What are the diagnostic criteria for pre-eclampsia and when to initiate magnesium sulfate (MgSO4) therapy?

Diagnosis of Pre-eclampsia and Magnesium Sulfate Therapy

Pre-eclampsia is diagnosed by new-onset hypertension (BP ≥140/90 mmHg) and proteinuria (≥0.3 g/24h or ≥+ on dipstick) after 20 weeks of gestation, though proteinuria is no longer absolutely required if other systemic features are present. 1

Diagnostic Criteria for Pre-eclampsia

Essential Criteria

• Hypertension: Systolic BP ≥140 mmHg and/or diastolic BP ≥90 mmHg after 20 weeks of gestation in a previously normotensive woman 2
• Proteinuria: ≥0.3 g/24h or ≥+ (300 mg/L) on dipstick testing or protein:creatinine ratio ≥30 mg/mmol 2

Alternative Criteria (if proteinuria absent)

New-onset hypertension with any of the following:

• Right upper quadrant/epigastric pain
• Headache or visual disturbances
• Thrombocytopenia (<100,000/μL)
• Elevated liver enzymes (>2× normal)
• Renal insufficiency
• Pulmonary edema
• HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets) 2, 1

Classification

1. Pre-existing hypertension: BP ≥140/90 mmHg before pregnancy or before 20 weeks gestation
2. Gestational hypertension: New hypertension after 20 weeks without proteinuria
3. Pre-eclampsia: New hypertension after 20 weeks with proteinuria or other systemic features
4. Superimposed pre-eclampsia: Worsening hypertension with new proteinuria or systemic features in women with pre-existing hypertension 2

Severity Assessment

Pre-eclampsia is considered severe with any of these features:

• Severe hypertension: SBP ≥160 mmHg and/or DBP ≥110 mmHg
• Thrombocytopenia <100,000/μL
• Liver enzymes >2× normal values
• Persistent epigastric/RUQ pain
• Renal failure
• Pulmonary edema
• Neurological symptoms (headache, visual disturbances)
• HELLP syndrome 1

Magnesium Sulfate Therapy

Indications for MgSO₄

• Severe pre-eclampsia (prevention of eclampsia)
• Eclampsia (treatment and prevention of recurrent seizures) 3

Dosage Regimen

For severe pre-eclampsia or eclampsia:

Loading dose:

• 4-5 g IV in 250 mL of 5% Dextrose or 0.9% Sodium Chloride over 15-20 minutes 3

Maintenance dose options:

1. IV regimen: 1-2 g/hour by continuous infusion 3
2. IM regimen: 5 g IM every 4 hours in alternate buttocks 3, 4

Duration of Treatment

• Continue until 24 hours after delivery or 24 hours after the last seizure in postpartum eclampsia 5, 6
• Recent evidence suggests that a 12-hour maintenance regimen may be as effective as the traditional 24-hour regimen 6
• Do not exceed total daily dose of 30-40 g 3
• Caution: Continuous use beyond 5-7 days can cause fetal abnormalities 3

Monitoring During MgSO₄ Therapy

• Patellar reflexes (first sign of toxicity when absent)
• Respiratory rate (should be >12/min)
• Urine output (should be >30 mL/hour)
• Serum magnesium levels if available (therapeutic range: 1.8-3.0 mmol/L) 4

Signs of Magnesium Toxicity

• Loss of patellar reflexes: 3.5-5 mmol/L
• Respiratory depression: 5-6.5 mmol/L
• Cardiac conduction abnormalities: >7.5 mmol/L
• Cardiac arrest: >12.5 mmol/L 4

Dose Adjustment

• In severe renal insufficiency: Maximum 20 g/48 hours with frequent monitoring of serum magnesium levels 3

Common Pitfalls and Caveats

1. Diagnostic challenges:

   • Pre-existing hypertension may be masked by physiological BP drop in early pregnancy
   • Proteinuria may be absent in up to 20% of women with pre-eclampsia

2. Treatment pitfalls:

   • Failure to monitor for magnesium toxicity
   • Inadequate loading dose leading to subtherapeutic levels
   • Continuing magnesium beyond necessary duration
   • Not adjusting dose in renal impairment

3. Monitoring issues:

   • Standard regimen may not achieve therapeutic levels in all patients 5
   • Total magnesium levels don't always correlate with ionized (active) magnesium 5

Remember that magnesium sulfate therapy does not treat the underlying condition but prevents complications. The definitive treatment for pre-eclampsia remains delivery of the placenta and fetus.