Thus far, few reports have mentioned angiographic features of retained placenta in affected patients [5–7]. In those reports, angiographic features of retained placenta have included tortuous dilated uterine artery flowing into a sac-like structure, intrauterine vascular lesion with or without arteriovenous (AV) shunt, focal contrast blush, and pseudoaneurysm. In the present study, most patients (91%) showed a characteristic finding of “serpiginous blood sinus” in the mid-arterial to capillary phase. The blood sinus drained into the uterine veins in the capillary to venous phase. The placenta consists of the chorionic and basal plates, and the intervillous space lies between these two plates. The main stem villi, consisting of chorionic veins and arteries, project into the intervillous space. Maternal endometrial arteries and veins penetrate the basal plate; exchange between fetal and maternal circulatory systems occurs between the main stem villi and the maternal endometrial vessels in the intervillous space [8–11] (Fig. 3). In addition, uterine arteries and veins are presumed to exhibit arteriovenous anastomosis separate from this intervillous short-circuit [12]. The retained placenta consists of intervillous space and decidua basalis. In cases of retained placenta, various extents of remnant intervillous space and arteriovenous anastomosis of endometrial arteries/veins could remain in the uterine cavity (Fig. 4). The angiographic finding of serpiginous blood sinus may correspond to remnant intervillous space. Furthermore, endometrial arteries and veins connecting to the intervillous space may represent one or more low-flow AV shunts mimicking arteriovenous malformation (AVM)-like findings. Uterine AVM is rare, and it involves abnormal vascular channels in the endometrium or myometrium with early venous filling during the early arterial phase [13–15]. Retained placenta can be incorrectly diagnosed as AVM. However, angiography in our study showed venous drainage from the serpiginous blood sinus of retained placenta was evident in the capillary to venous phase. This angiographic finding of apparently delayed venous drainage may differentiate retained placenta from uterine AVM.
Regarding TAE for the treatment of retained placenta with bleeding, only a few case series have been reported. Bazeries et al. [5] reported that TAE technical and primary clinical successes, using mainly microspheres (size: 700–1200 µm), were achieved in 90.3% (27/31) and 74.2% (23/31) of their patients. Kimura et al. [7] reported higher rates of TAE technical and clinical success using GS (93%, 13/14; 100%, 14/14). NBCA embolization of retained placenta increta was described in a case report; complete occlusion and cure was achieved with single embolization [16]. In our study, TAE for retained placenta was performed using GS and/or NBCA, according to the operator’s preference, and favorable outcomes were achieved.
Retained placenta can spontaneously resolve with conservative management. Hence, asymptomatic patients with small and non-hypervascularized retained placenta may be candidates for conservative management [17–19]. In this study, 15 patients underwent expectant management after TAE. Among these 15 patients, eight showed markedly reduced residual vascularity of retained placenta. Images collected at the 1-month follow-up showed vascular lesion disappearance in the uterus, and there were no cases of recurrent bleeding that required any treatment. The safety of uterine artery embolization has been indicated for resolution of post-partum hemorrhage [20]. Common adverse effects related to the TAE procedure include high fever, acute pelvic inflammation, and hip pain [21, 22]. However, excessive embolization may cause serious complications, such as uterine necrosis and endometrial atrophy [23, 24]. As described previously, retained placenta has a regressive nature. Progressive occlusion of the blood sinus of retained placenta can occur after TAE. Therefore, excessive embolization should be avoided when the characteristic finding of retained placenta, serpiginous blood sinus, is identified during angiography examination. This information is important for interventional radiologists to determine the procedural endpoint of TAE for retained placenta with abnormal bleeding.
This study had several limitations including its retrospective nature, limited case number, and short follow-up period (mean, 3.4 months; range, 1–17 months). Larger prospective studies are needed to confirm the safety and efficacy of the TAE procedure as a monotherapeutic approach for retained placenta with abnormal bleeding.
In summary, the characteristic angiographic feature of retained placenta with vaginal bleeding is a serpiginous blood sinus fed by multiple uterine arterial branches in the arterial to capillary phase, which drains into the uterine vein in the capillary to venous phase. TAE using GS and/or NBCA can be a safe and effective treatment for management of abnormal bleeding caused by retained placenta.