The traditional view has been that entry of amniotic fluid into the maternal circulation
resulted in obstruction of pulmonary capillaries by amniotic fluid emboli, leading to
cardiovascular collapse. However, the usual absence of physical evidence of pulmonary
vessel obstruction, the high degree in variability in clinical course, and the failure to
consistently reproduce the disease in animal models51,52 suggest that physical obstruction to
the circulation is not the main mechanism of AFE. Some investigators have suggested that
immunologic factors could be involved in the pathogenesis of this syndrome.53–56 In 1984,
Hammerschmidt et al53 found that amniotic fluid could activate complement when
incubated with normal plasma and postulated that complement and granulocyte activation by
amniotic fluid could contribute to the pulmonary collapse of this syndrome. In 1993,
Benson55 suggested that AFE might actually result from anaphylaxis to fetal material
leaking into the maternal circulation, which would stimulate a cascade of endogenousimmune
mediators, resulting in a reaction similar to anaphylaxis. However, in a further
study, this same investigator did not find evidence to support the role of mast cell
degranulation (anaphylaxis) in the pathophysiology of the disease.57 In contrast, all of the 8
women with AFE had abnormally low C3 and C4 complement concentrations, suggesting
that complement activation could play a role in the pathogenesis of AFE. This hypothesis
remains to be proved.