Amniotic Fluid Embolism (AFE) Background

Amniotic fluid embolism (AFE) is a rare obstetric emergency in which it is postulated that amniotic fluid, fetal cells, hair, or other debris enter the maternal circulation, causing cardiorespiratory collapse.

In 1941, Steiner and Luschbaugh described AFE for the first time after they found fetal debris in the pulmonary circulation of women who died during labor.

Current data from the National Amniotic Fluid Embolus Registry suggest that the process is more similar to anaphylaxis than to embolism, and the term anaphylactoid syndrome of pregnancy has been suggested because fetal tissue or amniotic fluid components are not universally found in women who present with signs and symptoms attributable to AFE.

The diagnosis of AFE has traditionally been made at autopsy when fetal squamous cells are found in the maternal pulmonary circulation; however, fetal squamous cells are commonly found in the circulation of laboring patients who do not develop the syndrome. In a patient who is critically ill, a sample obtained by aspiration of the distal port of a pulmonary artery catheter that contains fetal squamous cells is considered suggestive of but not diagnostic of AFE syndrome. The diagnosis is essentially one of exclusion based on clinical presentation. Other causes of hemodynamic instability should not be neglected.

For related information on pregnancy, see Medscape’s Pregnancy Resource Center.

Pathophysiology

The pathophysiology of AFE is poorly understood. Based on the original description, it was theorized that amniotic fluid and fetal cells enter the maternal circulation, possibly triggering an anaphylactic reaction to fetal antigens. However, fetal material is not always found in the maternal circulation in patients with AFE, and material of fetal origin is often found in women who do not develop AFE.

Benson et al tested 2 hyptheses concerning the pathophysiology of AFE: (1) Clinical symptoms result from mast cell degranulation with the release of histamine and tryptase, or (2) Clinical symptoms result from activation of the complement pathway. Nine women with AFE were compared with 22 women with normal labors. Serum from patients with AFE was collected within 14 hours of symptom onset and frozen. Urine was collected within 12-24 hours after symptom onset. Control patients had complement levels measured on admission, during labor, and the day after delivery.

Six of the 9 women with AFE died, and all 9 required blood transfusions for disseminated intravascular coagulation (DIC). Seven women had no evidence of mast cell degranulation (ie, either urinary histamine or serum tryptase). Compared with postpartum control patients, complement levels in the AFE group were severely depressed. C3 in the AFE group was 44 compared with 117.2 in the postpartum group. C4 was 10.7 in the AFE group versus 29.4 in the postpartum group. These differences were statistically significant. This suggests that complement activation may play an important role in the pathophysiology of AFE.

Farrar and Gherman reported the case of a 40-year-old multipara in active labor with acute onset of facial erythema, seizures, hypoxia, cardiac arrest, DIC, and ultimately death. Fetal squames and fibrin thrombi were found in the pulmonary tree at autopsy. Blood drawn 2 hours after symptom onset had a serum tryptase level of 4.7 ng/mL (normal <1 ng/mL).

A case reported by Marcus et al, in which AFE developed after a spontaneous rupture of membranes, demonstrated no increase in mast cells or degranulation in lung tissue as shown by Giemsa staining. Serum tryptase levels were 11.4 ng/mL (normal <11.4 ng/mL).

The initiating event is poorly understood. However, usually during labor or other procedure, amniotic fluid and debris, or some as yet unidentified substance, enters the maternal circulation; this may trigger a massive anaphylactic reaction, activation of the complement cascade, or both. Progression usually occurs in 2 phases. In phase I, pulmonary artery vasospasm with pulmonary hypertension and elevated right ventricular pressure cause hypoxia. Hypoxia causes myocardial capillary damage and pulmonary capillary damage, left heart failure, and acute respiratory distress syndrome. Women who survive these events may enter phase II. This is a hemorrhagic phase characterized by massive hemorrhage with uterine atony and DIC; however, fatal consumptive coagulopathy may be the initial presentation.

Frequency

United States

Incidence of AFE is estimated at 1 case per 8,000-30,000 pregnancies. The true incidence is unknown because of inaccurate diagnoses and inconsistent reporting of nonfatal cases.

International

Incidence is similar to that of the United States.

Mortality/Morbidity

Maternal mortality approaches 80%. Mortality was 61% in the US national registry, which listed 46 cases. Five to 10% of maternal mortality in the United States is due to AFE. Of patients with AFE, 50% die within the first hour of onset of symptoms. Of survivors of the initial cardiorespiratory phase, 50% develop a coagulopathy.

The United Kingdom AFE registry reports a mortality of 37%. Of those who survived AFE, 7% were neurologically impaired.

Survival is uncommon, although the prognosis is improved with early recognition and prompt resuscitation. Most women who survive have permanent neurologic impairment. Neonatal survival was 79% in the US registry and 78% in the UK registry.

Race

No racial or ethnic predilection exists.

Sex

AFE only occurs in women.

Age

Previously, advanced maternal age was believed to be a risk factor. No relationship to age has been found in the National Amniotic Fluid Embolus Registry.

read the completed about Amniotic Fluid Embolism HERE