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Chapter 37Placenta Previa, Placenta Accreta,Abruptio Placentae, and Vasa Previa                                           ...
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  1. 1. Chapter 37Placenta Previa, Placenta Accreta,Abruptio Placentae, and Vasa Previa Andrew D. Hull, MD, and Robert Resnik, MDBleeding in the later stages of pregnancy has been described as “third- the cervix dilates or the lower uterine segment effaces. The term “low-trimester bleeding” or “antepartum hemorrhage.” Late pregnancy lying placenta” should be reserved for cases in which the placenta isbleeding is a significant cause of maternal and fetal morbidity, fetal seen on transabdominal ultrasound to extend into the lower uterinemortality, and preterm delivery. Traditional accounts of such bleeding segment but its precise limits have not been defined, and for caseshave addressed placenta previa, abruptio placentae, and vasa previa, identified before the third trimester. TVUS allows location of the pla-although in fact the only thing that these clinical problems have in centa in relation to the internal cervical os with great precision. Whencommon is that they all are concerned, to a greater or lesser extent, such studies are performed, the placenta may be classified as a completewith hemorrhage. The etiology, management, and complications of previa if it completely covers the internal os. The term marginal pla-each are quite distinct. In the past, uncertainty in precise diagnosis of centa previa should be used if the placental edge lies 2.5 cm or closerthe cause of late pregnancy bleeding has led to these conditions being to the internal os. It has been shown that when the placenta is moreconsidered together, but the universal availability of ultrasound tech- than 2 to 3 cm from the cervix, there is no increased risk of bleeding.2nology has eliminated much of the diagnostic dilemma. A definitive diagnosis of placenta previa should be avoided in asymp- Bleeding during the second half of pregnancy complicates about tomatic patients before the third trimester, because many cases of pla-6% of all pregnancies. Placenta previa is ultimately documented in 7% centa previa identified early in pregnancy will resolve as pregnancyof cases, and evidence of significant placental abruption is found in advances.13%. In the remaining 80% of cases, the bleeding can be ascribed to Placenta previa affects about 1 (0.5%) of every 200 pregnancies ateither early labor or local lesions of the lower genital tract or no source term.3 There is some evidence that the incidence of placenta previa iscan be identified.1 Faced with a woman with late pregnancy bleeding, increasing.4 This increase may be related to the increasing rate ofthe clinician must rapidly reach a firm diagnosis and management plan cesarean section observed in all developed countries. A single priorto ensure the optimum outcome for mother and baby. Ultrasonogra- cesarean section or a prior pregnancy complicated by placenta previaphy, electronic fetal monitoring, and, frequently, evaluation of the increases the incidence of placenta previa in a subsequent pregnancyfunction of the maternal coagulation system make up the foundation to as high as 5%,1,5,6 rising even further with a history of more prioron which both diagnosis and management are developed. Clinical cesarean deliveries.7 Advanced maternal age increases the incidence ofassessment must occur simultaneously with imaging and fetal placenta previa to 2% after 35 years of age and 5% after age 40.7 Mul-assessment. tiparity, prior suction curettage, and smoking are all associated with In asymptomatic patients who are without antenatal bleeding but higher risks of placenta previa.8-10 The relative risks for these associatedhave been identified by prenatal ultrasound as having risk factors (pla- factors are summarized in Table 37-1.centa previa, placenta accreta, or vasa previa), timing of delivery is themost important clinical decision that has to be made. Pathogenesis The underlying cause of placenta previa is unknown. There is a clear association between placental implantation in the lower uterinePlacenta Previa segment and prior endometrial damage and uterine scarring from curettage, surgical insult, prior placenta previa, and multiple priorDefinition and Epidemiology pregnancies.Advances in the precision of sonographic diagnosis, particularly trans- At least 90% of placentas identified as being “low lying” in earlyvaginal ultrasound (TVUS) technology, as well as an increased under- pregnancy will ultimately resolve by the third trimester.11 The termstanding of the changing relationship between the placenta and the “placental migration” is widely used to describe this phenomenon. Theinternal cervical os as pregnancy advances, have rendered traditional placenta clearly does not move; rather, it is likely that the placentadefinitions and classifications of placenta previa obsolete. Placenta grows toward the better blood supply at the fundus, a process knownprevia exists when the placenta covers the cervix either completely or as trophotropism, leaving the distal portions of the placenta, closer topartially or extends close enough to the cervix to cause bleeding when the relatively poor blood supply of the lower segment, to regress and
