Magnesium Sulfate Dosing for Posterior Reversible Encephalopathy Syndrome (PRES)
Use the standard eclampsia regimen: 4-6 grams IV loading dose over 20-30 minutes, followed by 1-2 grams per hour continuous infusion for 24 hours postpartum. 1
Loading Dose Administration
- Administer 4-6 grams IV over 20-30 minutes as the initial loading dose 1
- The loading dose can alternatively be given as 4 grams IV plus 10 grams IM (5 grams in each buttock) using the Pritchard protocol, particularly useful in settings with limited IV access 1
- In the comparative study of eclamptic women with PRES, magnesium sulfate demonstrated significantly shorter treatment duration and better neurological recovery compared to mannitol 2
Maintenance Infusion
- Continue with 1-2 grams per hour by continuous IV infusion 1
- Evidence suggests 2 grams per hour is more effective than 1 gram per hour in achieving therapeutic levels, particularly in patients with BMI ≥25 kg/m² 1
- The therapeutic serum concentration target is 1.8-3.0 mmol/L for controlling eclamptic convulsions 3
Duration of Therapy
- Continue magnesium sulfate for 24 hours postpartum in most cases 1, 4
- The 24-hour protocol remains the safer standard despite some evidence suggesting women who received ≥8 grams before delivery may not benefit from continuing the full 24 hours 1
- Preeclampsia may worsen or appear de novo after delivery, particularly between days 3-6 postpartum, justifying the extended duration 4
Critical Safety Monitoring
- Monitor for loss of patellar reflexes (occurs at 3.5-5 mmol/L), respiratory rate, and urine output continuously 3
- Respiratory paralysis occurs at 5-6.5 mmol/L, and cardiac arrest can occur when concentrations exceed 12.5 mmol/L 3
- Have calcium chloride 10% (5-10 mL) or calcium gluconate 10% (15-30 mL) immediately available for IV administration over 2-5 minutes if magnesium toxicity develops 5
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 1
Special Considerations for PRES
- In the retrospective analysis of 62 eclamptic women with PRES, magnesium sulfate achieved better neurological recovery than mannitol (p = 0.039) 2
- All patients in the PRES study underwent FLAIR MRI showing parietooccipital region and cerebellum as the most commonly affected areas 2
- Notably, 29-35.7% of eclamptic patients with PRES were normotensive at admission, indicating PRES can occur without severe hypertension 2
Critical Pitfalls to Avoid
- Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) due to risk of severe hypotension and myocardial depression 1
- Do not delay calcium administration while waiting for laboratory confirmation when clinical signs strongly suggest magnesium toxicity 5
- Avoid NSAIDs for postpartum pain in preeclamptic patients as they worsen hypertension and increase acute kidney injury risk 1
- Do not continue magnesium administration beyond 5-7 days as prolonged use can cause fetal abnormalities; maximum dosage is 20 grams/48 hours in severe renal insufficiency 4