What are the guidelines for administering magnesium sulfate via intramuscular (IM) injection to a pregnant woman diagnosed with preeclampsia?

Magnesium Sulfate IM Administration for Preeclampsia

Standard IM Regimen (Pritchard Protocol)

For preeclampsia with severe features or eclampsia, administer magnesium sulfate as a combined loading dose of 4 grams IV over 20-30 minutes plus 10 grams IM (5 grams in each buttock), followed by 5 grams IM every 4 hours in alternate buttocks for 24 hours postpartum. 1, 2

This IM protocol is particularly valuable in resource-limited settings with limited IV access or where continuous infusion pumps are unavailable. 1, 3

Loading Dose Administration

• Initial dose: 4 grams IV (given as 20% solution over 10-15 minutes) PLUS 10 grams IM simultaneously (5 grams deep IM in each buttock using undiluted 50% solution). 2, 4

• The IM injection of undiluted 50% solution achieves therapeutic plasma levels within 60 minutes, whereas IV dosing provides immediate therapeutic levels. 2

• For children, dilute the 50% solution to 20% or less concentration before IM injection. 2

Maintenance Dosing

• Standard maintenance: 5 grams IM every 4 hours in alternate buttocks for 24 hours postpartum. 1, 2

• Continue therapy for 24 hours after delivery in most cases, or 24 hours after the last seizure in postpartum eclampsia. 1, 5

• Some evidence suggests women who received ≥8 grams before delivery may not require the full 24-hour postpartum course, though the 24-hour protocol remains the safer standard. 1, 5

Clinical Indications for IM Protocol

• Severe preeclampsia with blood pressure ≥160/110 mmHg with proteinuria, OR moderate hypertension with proteinuria plus neurological symptoms (severe headache, visual disturbances, hyperreflexia, clonus). 6, 5, 3

• Eclampsia (active seizures). 3, 2

• The IM route is appropriate when IV access is limited or continuous infusion monitoring is unavailable. 1, 3

Critical Safety Monitoring Before Each IM Dose

You must verify ALL three criteria before administering each maintenance dose: 5, 2, 7

• Patellar reflexes present — Loss of reflexes indicates impending toxicity at magnesium levels of 3.5-5 mmol/L. 5, 7

• Respiratory rate >12 breaths/minute — Respiratory paralysis occurs at 5-6.5 mmol/L. 1, 5, 7

• Urine output ≥30 mL/hour — Magnesium is renally excreted; oliguria dramatically increases toxicity risk. 1, 5, 7

If any of these criteria are not met, hold the dose and check serum magnesium level immediately. 2, 7

Therapeutic Magnesium Levels

• Target therapeutic range: 1.8-3.0 mmol/L (4-6 mEq/L) for seizure prevention. 7, 4

• Optimal level for seizure control: 6 mg/100 mL (approximately 2.5 mmol/L). 2

• Routine serum magnesium monitoring is not necessary with proper clinical monitoring, but should be checked in renal impairment or suspected toxicity. 1, 3

Toxicity Recognition and Management

Progressive toxicity occurs at predictable serum levels: 7

• 3.5-5 mmol/L: Loss of patellar reflexes (first warning sign)
• 5-6.5 mmol/L: Respiratory depression/paralysis
• >7.5 mmol/L: Altered cardiac conduction
• >12.5 mmol/L: Cardiac arrest

Antidote: Calcium gluconate 1 gram (10 mL of 10% solution) IV over 3 minutes reverses magnesium toxicity. 2

Absolute Contraindications and Critical Precautions

• Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) — this causes severe myocardial depression and precipitous hypotension. 1, 5, 3

• Myasthenia gravis is a contraindication due to neuromuscular blockade risk. 2

• Severe renal insufficiency: Maximum dose is 20 grams/48 hours with frequent serum level monitoring. 2

• Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak. 1, 5

Maximum Dosing Limits

• Do not exceed 30-40 grams total in 24 hours. 2

• Do not continue magnesium sulfate beyond 5-7 days in pregnancy, as prolonged use causes fetal abnormalities. 2

Common Injection Site Side Effects

• Pain at injection site occurs in >90% of patients receiving IM magnesium sulfate. 8

• Inflammation (40-62%) and bleeding/bruising (14-25%) at injection sites are common. 8

• These side effects are inherent to the IM route but do not contraindicate continued therapy if clinically indicated. 8

When to Repeat Loading Dose

• If seizures recur after the initial loading dose, administer an additional 2 grams IV over 3-4 minutes. 2, 4

• Most seizures terminate after the initial loading dose; only 2-5% of patients require repeat dosing. 4

Alternative Shortened Protocol

• A 12-hour IM regimen (same loading dose, then 5 grams IM every 4 hours for only 3 maintenance doses) showed equivalent seizure prevention with fewer side effects and shorter hospitalization in a recent randomized trial. 8

• However, the standard 24-hour protocol remains the established guideline recommendation. 1, 5

Practical Administration Considerations

• IM administration can be performed by trained midwives or nursing staff, making it suitable for settings where specialist care may be delayed. 3, 9

• The IM route eliminates the need for continuous infusion pumps and intensive monitoring required for IV maintenance therapy. 3

• Despite ease of administration, close maternal surveillance remains crucial as progression to severe complications cannot be predicted without ongoing monitoring. 9