renal impairment combined with a magnesium load, or from iatrogenic administration.

The classic contexts are: magnesium-containing antacids or laxatives in a patient with chronic kidney disease; therapeutic magnesium infusion in pre-eclampsia and eclampsia, where it is the intended treatment and toxicity is a recognised complication requiring active monitoring; treatment of severe asthma or arrhythmia; and, rarely, massive magnesium ingestion.

Because iatrogenic causes dominate, hypermagnesaemia is largely a preventable and anticipatable problem rather than a diagnostic puzzle.

Features progress fairly predictably with rising levels, which is what makes clinical monitoring during magnesium infusion so effective.

• Loss of deep tendon reflexes is the earliest and most useful clinical sign, and is the reason reflexes are checked hourly during magnesium infusion in obstetric practice. Their loss precedes respiratory and cardiac toxicity
• Nausea, vomiting, flushing and a sensation of warmth
• Lethargy, drowsiness and weakness
• Hypotension and bradycardia
• Respiratory depression progressing to respiratory arrest as levels rise further
• Cardiac conduction abnormalities: prolonged PR interval, widened QRS, prolonged QT, and complete heart block and cardiac arrest

In obstetric practice the monitoring triad is therefore reflexes, respiratory rate and urine output, the last because magnesium is renally cleared and oliguria causes accumulation.

• Stop all magnesium administration and all magnesium-containing medications immediately, including antacids and laxatives, which are easily overlooked
• Intravenous calcium gluconate (10 mL of 10% solution over 10 minutes, repeated as needed) is the immediate antidote for significant cardiovascular or respiratory toxicity. It directly antagonises magnesium at the neuromuscular junction and myocardium and acts within minutes
• Intravenous fluids with a loop diuretic promote renal magnesium excretion in patients with adequate renal function and volume status
• Haemodialysis is definitive and is indicated for severe hypermagnesaemia, particularly in renal failure where renal clearance cannot be relied upon
• Supportive care: airway protection and ventilatory support for respiratory depression, cardiac monitoring, and treatment of bradyarrhythmia
• Monitor magnesium, calcium, potassium and renal function

In the obstetric setting: hourly assessment of deep tendon reflexes, respiratory rate and urine output during magnesium infusion, with calcium gluconate immediately available at the bedside. Reduce the dose or stop the infusion for oliguria or lost reflexes rather than waiting for respiratory compromise.

Prevention: avoid magnesium-containing preparations in chronic kidney disease, and review medication lists specifically for them in any patient with renal impairment.

Referral: nephrology for dialysis; obstetrics and critical care for magnesium toxicity in pre-eclampsia.