  2. 2. 726 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previaatrophy. As the uterus grows and expands to accommodate the devel- onset of labor. Some women experience pain secondary to uterineoping fetus, there is differential growth of the lower segment, and this contractions. Bleeding may be provoked by labor, examination, ormay further increase the distance between the lower edge of the pla- intercourse but it usually has no identifiable precipitating cause. Thecenta and the cervix. patient is more likely to have a fetus with an abnormal lie, inasmuch Bleeding from placenta previa may occur before labor as a result of as the placenta previa may prevent the fetus from establishing normaldevelopment of the lower uterine segment and effacement of the cervix polarity. All women presenting with painless vaginal bleeding after 20with advancing gestation. Prelabor uterine contractions may also weeks’ gestation should be assumed to have a placenta previa untilproduce bleeding, as may intercourse or injudicious vaginal examina- proven otherwise. Transabdominal ultrasound should be quickly uti-tion. Once labor begins, significant bleeding will occur as the cervix lized to screen for placenta previa. Unless the placenta is clearly fundaldilates and the placenta is forced to separate from the underlying and the lower segment is clear, TVUS should then be performed.decidua. Transabdominal ultrasound has been shown to be inferior to TVUS for definitive placental localization.12,13 Concerns regarding the poten- tial for TVUS to provoke bleeding are unfounded, and several studiesDiagnosis have confirmed the safety of a careful TVUS approach (Fig. 37-1).7,13,14The classic history for placenta previa is that of painless third-trimester The placement of the transvaginal probe should be observed continu-bleeding. Several small “herald bleeds” may occur in advance of major ously on the ultrasound monitor during insertion, to avoid placing thehemorrhage, but in up to 10% of cases there is no bleeding until the probe into a potentially dilated cervix. If a transvaginal probe is unavailable, translabial imaging using a regular abdominal probe can produce excellent results, with better visualization of the relationship TABLE 37-1 RISK FACTORS AND RELATIVE between the cervix and placenta than is obtained from transabdominal scanning.15 A digital or speculum examination to inspect the cervix for RISKS OF PLACENTA PREVIA local causes of bleeding should not be performed until placenta previa Risk Factor Increased Risk Reference has been excluded by ultrasonography. In the unusual setting of significant late pregnancy bleeding where Previous placenta 83× Monica and Lilja, 1995 ultrasound is not available and the diagnosis is not clear, there is still previa a place for the “double-setup” examination. The patient is taken to the Previous cesarean 1.5-153× Herschkowitz et al, 1995; section Hemminki and operating room, where preparations are made for a cesarean delivery. Merilainen, 1996 A vaginal examination is then performed, beginning in the vaginal Previous suction 1.33× Taylor et al, 1994 fornices and avoiding placing the fingers directly in the cervix. If a curettage for placenta previa is detected, cesarean section is then performed. If no abortion placenta previa is found, a search for other causes of third-trimester Age > 35 yr 4.73× Iyasu et al, 1993 bleeding ensues. Age > 40 yr 93× Ananth et al, 1996 Multiparity 1.1-1.73× Williams and Mittendorf, Implications of Early Pregnancy Diagnosis 1993 The routine use of ultrasonography in the first and second trimesters Nonwhite race (all) 0.33× Iyasu et al, 1993 of pregnancy has led to the frequent observation of a low-lying pla- Asian race 1.93× Iyasu et al, 1993 Cigarette smoking 1.4-33× Handler et al, 1994; centa or a previa. Ananth, Savits, and Transabdominal ultrasound tends to over-diagnose low-lying pla- Luther, 1996; Chelmow centa, especially when the bladder is empty.16 Even with TVUS, the et al, 1996 findings may not correlate with the placental position at term. Several reports confirm that up to 10 times as many women are found to have B A FIGURE 37-1 Ultrasound study performed at 18 weeks’ gestation for fetal anatomy survey. A, Transabdominal ultrasound shows an apparent “low-lying placenta.” B, Transvaginal ultrasound shows that the placenta completely covers the cervix.
