What are the signs of magnesium excess in patients, particularly those with impaired renal function or taking magnesium supplements?

Signs of Magnesium Excess

Magnesium toxicity presents with a predictable progression of cardiac, neurological, and gastrointestinal manifestations that correlate directly with serum magnesium levels, requiring immediate recognition and intervention particularly in patients with renal impairment or receiving magnesium-containing medications.

Clinical Manifestations by Severity

Mild to Moderate Toxicity (2.5-5 mmol/L)

Cardiac effects:

• Prolonged PR interval 1, 2
• Widened QRS complex 1, 2
• Prolonged QT interval 1, 2

Neurological effects:

• Loss of deep tendon reflexes (typically first sign) 1, 2
• Lethargy and headache 2
• Sedation 1

Other manifestations:

• Nausea and vomiting 1, 2
• Flushing of the skin 1, 2
• Hypophosphatemia 1
• Hyperosmolar dehydration 1

Severe Toxicity (6-10 mmol/L)

Life-threatening cardiac effects:

• AV nodal conduction block 1
• Bradycardia 1
• Hypotension 1, 2
• Cardiac arrest 1, 2

Severe neurological effects:

• Severe muscular weakness 1
• Respiratory depression 1, 2
• Altered consciousness 1
• Flaccid paralysis 1

Gastrointestinal effects:

• Paralytic ileus (less commonly recognized but documented) 3

High-Risk Populations

Patients with renal impairment are at substantially higher risk:

• Can develop toxicity after relatively lower magnesium doses 1, 2
• Magnesium is primarily excreted renally, making renal dysfunction the most important risk factor 4, 5
• Fatal cases have been reported even with standard doses in patients with end-stage renal disease 4

Obstetric patients:

• Iatrogenic overdose is particularly common in pregnant women receiving magnesium sulfate for preeclampsia/eclampsia 1, 2
• Risk increases significantly if the patient becomes oliguric 1

Important caveat: Fatal hypermagnesemia can occur even in patients with normal renal function when excessive amounts are consumed, as documented in case reports of patients taking magnesium hydroxide for constipation 4.

Critical Assessment Points

Physical examination priorities:

• Check deep tendon reflexes immediately—loss of reflexes is typically the earliest clinical sign and occurs at 4-5 mmol/L 1, 2
• Assess respiratory rate and effort for signs of respiratory depression 1, 2
• Monitor blood pressure and heart rate for hypotension and bradycardia 1, 2

ECG findings to identify:

• Obtain 12-lead ECG to assess for progressive conduction abnormalities 2
• Early changes include peaked T waves, then flattened/absent T waves 1
• Progressive widening of PR interval, QRS complex, and QT interval 1, 2
• Severe cases show idioventricular rhythms, sine-wave pattern, and asystole 1

Laboratory evaluation:

• Serum magnesium level is diagnostic 2, 4
• Check concurrent electrolytes (calcium, potassium) as abnormalities commonly coexist 2, 6
• Assess renal function (creatinine) 2

Common Pitfalls

Do not overlook magnesium toxicity in patients with normal renal function who are taking over-the-counter magnesium-containing laxatives or antacids—fatal cases have been documented 4.

Recognize that serum magnesium represents less than 1% of total body stores, so toxicity assessment relies on clinical manifestations combined with serum levels 6.

Be aware that certain medications potentiate magnesium toxicity, including diuretics and digoxin 2.