Uterine atony is a major cause of postpartum hemorrhage. We recently proposed the new histological concept of postpartum acute myometritis (PAM) for the pathophysiology of refractory uterine atony of unknown etiology, which is characterized by the diffuse activation of mast cells and the complement system as well as the massive infiltration of macrophages and neutrophils into the uterine body. We herein focused on the uterine isthmus just adjacent to the body. The isthmus becomes significantly elongated throughout pregnancy. It is composed of myocytes and fibroblasts with an extracellular matrix that forms a passive lower segment during labor. The aim of this study was to histologically examine the uterine isthmus in cases of PAM in the uterine body. Under the amniotic fluid embolism-registry program in Japan, we selected PAM cases from uterine samples obtained by cesarean hysterectomy and delivered to us for analyses between 2011 and 2017. Control tissues were collected during elective cesarean section. We investigated the isthmus tissues of these cases and performed immunohistochemistry for inflammatory cell markers, i.e. neutrophil elastase, mast cell tryptase, CD68, CD3, and C5a receptor (C5aR). The numbers of tryptase-positive degranulating mast cells, elastase-positive neutrophils, CD68-positive macrophages, and C5aR-positive cells in the isthmus were significantly higher in uteri with PAM in the body than in controls without PAM. CD3 was negative in both groups. In conclusion, inflammation and an anaphylactoid reaction were histologically detected not only in the uterine body, but in the isthmus among cases of refractory PPH of unknown etiology after cesarean section.

中文翻译：

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子宫峡部的急性炎症与子宫体的产后急性子宫肌炎同时发生，包括剖宫产后病因不明的难治性产后出血。

子宫收缩乏力是产后出血的主要原因。我们最近针对病因不明的难治性子宫收缩乏力的病理生理提出了产后急性子宫肌炎 (PAM) 的新组织学概念，其特征是肥大细胞和补体系统的弥漫性激活以及巨噬细胞和中性粒细胞大量浸润子宫体。我们在此关注紧邻身体的子宫峡部。峡部在整个怀孕期间显着拉长。它由肌细胞和成纤维细胞组成，细胞外基质在分娩过程中形成被动的下段。本研究的目的是对子宫体 PAM 病例的子宫峡部进行组织学检查。根据日本的羊水栓塞登记计划，我们从剖宫产子宫切除术获得的子宫样本中选择 PAM 病例，并在 2011 年至 2017 年期间交付给我们进行分析。在选择性剖宫产期间收集对照组织。我们研究了这些病例的峡部组织并对炎症细胞标志物进行了免疫组织化学，即中性粒细胞弹性蛋白酶、肥大细胞类胰蛋白酶、CD68、CD3 和 C5a 受体 (C5aR)。体内有 PAM 的子宫中，峡部中类胰蛋白酶阳性脱粒肥大细胞、弹性蛋白酶阳性中性粒细胞、CD68 阳性巨噬细胞和 C5aR 阳性细胞的数量显着高于没有 PAM 的对照组。两组的 CD3 均为阴性。总之，炎症和类过敏反应不仅在子宫体中被组织学检测到，