Histological characteristics of the myometrium in the postpartum hemorrhage of unknown etiology: a possible involvement of local immune reactions

- We provide the pathological features of the myometrium in postpartum hemorrhage (PPH) of unknown etiology.
- Massive infiltration of inflammatory and immune cells is significant in myometrium of PPH.
- Postpartum acute myometritis (PAM) could be a pathological concept of PPH of unknown etiology.
- Fetal components are a major candidate for the causes of PAM.

The aim of this study was to evaluate the histological characteristics of the myometrium obtained in postpartum hemorrhage (PPH) of unknown etiology secondary to uterine atony. These characteristics were selected from among registered cases of clinically suspected amniotic fluid embolism (AFE) and classified as PPH of unknown etiology because of no obvious cause of PPH at Hamamatsu University School of Medicine, a registration center for clinical AFE in Japan. Immunohistochemical studies were performed on myometrium using anti-mast cell tryptase, anti-neutrophil elastase, anti-CD68, anti-CD88, anti-CD3, and anti-ZnCP-1 antibodies. Massive infiltrations of inflammatory cells with mast cell degranulation within the myometrium secondary to complement activation were observed in PPH of unknown etiology (n = 34), but not in control pregnant women (n = 15) or after delivery in women without PPH (n = 18). The concomitant immunohistochemical detection of meconium in myometrium suggests that amniotic fluids or fetal materials are one of the candidates for inducing maternal local immune activation in the PPH of unknown etiology. Postpartum acute myometritis in the absence of an infective etiology may be a histological characteristic of PPH of unknown etiology.