  3. 3. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 727a placenta previa in the second or first trimester than at delivery.14,17-21 the less likely it is that a significant prolongation of pregnancy will beThe earlier in pregnancy a diagnosis of placenta previa is made, the gained.26 Betamethasone to enhance fetal lung maturation should beless likely it is that the finding will persist at delivery.22 The likelihood administered to patients who are at less than 34 weeks’ gestation ifof persistence to term of a placenta previa found in the second trimes- expectant management is planned.ter is also related to the degree to which the placenta overlies the There is controversy regarding the role of tocolytics in the settingcervix14,17-21 and the thickness of the placental edge.23 It is recom- of hemorrhage from placenta previa. Both β-mimetics and magnesiummended that a follow-up ultrasound study be performed between 28 sulfate27-29 have been used in this setting and appear to be associatedand 32 weeks’ gestation to further evaluate the placental position. If with significant prolongation of pregnancy without adverse effects.there appears to be a significant change in the position of the placental After the initial presentation with bleeding, patients should remainedge over time, a final study should be done at 36 weeks, before making in hospital until they are free of bleeding for at least 48 hours. Somea final decision as to the appropriate route of delivery. may then be considered for home management. Several studies have addressed the issue of safety of outpatient management in a controlled setting at home.25,30-33 With the exception of one report of an increaseManagement in perinatal mortality and morbidity and earlier gestational age,34 itAny woman with vaginal bleeding after 20 weeks’ gestation should be appears to be a safe approach. Patients selected for home managementassessed on a labor and delivery unit. The primary focus should be on should be asymptomatic with regard to bleeding and abdominal pain,hemodynamic assessment of the mother and assessment of fetal well- be able to remain at home with limited activity, and have adequatebeing. Vital signs are obtained, and electronic fetal monitoring initi- support as well as adequate access to transport to a nearby hospital ifated. One or two large-gauge intravenous lines should be placed, and bleeding recurs. A second significant bleeding episode usually resultsmaternal blood should be sent for determination of the hematocrit in readmission until delivery.and type and screen. For substantial bleeding episodes, 2 to 4 units of Several strategies have been proposed to reduce the risk of hemor-blood should be cross-matched. Obstetric units might consider the use rhage in women with a known placenta previa. Bed rest, reducedof an “Obstetric Hemorrhage Protocol” to facilitate access to the activity, and avoidance of intercourse are commonly mandated andresources of the hospital blood bank for this and any other obstetric seem logical, although there is no conclusive evidence to support thesehemorrhage (Table 37-2). Rh immune globulin is administered, when measures. Cervical cerclage was evaluated in two small prospectiveappropriate, to Rh-negative, nonimmunized women. studies35,36 without clear benefit and is not recommended. Once the patient is stabilized and fetal condition has been assessed, All women whose placenta lies within 2 cm of the cervix, as docu-the definitive cause of the bleeding can be addressed. If the diagnosis mented by a late third-trimester TVUS scan, should be delivered byis clearly placenta previa and the patient is at or beyond 36 weeks’ cesarean section.37-39 An asymptomatic woman whose placenta liesgestation, delivery is appropriate. If bleeding is excessive or continues, more than 2 cm from the cervical os can be allowed to labor safely.16or if there are concerns about the condition of the fetus, the patient It should be noted that the presence of a low-lying placenta, even if itshould be delivered regardless of gestational age. In all other cases, does not cause intrapartum bleeding, increases the risk of postpartummanagement may be conservative and has been shown to be safe,24,25 hemorrhage because of lower uterine segment atony.with prolongation of pregnancy by an average of 4 weeks after the Cesarean section for placenta previa should be performed by theinitial bleeding episode. The closer it is to a gestational age of 36 weeks, most experienced team available because of the substantial risk of intraoperative hemorrhage.40 In most instances, a lower uterine segment incision is appropriate. If the placenta is anterior, it is neces- sary to clamp the umbilical cord immediately to prevent excessive TABLE 37-2 OBSTETRIC HEMORRHAGE blood loss caused by disruption of the placenta during entry. A vertical PROTOCOL incision is also reasonable in such cases and may be preferable if the Blood is immediately drawn and set up for fetus is premature or if a transverse lie exists.41 Postpartum hemor- Type and cross-matching rhage may occur from the placental implantation site secondary to Hematocrit atony