2 Uterovaginal Blood Supply: the S1 and S2 Segmental Concepts and their Clinical Relevance J. M. Palacios-Jaraquemada, M. Karoshi and L. G. Keith ORIGIN OF THE CONCEPT The S1 segment is supplied by ascending branches of the uterine artery and, to a lesser extent, by the Recent careful and precise anatomic studies in cadav- descending branches of the ovarian artery. Rarely, the ers demonstrate that the female reproductive system round ligament artery contributes to the collateral has two separate and completely distinguishable blood blood supply of the uterus. In contrast, the S2 segment supplies. These investigations confirm earlier observa- is supplied by branches of the uterine, cervical, upper tions1 that the blood supplies are anastomotic in vesical, vaginal and pudendal arteries. All of these blood nature2,3. The distinctive natures of the two areas and vessels are located subperitoneally. their differing blood supplies are shown in the sagittal A thorough understanding of the differences in the view of the pelvis as depicted in Figure 1. In practical blood supply to both uterine segments (S1 and S2) terms, the two areas can be distinguished by drawing a underlies the proper selection of a therapeutic inter- line perpendicular to the posterior wall of the bladder. vention, be it surgical or radiological, for the treatment As defined in the writings of Palacios-Jaraquemada, of postpartum hemorrhage (PPH). It is possible that who first used this terminology in 2005, the S1 the recent increases in maternal mortality in several segment comprises the body of the uterus2. In this countries are related to the changes in the rates of construct, the S2 segment corresponds to the lower PPH, with more severe bleeding related the increased uterine segment, cervix, upper part of the vagina and incidence of adherent placentas. the respective parametria. ANATOMIC EVIDENCE Figure 2 shows a fetal angiographic preparation which facilitates an understanding of the rich arterial anastomotic system between the left uterine artery and the corresponding left lower and middle vaginal arter- ies1. These arteries also anastomose with the descend- ing branches of the uterine artery on the ipsilateral side. In practical terms, this means that the whole of one half of the lower part of the uterus and the upper part of the vagina receives blood from an intercon- nected system. In this figure, the uterine artery and the middle left vaginal artery are branches of the anterior division of the internal iliac artery, whereas the lower left vaginal artery is a branch of the posterior division of the internal iliac artery. (Note: it could also arise from the internal pudendal artery, which in turn is a branch of the posterior division of internal iliac.) It is crucial to understand that the system illustrated in this fetal angiogram is fully maintained in the adult and is the system responsible for continuation of uter- ine blood supply when the uterine arteries are embolized. Figure 1 Sagittal line demarcating the S1 and S2 areas of blood The cross anastomoses between the right and the supply to the uterus and upper vagina at term. left uterine arteries over the uterine surface shown in 19 POSTPARTUM HEMORRHAGE Figure 3 are not evident to the naked eye in either the and 3 which derive from a fetus. First, the anatomy non-pregnant or the normal pregnant uterus. How- present in the fetus is present and persists through adult ever, in the presence of abnormal placentation, they become life. Second, the bilateral anastomotic flow pathways grossly enlarged and engorged, and are visible on the uterine between the vaginal and uterine arteries remain surface, especially in the lower segment. This is because the visible. Third, the side-to-side anastomoses between lower segment of pregnant uterus is stretched and both uterine arteries are present. This latter point has thinned. exceedingly practical implications, because if one is Figure 4 is a rare adult angiographic preparation going to perform stepwise devascularization in cases of major showing the features previously described in Figures 2 PPH, it must be bilateral (as is the case in bilateral uterine artery embolization) or the procedure will be less than effective as a result of the cross anastomoses. In contrast, vertical uterine compression sutures (B-Lynch, Hayman) act over both systems. REASONS FOR FAILURE OF SOME INTERVENTIONAL RADIOLOGICAL PROCEDURES Figure 4 demonstrates the anatomic basis for the failures of some uterine embolization procedures. It also provides an explanation of why non-target embolizations occur, especially in the bladder where they may cause necrosis. In the majority of instances, the interventional radiologists cannot explain these occurrences, because the embolization material has been injected into the uterine artery and the damage is in the area supplied by the vesical artery. Under normal circumstances, the connection between the uterine artery and the vesical artery is Figure 2 Fetal cadaveric angiographic preparation microscopic; however, in the presence of abnormal demonstrating the rich arterial anastomotic