How to correct hypomagnesemia?

How to Correct Hypomagnesemia

For mild hypomagnesemia, start with oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium), preferably given at night when intestinal transit is slowest, but first correct any volume depletion with IV saline to eliminate secondary hyperaldosteronism that causes renal magnesium wasting. 1, 2

Step 1: Assess Severity and Check Renal Function

• Measure serum magnesium level to determine severity: mild (1.2-1.7 mg/dL), moderate (0.7-1.2 mg/dL), or severe (<0.7 mg/dL or <1.2 mg/dL with symptoms) 3, 4
• Check creatinine clearance before any supplementation - magnesium is absolutely contraindicated if CrCl <20 mL/min due to life-threatening hypermagnesemia risk 1, 2
• Use extreme caution between CrCl 20-30 mL/min; only supplement in emergencies with close monitoring 1
• Measure fractional excretion of magnesium (FEMg): <2% indicates gastrointestinal losses, >2% indicates renal wasting 4
• Check potassium and calcium levels simultaneously - hypomagnesemia causes refractory hypokalemia and hypocalcemia that won't correct until magnesium is normalized 1, 3, 5

Step 2: Correct Volume Depletion FIRST (Critical Step)

This is the most commonly missed step that leads to treatment failure. 1

• Administer IV normal saline to correct sodium and water depletion before starting magnesium supplementation 1, 3
• Volume depletion triggers secondary hyperaldosteronism, which increases renal retention of sodium at the expense of magnesium and potassium, creating ongoing losses that exceed supplementation 1
• In patients with high-output stomas or diarrhea, each liter of jejunostomy fluid contains ~100 mmol/L sodium, causing significant volume depletion 1, 3
• Failure to correct volume status first will result in continued magnesium losses despite supplementation 1

Step 3: Choose Route Based on Severity

For Mild-Moderate Hypomagnesemia (Asymptomatic, Mg >1.2 mg/dL):

Oral magnesium oxide is first-line: 1, 2, 6

• Start with 12 mmol (approximately 480 mg elemental magnesium) given at night when intestinal transit is slowest to maximize absorption 1, 2
• Increase to 24 mmol daily (divided doses or all at night) if needed based on response 1, 2
• For FDA-labeled dosing: 1 g magnesium sulfate (8.12 mEq) IM every 6 hours for 4 doses equals 32.5 mEq per 24 hours 6
• Alternative organic salts (magnesium citrate, aspartate, lactate) have better bioavailability than oxide and may be preferred in malabsorption 1, 2
• Liquid or dissolvable forms are better tolerated than pills 1

For Severe Hypomagnesemia (Symptomatic or Mg <1.2 mg/dL):

IV magnesium sulfate is required: 6, 7

• For severe symptomatic cases: 1-2 g IV magnesium sulfate over 15 minutes, followed by continuous infusion 3, 6
• FDA dosing for severe hypomagnesemia: up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours, or 5 g (40 mEq) added to 1 liter D5W or NS infused over 3 hours 6
• Maximum rate: 150 mg/minute (1.5 mL of 10% solution) except in life-threatening emergencies 6
• For cardiac emergencies (torsades de pointes, life-threatening arrhythmias): 1-2 g IV bolus over 5 minutes regardless of measured magnesium level 2, 3, 7

Step 4: Address Concurrent Electrolyte Abnormalities

Always replace magnesium BEFORE attempting to correct calcium or potassium: 3, 5

• Hypomagnesemia causes dysfunction of potassium transport systems, making hypokalemia resistant to potassium supplementation alone 1, 5
• Hypocalcemia will not correct until magnesium is normalized - expect calcium normalization within 24-72 hours after magnesium repletion begins 1, 3
• Separate calcium and magnesium supplements by at least 2 hours as they inhibit each other's absorption 3

Step 5: Monitor Response and Adjust

Monitoring schedule: 1

• Recheck magnesium 2-3 weeks after starting supplementation or any dose change 1
• Once stable, monitor every 3 months 1
• In high-risk patients (short bowel syndrome, high GI losses, CRRT), check every 2 weeks initially 1
• Target serum magnesium >0.6 mmol/L (>1.5 mg/dL) 2
• Monitor for toxicity signs: loss of patellar reflexes, respiratory depression, hypotension, bradycardia 3
• Have calcium chloride available to reverse magnesium toxicity if needed 1, 3

Common Pitfalls to Avoid

• Not checking renal function first - can cause fatal hypermagnesemia in renal insufficiency 1, 2
• Supplementing magnesium without correcting volume depletion - leads to continued renal losses and treatment failure 1, 3
• Attempting to correct hypokalemia or hypocalcemia before magnesium - these will remain refractory until magnesium is normalized 1, 3, 5
• Using magnesium oxide in patients with diarrhea or high-output stomas - poorly absorbed and may worsen diarrhea; consider organic salts instead 1, 2
• Rapid IV infusion - can cause hypotension and bradycardia; respect maximum infusion rates 1, 6
• Assuming serum magnesium reflects total body stores - less than 1% of total body magnesium is in serum; symptoms may occur despite "normal" levels 1

Special Situations

Short Bowel Syndrome/Malabsorption:

• Require higher doses (12-24 mmol daily) due to significant losses 1, 2
• If oral therapy fails, consider oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) to improve magnesium balance, but monitor calcium closely 1, 2
• May require IV or subcutaneous magnesium sulfate (4-12 mmol added to saline) 1-3 times weekly 1, 2

Continuous Renal Replacement Therapy:

• Hypomagnesemia occurs in 60-65% of critically ill patients on CRRT 1
• Use dialysis solutions containing magnesium to prevent ongoing losses 1, 3
• Regional citrate anticoagulation increases magnesium losses through chelation 1

Pregnancy (Pre-eclampsia/Eclampsia):

• Initial dose: 4-5 g IV in 250 mL D5W or NS, plus 10 g IM (5 g in each buttock) 6
• Maintenance: 4-5 g IM every 4 hours or 1-2 g/hour continuous IV infusion 6
• Target serum level: 6 mg/100 mL for seizure control 6
• Do not exceed 30-40 g per 24 hours 6
• Continuous use beyond 5-7 days can cause fetal abnormalities 6

Maximum Dosing in Renal Insufficiency:

• Maximum 20 grams/48 hours with frequent serum monitoring 1, 6