and may require the use of additional pharmacologic agents to Coagulation studies (PT/PTT/fibrinogen) control blood loss, such as methylergonovine maleate (Methergine), Wall clot (blood is drawn into a plain tube and set aside—should 15-methyl prostaglandin F2α (Hemabate), and high-dose oxytocin, clot within 6 min) used either singly or in combination. The B-Lynch suture42 or local An ABG determination may be requested to assess acute blood suturing of the placental bed may be needed to control bleeding. In loss (typically, every increment in base deficit of −1 to −2 rare cases of refractory hemorrhage, hysterectomy may be required. requires 1 unit of PRBCs to correct it) Among women known to have a placenta previa who do not require Four units of type-specific or O-negative blood are made very early delivery, elective delivery should be performed before sig- immediately available. The laboratory immediately starts to cross-match 4 units of blood nificant bleeding has occurred. It is reasonable to plan on delivery at and stays 4 units ahead of blood use. or just after 36 weeks’ gestation, because there is little fetal advantage Two units of FFP are thawed and made available. after that time, when weighed against the risk of a sudden and possibly One 10-pack of platelets is made available. excessive bleeding episode. The alternative is to perform an amniocen- The blood bank is alerted to provide further units of blood, FFP, tesis to confirm lung maturity before delivery, but the risk of hemor- and platelets as needed. rhage with delayed delivery usually outweighs the risk of fetal lung Further samples for ABGs and other laboratory studies are drawn immaturity at that gestational age. as required. The selection of anesthesia to be used for cesarean section in cases ABGs, arterial blood gases; FFP, fresh-frozen plasma; PRBCs, packed of placenta previa should be decided by the obstetrician and anesthe- red blood cells; PT, prothrombin time; PTT, partial thromboplastin siologist involved with the delivery, in concert with the patient. In the time. United Kingdom, regional anesthesia was preferred by most obstetric
  4. 4. 728 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previaanesthesiologists in a survey43 and was used in 60% of cases in a ret- with placenta previa and prior uterine surgery.46,47 In patients withrospective series.44 Regional anesthesia is associated with lower opera- placenta previa, the risk of accreta is 10% to 25% with one prior cesar-tive blood loss and less need for transfusion than general anesthesia,3,45 ean section and exceeds 50% with two or more prior cesareans.48-50probably because many inhaled anesthetics cause uterine relaxation. Prevalence appears to be similar in women with these risk factors undergoing second-trimester pregnancy termination.51 The diagnosis of placental invasion of the myometrium usually canComplications be made by ultrasound,52-54 with a reported sensitivity and specificity for the diagnosis of approximately 0.8 and 0.95, respectively.53,55,56 Mag-Placenta Accreta netic resonance imaging (MRI) has also been used to confirm theOne of the most serious complications of placenta previa is the devel- diagnosis or better delineate the presence or extent of accreta.53 MRIopment of placenta accreta. This condition involves trophoblastic inva- is also useful in the presence of a posterior placenta and in the assess-sion beyond the normal boundary established by the Nitabuch fibrinoid ment of deep myometrial, parametrial, and bladder involvement.55,57layer. If invasion extends into the myometrium, the term placenta The ultrasound appearance of a normal placental attachment siteincreta is used; placental invasion beyond the uterine serosa (at times is shown in Figure 37-3A. Normal attachment is characterized by ainvolving the bladder or other pelvic organs and vessels) is termed homogeneous appearance of the placenta and a hypoechoic boundaryplacenta percreta. Histologic examples of normal placental implanta-tion and placenta accreta are shown in Figure 37-2. Placenta accreta is associated positively with advanced maternalage, smoking, and parity, but the strongest recognized association is B P C P A B P C B FIGURE 37-3 Ultrasound appearance of a normal placental attachment site and placenta accreta. A, Normal placental attachment in an anterior placenta previa. A hypoechoic areaFIGURE 37-2 Histologic appearance of normal placental separates the bladder wall and the placental tissue, representingimplantation and placenta accreta. A, Histologic section of a myometrium and myometrial vasculature. B, Characteristic ultrasoundnormal placental attachment site. Trophoblastic tissue with anchoring appearance of placenta accreta. Note the lack of a hypoechoic area,villi encroach but do not go through the Nitabuch membrane. as well as obliteration of the well-delineated bladder wall. In addition,B, Representative histologic section of placenta accreta, there are intraplacental sonolucent spaces (arrows) adjacent to thedemonstrating invasion of trophoblasts into the myometrial tissue. involved uterine wall. B, bladder; C, cervix; P, placenta.
