Toxicology – Magnesium Toxicity

Sources

Hypermagnesemia may result from excessive exposure to magnesium-containing products such as:

• Magnesium sulfate
• Magnesium citrate
• Milk of magnesia
• Other magnesium-containing antacids or laxatives

Risk is increased in patients with renal impairment, because magnesium is primarily eliminated by the kidneys.

Typical Presentation

A patient receiving magnesium therapy, such as during treatment of preeclampsia, develops:

• Weakness
• Nausea
• Flushing
• Diminished reflexes
• Hypotension
• Bradycardia

Severe toxicity may progress to respiratory depression and cardiac conduction abnormalities.

Clinical Features

As magnesium levels rise, findings may include:

• Nausea and vomiting
• Flushing
• Thirst
• Generalized weakness
• Lethargy
• Decreased or absent deep tendon reflexes
• Hypotension
• Bradycardia
• Respiratory depression
• Prolonged cardiac conduction
• Wide QRS complexes
• Complete heart block
• Cardiac arrest in extreme cases

Loss of deep tendon reflexes is an important early bedside clue during magnesium therapy.

Mechanism of Action

Excess magnesium depresses neuromuscular and cardiac function.

It:

• Reduces acetylcholine release at the neuromuscular junction
• Interferes with calcium-dependent processes
• Alters sodium and potassium channel activity
• Slows cardiac conduction
• Produces peripheral vasodilation

Management

Treatment includes:

• Immediately stopping the magnesium source
• Airway and respiratory support when needed
• IV fluids and cardiovascular support
• Continuous ECG monitoring in significant toxicity

IV calcium, usually calcium gluconate, can temporarily antagonize the cardiac and neuromuscular effects of magnesium.

Patients with adequate renal function may eliminate excess magnesium with supportive care. Hemodialysis is particularly useful in severe toxicity or when renal failure prevents magnesium excretion.

Key Points

• Think of weakness + diminished reflexes + hypotension/bradycardia in a patient receiving magnesium.
• Renal failure greatly increases the risk of magnesium accumulation.
• Calcium antagonizes the physiologic effects of magnesium and is used for significant toxicity.
• Severe hypermagnesemia can cause respiratory failure and heart block.
• Magnesium-containing laxatives should be used cautiously in patients with impaired renal function.