Severe Hypermagnesemia: Definition, Pathophysiology, and Clinical Manifestations

Definition and Pathophysiology

Severe hypermagnesemia is a life-threatening electrolyte disorder defined as serum magnesium concentration >2.2 mEq/L, most commonly occurring in the obstetric setting with IV magnesium therapy for preeclampsia/eclampsia, or in patients with renal dysfunction taking magnesium-containing medications 1, 2
Hypermagnesemia represents an elevated serum magnesium concentration above the normal range of 1.3-2.2 mEq/L 2, 3
Magnesium is essential for ATPase function, sodium-potassium-calcium transport across cell membranes, and stabilization of excitable membranes 2, 3
At extremely elevated levels, hypermagnesemia disrupts normal cellular excitability and conduction, leading to cardiovascular and neuromuscular collapse 1

Muscular weakness and paralysis can occur in patients with severe hypermagnesemia 2, 3
Ataxia and drowsiness are also common neurological manifestations of severe hypermagnesemia 2, 3
Confusion progressing to altered consciousness and coma can occur in severe cases of hypermagnesemia 1
Vasodilation and hypotension are common cardiovascular manifestations of severe hypermagnesemia 2, 3
Bradycardia and ventricular arrhythmias can occur in patients with severe hypermagnesemia 1, 4
Cardiorespiratory arrest and asystole can be fatal complications of severe hypermagnesemia 2, 3
Hypoventilation leading to respiratory failure can occur in patients with severe hypermagnesemia 2, 3

The most likely scenario for severe hypermagnesemia is in pregnant patients receiving IV magnesium sulfate for preeclampsia or eclampsia treatment, according to the American Heart Association guidelines 1, 4, 5

For cardiac arrest with known or suspected hypermagnesemia, empirical IV calcium administration may be reasonable in addition to standard ACLS care (Class 2b recommendation), as recommended by the American Heart Association 1, 4
Calcium chloride 10%: 5-10 mL IV over 2-5 minutes can be administered to treat severe hypermagnesemia 2, 3
OR calcium gluconate 10%: 15-30 mL IV over 2-5 minutes can be administered to treat severe hypermagnesemia 2, 3

Loss of deep tendon reflexes is a critical bedside indicator of magnesium toxicity, typically occurring at magnesium levels of 4-5 mmol/L, and should prompt immediate intervention, according to the American Heart Association 6
Loss of patellar (knee-jerk) reflexes specifically occurs at magnesium levels of 4-5 mmol/L, making this a crucial monitoring parameter during magnesium therapy 6

At 4-5 mmol/L, hypermagnesemia produces a predictable sequence of neurological findings, including loss of tendon reflexes, sedation, drowsiness, and muscular weakness 6
At 6-10 mmol/L, severe muscular weakness progresses to flaccid paralysis, respiratory depression, and hypoventilation 6, 7

When deep tendon reflexes are lost, immediately administer intravenous calcium as a direct antagonist to magnesium's effects on excitable membranes, with a recommended dose of calcium chloride 10%: 5-10 mL IV over 2-5 minutes or calcium gluconate 10%: 15-30 mL IV over 2-5 minutes 6

Obstetric patients receiving IV magnesium sulfate for preeclampsia/eclampsia represent the highest risk group, and monitoring patellar reflexes hourly during magnesium infusion is crucial, with loss of reflexes mandating immediate cessation of magnesium and empirical calcium administration 6
Oliguria dramatically increases toxicity risk in obstetric patients receiving IV magnesium sulfate 6