Pathophysiology

It is not uncommon for amniotic fluid and fetal squames to enter the maternal circulation without ill effect. In certain susceptible women, however, it seems that the presence of fetal cells and/or other components of amniotic fluid may trigger a complex pathophysiological cascade similar to that seen with anaphylaxis and septic shock. The initial pathophysiological mechanism is of acute pulmonary vascular obstruction and hypertension leading to cor pulmonale. This is quite transient and soon followed by left ventricular failure leading to profound hypotension and shock. An acute inflammatory response disrupts the pulmonary capillary endothelium and alveoli leading to a ventilation–perfusion imbalance – resulting in severe hypoxia, convulsions and coma. If the patient survives for more than 1 hour it is virtually inevitable that she will develop disseminated intravascular coagulation due to the activation of coagulation factors by the amniotic fluid (which contains tissue factor) and fetal cells, in addition to the profound shock.

Diagnosis

The clinical diagnosis is based on the sudden development of acute respiratory distress and cardiovascular collapse in a patient in labour or recently delivered. In some cases the signs and symptoms occur within minutes of the triggering event, such as amniotomy or caesarean section.

The current criteria for defining a case in the UK are:

Either

In the absence of any other clear cause, acute maternal collapse with one or more of the following features:

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  Acute fetal compromise

  
  
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  Cardiac arrhythmias or arrest

  
  
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  Coagulopathy

  
  
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  Convulsion

  
  
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  Hypotension

  
  
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  Maternal haemorrhage

  
  
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  Premonitory symptoms, e.g. restlessness, numbness, agitation, tingling

  
  
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  Shortness of breath

Or

Women in whom the diagnosis was made at postmortem examination by finding fetal squames or hair in the lungs.

The differential diagnosis includes other acute catastrophies that may present with similar features. These would include cardiac causes (myocardial infarction, cardiomyopathy, heart failure secondary to volume overload, valvular disease), respiratory (pulmonary oedema secondary to volume overload, acute asthma, pulmonary embolus), infectious (severe sepsis, chest, chorioamnionitis, endocarditis), pregnancy complications (pre-eclampsia and eclampsia, HELLP (Haemolysis, Elevated Liver enzymes and Low Platelets) syndrome, ante- and postpartum haemorrhage) and others (anaphylaxis, air embolus, local anaesthetic toxicity).

The clinical features and context should help differentiate between these causes but, in any event, the initial management – cardiopulmonary resuscitation – may be required for all of these conditions. The initial clinical diagnosis of AFE, then, is really a combination of the above clinical features without any of the other obvious clinical causes.