Amsterdam Placental Workshop Group Consensus Statement definitions revisited: Basal chronic villitis.

Villitis of unknown etiology (VUE) can be categorized as distal, proximal, or basal, depending on the type of chorionic villi involved. Either terminal or mature intermediate villi are affected in the distal type, while stem villi are affected in the proximal type. The Amsterdam Placental Workshop Group Consensus Statement did not amplify on the definition of basal VUE. We review the literature to explore the entity, focusing on the terminology, inflammatory infiltrate, involved structures, prevalence, and clinical significance. The prevalence of the lesion, variously defined, in placentas with differing indications for pathological examination and which could include cases with concurrent intraparenchymal VUE ranges from 6.6 to 28.3% of VUE; however, the prevalence of VUE confined only to basal and parabasal villi could be as low as 3%. We propose that the villi that are inflamed must be clearly identified as anchoring or anchored villi; that the preferred term is basal chronic villitis; and that it is diagnosed when chorionic inflammation of anchoring villi, or of villi anchored within the basal plate is seen. No studies have looked specifically at the clinical significance of basal villitis; associations with donor oocyte IVF pregnancies, fetal growth restriction, hypertensive disorders of pregnancy, pre-eclampsia, and morbidly adherent placenta have been reported by some but not confirmed by others. VUE should continue to be reported as intraparenchymal, basal, or mixed until such time that the accumulated experience of the clinical correlates suggests there is no merit in doing so.