Perivillous fibrinoid, I think, can best be understood as the consequence of an injury to the syncytiotrophoblast, whether ischemic, immune or metabolic, that spares the underlying cytotrophoblast. The now freed cytotrophblast do what they do at the placental margins, they secrete matrix composed of annexins, fetal fibronectin etc. These substances, along with exposed villous stroma, induce thrombogenesis from the maternal blood. Together the fibrin deposition and matrix produce perivillous fibrinoid. Diffuse syncytial injury of this type results in massive perivillous fibrinoid deposition, aka maternal floor infarction or to keep the MFI, massive fibrinoid infiltration. The diagnostic problem is discovering the various causes of this syncytial injury.
I developed this concept some years ago on my blog. When I was in Dublin, I think I heard that others have also come to a similar understanding, probably independently from me. I have now posted that older page on MFI with copious illustrations that I think supports this view of perivillous fibrinoid deposition. I added the figures within the text, rather than at the end, because there were so many.
obstetrical pathology 
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