Management of Abnormal Magnesium Serum Levels
Hypomagnesemia Management
For mild hypomagnesemia (serum Mg 1.2-1.8 mg/dL), start with oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium), preferably administered at night when intestinal transit is slowest to maximize absorption. 1
Initial Assessment and Correction of Underlying Factors
First, correct water and sodium depletion with intravenous normal saline before initiating magnesium supplementation. 2, 1 This critical step addresses secondary hyperaldosteronism, which drives renal magnesium wasting—the more sodium-depleted the patient, the more aldosterone secreted, and the more magnesium lost in urine despite total body depletion. 3
Measure fractional excretion of magnesium (FEMg) to determine the cause: FEMg <2% indicates gastrointestinal losses, while >2% indicates renal magnesium wasting. 4
Check renal function before any magnesium supplementation—avoid supplementation if creatinine clearance <20 mL/min due to hypermagnesemia risk. 3
Oral Magnesium Therapy for Mild-Moderate Deficiency
Magnesium oxide is the preferred oral formulation as it contains more elemental magnesium than other salts and converts to magnesium chloride in the stomach. 1
Start with 12 mmol (480 mg elemental magnesium) at night, increasing to 24 mmol daily if needed based on response and tolerance. 1
Organic magnesium salts (aspartate, citrate, lactate) have higher bioavailability than magnesium oxide and should be considered as alternatives, particularly in patients with malabsorption. 1, 3
Divide doses throughout the day for continuous repletion in patients with ongoing losses. 3
Parenteral Therapy for Severe Hypomagnesemia
For severe hypomagnesemia (<1.2 mg/dL) or symptomatic patients with cardiac arrhythmias, administer 1-2 g IV magnesium sulfate as a bolus over 5-15 minutes. 2, 5
The FDA-approved dosing for severe hypomagnesemia is up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary, or 5 g (40 mEq) added to 1 liter of fluid for slow IV infusion over 3 hours. 5
For cardiac arrhythmias associated with hypomagnesemia, particularly torsades de pointes, give IV magnesium 1-2 g bolus regardless of measured serum levels—magnesium suppresses episodes even when serum magnesium is normal. 2
Repeated doses may be needed, titrated to suppress ectopy while precipitating factors are corrected. 2
The rate of IV injection should generally not exceed 150 mg/minute except in severe eclampsia with seizures. 5
Refractory Cases and Special Considerations
If oral magnesium fails to normalize levels after adequate sodium repletion, add oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses to improve magnesium balance. 1, 3 Monitor serum calcium regularly to avoid hypercalcemia. 1
For patients with short bowel syndrome or high-output stomas requiring long-term supplementation, subcutaneous administration of 4 mmol magnesium sulfate added to saline 1-3 times weekly is an option. 1, 3
Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output—this is a critical pitfall in patients with gastrointestinal disorders. 1, 3
Concurrent Electrolyte Abnormalities
Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment until magnesium is corrected. 2, 3, 6
Normalize serum magnesium before or simultaneously with potassium supplementation—potassium therapy will fail if magnesium remains low. 3
Maintain serum potassium between 4.5-5.0 mEq/L in patients with torsades de pointes, as this shortens QT interval and reduces recurrence risk. 2
Target Levels and Monitoring
Target serum magnesium >0.85 mmol/L (2.07 mg/dL), as values below this are associated with increased health risks. 7
A reasonable minimum target is >0.6 mmol/L (1.46 mg/dL) for patients with ongoing losses. 1, 3
Effective anticonvulsant serum levels range from 2.5-7.5 mEq/L when magnesium is used therapeutically. 5
Monitor for magnesium toxicity: deep tendon reflexes disappear at plasma levels approaching 10 mEq/L, respiratory paralysis may occur at this level, and levels >12 mEq/L may be fatal. 5
Hypermagnesemia Management
For cardiac arrest or severe cardiotoxicity due to hypermagnesemia (>2.2 mEq/L), administer calcium chloride 10% 5-10 mL or calcium gluconate 10% 15-30 mL IV over 2-5 minutes. 2
Clinical Manifestations
Neurological symptoms include muscular weakness, paralysis, ataxia, drowsiness, and confusion. 2
Cardiovascular effects include vasodilation, hypotension, bradycardia, cardiac arrhythmias, and potential cardiorespiratory arrest at extremely high levels. 2
Deep tendon reflexes decrease first, then disappear as plasma magnesium approaches 10 mEq/L. 5
Treatment Algorithm
Immediately discontinue all magnesium-containing medications and supplements. 2
Administer IV calcium as an antagonist to the peripheral and central effects of magnesium. 2, 5
Ensure adequate hydration and consider loop diuretics to enhance renal magnesium excretion in patients with normal renal function. 2
For patients with renal insufficiency causing hypermagnesemia, hemodialysis may be necessary for rapid magnesium removal. 2
Prevention in High-Risk Populations
Avoid magnesium supplementation in patients with creatinine clearance <20 mL/min. 3
In patients receiving continuous renal replacement therapy, use dialysis solutions containing appropriate magnesium concentrations to prevent both hypo- and hypermagnesemia. 3
Maximum dosage in severe renal insufficiency is 20 g magnesium sulfate per 48 hours with frequent serum monitoring. 5