Anaphylactoid Syndrome/Amniotic Fluid Embolism

Description

• Amniotic fluid embolism (AFE) is a syndrome associated with cardiorespiratory collapse during pregnancy or shortly after delivery.
• Classically includes a triad of hypoxia, hypotension, and coagulopathy surrounding labor and delivery

Epidemiology

Incidence

• Varies widely because of lack of diagnostic criteria
• 1.2 to 7.7 per 100,000 deliveries in the United Kingdom, Canada, and United States

Prevalence
Difficult to assess because this is a rare event

Etiology and Pathophysiology

• Incompletely understood
• Amniotic fluid enters maternal circulation through disruption of uterine vessels.
  • Causes an abnormal activation of proinflammatory mediators in susceptible patients, leading to cardiopulmonary arrest similar to anaphylaxis or systemic inflammatory response syndrome
  • However, fetal cells can also be found in maternal pulmonary artery samples following normal delivery without AFE.
• Hemodynamic response not caused by pulmonary vasculature obstruction. Instead, an initial period of pulmonary and systemic hypertension (HTN) is followed by:
  • Pulmonary vasoconstriction, resulting in severe pulmonary HTN, VQ mismatch, and hypoxia
  • Acute cor pulmonale: right ventricular failure with ventricular dilation and deviation of the interventricular septum
  • Impaired left ventricular filling and myocardial ischemia causing left ventricular failure as the dominant hemodynamic alteration
• Coagulopathy present in 83% of patients (1), but incompletely understood
  • Amniotic fluid can induce platelet aggregation, lead to release of platelet factor III, activate factor X, and the complement cascade.
  • Tissue factor in amniotic fluid can induce thrombocytopenia in animal models, but it is unclear if any of these factors are present in large enough quantities to induce such a significant coagulopathy.
  • Disseminated intravascular coagulation (DIC) results with potential for massive hemorrhage.

Genetics
A genetic component has not been identified.

Risk Factors

Large population-based studies have identified several risk factors:

• Medical induction of labor
• Maternal age ≥35 years old
• Multiple gestation
• Cesarean section prior to development of symptoms
• Cervical laceration or uterine rupture
  • Causality of association with operative vaginal delivery or cesarean section is not always clear.
• Placental abruption

General Prevention

No identified measure known to reduce the risk of AFE

Commonly Associated Conditions

• Cesarean section: Strong association with AFE as emergent cesarean may be required for delivery given fetal distress; may be causative in some cases
• DIC: consumptive coagulopathy that is often a component of the initial presentation of AFE