Magnesium toxicity: Signs

Parenteral magnesium sulfate is commonly used in obstetrics for tocolysis to halt preterm labor, and as an anticonvulsant in pre-eclampsia/eclampsia. Magnesium plays an important role in determining the electrical potential across cell membranes. It is a co-factor in the Na/K ATPase system and is required for the generation of cAMP. Magnesium also antagonizes calcium at the presynaptic junction as well as within myocytes. The net effect is that magnesium decreases excitability of nerves and muscles.

While these properties explain magnesium’s therapeutic benefits, they are also responsible for the serious side effects resulting from magnesium overdose. Supratherapeutic serum magnesium levels can lead to impaired muscle strength to the point of respiratory paralysis, and precipitate bradycardia, heart block, and cardiac arrest.

Magnesium levels and effects (see table below).

Magnesium also reduces MAC of general anesthetics and potentiates both depolarizing and non-depolarizing muscle relaxants, prolonging their duration of action. Magnesium also increases risk for post-partum hemorrhage due to its utero-relaxant effects. It can also cross the placenta and lead to neonatal hypotonia, hyporeflexia, and respiratory depression.

The treatment of magnesium toxicity is IV calcium, loop diuretics, and supportive care from a cardiopulmonary standpoint.

Updated definition 2020:

Hypermagnesemia is a potentially life threatening abnormality, typically presenting in selected patient populations. Magnesium homeostasis is managed both by GI absorption and kidney excretion; changes in the rates of either can alter homeostasis, as can exogenous intake. At risk groups include:

• Patients with renal failure, due to reduced ability to excrete magnesium
• Pre-eclamptic patients undergoing tocolytic therapy
• Patients with unusually heavy antacid use

Normal serum magnesium levels in the general population are 1.4-2.1 mEq/L (1.7-2.5 mg/dL). Increasing levels of hypermagnesemia often (but not always) correlate with worsening adverse effects:

• 1.4-2.1 mEq/L: normal range
• 3-4 mEq/L: flushing, mild increases in PR or QRS intervals
• 5-6 mEq/L: reduced heart rate and/or blood pressure; mild reduction in FEV1 and FVC; blurred vision; lethargy
• 10 mEq/L: loss of deep tendon reflexes
• 20 mEq/L: respiratory arrest; AV conduction block; progressive bradycardia and QRS widening
• Greater than 25 mEq/L: cardiac arrest

In healthy individuals, the kidney is able to drastically adjust to changing levels of magnesium in the body; however, administering exogenous magnesium to patients with renal dysfunction is relatively contraindicated.