system between the left placentation and also because of the effect of vascular uterine artery and the left lower and middle vaginal arteries. LIIA, growth factors, these vessels enlarge and represent left internal iliac artery; LLVA, lower left vaginal artery; LUA, left neovascularization as they lack a tunica media uterine artery; MLVA, middle left vaginal artery; MRVA, middle which in turn allows them to become high flow low right vaginal artery; RLA, round ligament artery; ROA, right resistance reservoirs. ovarian artery; RUA, right uterine artery; UB, uterine body; VA, 3 When patients undergo interventional radiological vagina. From Palacios-Jaraquemada et al., 2007 , with permission procedures in situations involving abnormal placentation, it is important to be aware of the potential for abnormal connections between uterine and vesical arteries. If the embolization material is injected under pressure and if the particle size is small (usually less than 700 µm), then it is possible to have non-target embolization to the bladder which occasionally becomes necrotic (see Chapter 49 for further reading). Figure 3 Another view of the preparation shown in Figure 2 but demonstrating cross anastomoses between the right and the left uterine arteries over the uterine surface. LLVA, left lower vaginal artery; LUA, left uterine artery; RIIA, right internal iliac artery; Figure 4 Adult angiographic preparation which shows the RUA, right uterine artery; UB, uterine body; VA, vagina. From features previously described in Figures 2 and 3. From Rohen et al., Palacios-Jaraquemada et al., 20073, with permission 20035, with permission 20 Uterovaginal Blood Supply: the S1 and S2 Segmental Concepts and their Clinical Relevance The radiograph shown in Figure 5 illustrates the COMMON PERCEPTIONS AND MISPERCEPTIONS dense anastomotic system present between the vaginal Students of history are well aware that if one does not and uterine arteries. In this patient, the catheter was learn from the mistakes of the past, one is bound to placed at the level of the internal pudendal artery. The repeat them. The same is true in anatomy, where 18th contrast material was injected and passed as high as the century dissections provide important clues to the 21st uterus via intercommunicating anastomoses. century dilemmas. Figure 6, to the best of the senior This radiograph can also explain two common author’s (J.P.J.) knowledge, is the first illustration clinical scenarios. In the first, where a patient suffers depicting the relationship between the relative dia- severe PPH following normal vaginal delivery, the meters of the vaginal and uterine arteries. The com- interventional radiologist performs embolization of mon perception is that the diameter of the uterine uterine artery and a second embolization attempt is artery is larger than that of the vaginal artery. made at the anterior division of internal iliac arteries. As shown in Figure 6, however, this is not always Both may fail to produce significant reduction in the the case, and the French anatomist’s observation4 has intensity of bleeding. This failure can be explained by been duplicated by later authors using specialized dis- the presence of rich anastomoses between the internal section techniques3. In a series of 39 cadaver dissec- pudendal anastomotic branches (S2 segment) and tions, the first author demonstrated that the diameter the uterine artery (S1) and also by the coexisting of the inferior vaginal artery was 33% thicker than that uncorrected clotting abnormalities. of the uterine artery (Table 1). In the second clinical scenario, a traumatic instru- This important difference between the major blood sup- mental delivery results in deep vaginal tears or causes a plies of the S2 segment and the S1 segment should not be retroperitoneal hematoma. When the bleeding cannot be controlled by routine measures, the clinician often elects to perform hysterectomy or uterine artery embolization, either one of which has its main effect on the S1 segment. Both can fail, however, because the origin of the bleeding is from the vessels that arise in the S2 segment shown in Figure 5 (internal pudendal artery). Figure 6 Eighteenth century illustration of the relative diameters of the uterine and vaginal arteries, respectively. From Bourgery, 18334 Table 1 Lower uterine vascular supply. From Palacios-Jaraquemada et al., 20073, with permission Upper pedicle Uterine artery 100% from IIA Middle pedicle Cervical artery 67% from UA 23% from Vas 10% from LVeA Lower pedicle UVA: 18% from UA MVA: 11% from IIA LVA: 71% from PIA 75% as descending branch 25% as ascending branch Sectional diameters UA: 1.81 mm MaVA: 1.88 mm IIA, iliac internal artery; UA, uterine artery; Vas, vaginal arteries; LVeA, Figure 5 Angiographic illustration of the dense anastomotic lower vesical artery; UVA, upper vaginal artery; MVA, middle vaginal system between the vaginal and uterine arteries. From artery; LVA, lower vaginal artery; PIA, pudendal internal artery; MaVA, Palacios-Jaraquemada et al., 20073, with permission main vaginal artery 21 POSTPARTUM HEMORRHAGE underestimated.