What is the recommended workup and treatment for a patient suspected of having magnesium deficiency (Mg) with symptoms such as muscle cramps, weakness, or cardiac arrhythmias?

Magnesium Deficiency Workup and Treatment

Initial Diagnostic Approach

For suspected magnesium deficiency, immediately obtain a serum magnesium level and ECG, particularly if the patient has cardiac symptoms, arrhythmias, or concurrent use of diuretics or digoxin. 1

Laboratory Evaluation

• Serum magnesium level: Hypomagnesemia is defined as <1.3 mEq/L (normal range: 1.3-2.2 mEq/L) 1, 2
• Concurrent electrolytes: Always check potassium and calcium levels, as hypomagnesemia commonly causes refractory hypokalemia and hypocalcemia 1, 3, 4
• Renal function: Document creatinine/GFR before initiating treatment, as this determines maximum safe dosing 5, 4
• ECG: Obtain immediately if QTc prolongation, arrhythmias, heart failure, or digoxin therapy is present 3, 6

Critical caveat: A normal serum magnesium does not exclude intracellular magnesium depletion, which may be present in up to 10% of hospitalized patients despite normal serum levels 7, 8. The magnesium tolerance test is more sensitive but rarely practical in acute settings 7.

Identify Underlying Etiology

Common causes to investigate include 1, 2, 4:

• Gastrointestinal losses: Diarrhea, malabsorption, short bowel syndrome, high-output stomas
• Renal losses: Diuretics (furosemide, thiazides), aminoglycosides, amphotericin B, cisplatin, calcineurin inhibitors, proton pump inhibitors 3
• Endocrine disorders: Diabetes mellitus, hyperthyroidism, hyperaldosteronism 2
• Alcohol use disorder: Combination of decreased intake and increased renal losses 4, 7

Treatment Algorithm

Severe Symptomatic Hypomagnesemia (Emergency)

For life-threatening presentations including torsades de pointes, ventricular arrhythmias, seizures, or cardiac arrest, give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline magnesium level. 1, 2, 3

• This is a Class I recommendation from the American Heart Association 1
• Follow with continuous infusion if needed 3
• Have IV calcium immediately available to reverse potential toxicity (calcium chloride 10% 5-10 mL or calcium gluconate 10% 15-30 mL over 2-5 minutes) 1, 2, 6

Moderate to Severe Hypomagnesemia (<0.50 mmol/L or symptomatic)

Administer parenteral magnesium sulfate as first-line therapy: 3, 5

• Initial dose: 1-2 g (8-16 mEq) IV bolus over 15-30 minutes 2, 5
• Maintenance: 5 g (approximately 40 mEq) added to 1 liter of D5W or normal saline, infused over 3 hours 5
• Alternative: 4-6 hour infusion of 500 mL of 2% magnesium sulfate in D5W 9
• Total daily dose: Approximately 1.0 mEq/kg on day 1, then 0.3-0.5 mEq/kg per day for 3-5 days 4

Maximum rate: Do not exceed 150 mg/minute (1.5 mL of 10% solution) except in severe eclampsia with seizures 5

Mild Hypomagnesemia (asymptomatic, >0.50 mmol/L)

Start with oral magnesium oxide 12-24 mmol daily (approximately 300-600 mg elemental magnesium). 3, 8

• Administer at night when intestinal transit is slowest to maximize absorption 3
• Important caveat: Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders 3

Special Populations and Adjustments

Renal insufficiency: 5, 4

• Maximum dose is 20 grams/48 hours in severe renal insufficiency
• Requires frequent serum magnesium monitoring
• Smaller doses with close observation mandatory

Short bowel syndrome/malabsorption: 3

• May require subcutaneous magnesium sulfate (4-12 mmol added to saline bags) 1-3 times weekly
• Consider oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses to improve magnesium balance
• Monitor serum calcium regularly to avoid hypercalcemia

Volume depletion: 3

• First correct sodium and water depletion with IV saline to eliminate secondary hyperaldosteronism, which increases renal magnesium wasting
• Each liter of jejunostomy fluid contains ~100 mmol/L sodium

Electrolyte Replacement Sequence

Critical principle: Always replace magnesium BEFORE attempting to correct hypocalcemia or hypokalemia, as these will be refractory to treatment until magnesium is normalized. 1, 3, 4

1. Magnesium first 3
2. Potassium second (hypomagnesemia causes dysfunction of potassium transport systems) 3, 4
3. Calcium last (calcium normalization typically occurs within 24-72 hours after magnesium repletion begins) 3

Absorption caveat: Do not administer calcium and iron supplements together with magnesium; separate by at least 2 hours as they inhibit each other's absorption 3

Monitoring During Treatment

Clinical Parameters

• Patellar reflexes: Loss indicates magnesium toxicity 5
• Respiratory rate: Decreased rate suggests impending respiratory depression 5, 9
• Vital signs: Monitor for hypotension and bradycardia, especially with rapid infusion 2, 5

Laboratory Monitoring

• Serum magnesium: Check frequently during IV replacement 5, 4
• Target level: Maintain >1.3 mEq/L; therapeutic levels for arrhythmia control are 2.5-5 mEq/L 2, 6
• Avoid levels >5.5 mEq/L to prevent toxicity 9
• Concurrent electrolytes: Monitor potassium and calcium closely 3

Signs of Magnesium Toxicity

• Loss of deep tendon reflexes 5, 9
• Respiratory depression 3, 5
• Hypotension 3, 5
• Bradycardia 2, 5

Common Pitfalls to Avoid

1. Do not give bolus potassium for cardiac arrest suspected to be secondary to hypokalemia (Class III recommendation) 1
2. Do not continue maternal magnesium sulfate beyond 5-7 days in pregnancy as it can cause fetal abnormalities 5
3. Do not mix magnesium sulfate with vasopressors or calcium in the same solution 3
4. Do not assume normal serum magnesium excludes deficiency - intracellular depletion may be present 7, 8
5. Do not attempt to correct hypocalcemia or hypokalemia before repleting magnesium - they will be refractory to treatment 1, 3, 4

Long-Term Management

For ongoing magnesium wasting conditions, daily oral calcium and magnesium supplementation is recommended. 2

• Identify and treat underlying causes (endocrine disorders, medication adjustments) 2
• For post-transplant patients on calcineurin inhibitors, increased dietary magnesium is typically insufficient; supplements are usually necessary 3
• Monitor magnesium levels regularly based on the underlying condition 2