  5. 5. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 729between the placenta and the bladder that represents the myometriumand the normal retroplacental myometrial vasculature. The bladderwall is intact throughout. In contrast, placental accreta is associated Vasa Previawith loss of the normal hypoechoic boundary, and there are usuallyintraplacental sonolucent spaces adjacent to the involved uterine wall Definition and Epidemiology(see Fig. 37-3B). Color-flow and power Doppler sonography have also Vasa previa is a rare but potentially catastrophic complication in whichbeen reported to facilitate the diagnosis.58-60 Chou and associates60 fetal vessels run through the fetal membranes and are at risk of ruptureevaluated 80 women with placenta previa to determine the accuracy with consequent fetal exsanguination. It is estimated that vasa previaof color-flow Doppler ultrasonography in distinguishing between affects between 1 in 1275 and 1 in 8333 pregnancies.73,74uncomplicated placenta previa and placenta accreta. Using their crite-ria, the antepartum diagnosis of accreta was made in 16 of the 80women studied and was confirmed histopathologically in 14. The sen- Pathogenesissitivity and specificity for diagnosis were 0.82 and 0.97, respectively. Vasa previa may occur because the insertion of the umbilical cord intoAlthough it is clear that larger numbers of patients must be studied by the placenta is velamentous, with the umbilical vessels coursing throughthese various modalities to more accurately determine the sensitivities the fetal membranes before inserting into the placental disk and theand specificities of diagnosis, various types of ultrasonography and unsupported vessels then overlying the cervix. It may also result fromMRI appear to hold promise in making or excluding the diagnosis in the presence of a bilobed or succenturiate placenta with the vesselsmost cases. connecting the placenta similarly overlying the cervix.16 If the condi- It is important, if at all possible, to make the diagnosis before deliv- tion goes unrecognized, it is associated with a fetal mortality rate ofery, because intraoperative hemorrhage can be massive, and placenta almost 60%. In addition to a succenturiate placenta75 and velamentousaccreta has been reported to be the most common indication for emer- insertion, other risk factors include a low-lying placenta observed ingency peripartum hysterectomy.61,62 In an effort to diminish blood loss, the second trimester,76 multiple gestation, and in vitro fertilization.77it is recommended that delivery be accomplished through a fundalincision followed by clamping of the cord. The placenta is allowed toremain in situ while the surgeons proceed to a total abdominal hyster- Diagnosisectomy. This may require very complex surgical technique and plan- The key to reducing fetal loss from vasa previa is prenatal diagnosis.16ning, and a pelvic surgeon capable of wide resection of the lower Many cases of vasa previa are identified only at the time of vesseluterine segment and parametrial areas should be available, as well as rupture in labor. Vaginal bleeding is followed by fetal distress and deathample transfusion capability. if emergent delivery cannot be effected in time. Because the entire fetal Although published reports are not extensive, it has been suggested cardiac output passes through the cord, it can take less than 10 minutesthat balloon occlusion of the aorta or internal iliac vessels may help to for total exsanguination to occur. Electronic fetal monitoring mayprevent excessive blood loss during resection of the lower uterine show an initial tachycardia, rapidly followed by decelerations, brady-segment. This involves preoperative placement of balloon-tipped cath- cardia, and a preterminal sinusoidal rhythm.78 If a cesarean deliveryeters retrograde through the femoral arteries immediately before can be accomplished immediately and with sufficient rapidity, goodsurgery. The catheters are guided under fluoroscopic direction into the newborn outcome can be obtained by aggressive postnatal transfusioninternal iliac arteries and inflated during the dissection. However, the therapy.79value and safety of this approach have been challenged with the recent It has been suggested that the blood from the vagina may be testedreports of no proven benefit and embolic complications.63,64 Neverthe- to confirm its fetal origin, using the Apt or Kleihauer-Betke tests orless, the placement of catheters does provide the opportunity to manage electrophoresis.80 In actual practice, such tests are either unavailable orpotential postoperative bleeding with angiographic embolization cannot be done quickly enough to be of any value. Occasionally, fetalrather than reexploration. vessels have reportedly been felt through the membranes during Conservative management may be an option if there is suspicion vaginal examination or visualized on amnioscopy; such observationsof a small focal accreta, if there is a fundal location after a myomectomy are really only of historical interest in modern practice.or classic cesarean section, or with a posteriorly implanted placenta. A It is now well established that vasa previa may be diagnosed prena-few reports have suggested leaving the uterus and placenta in situ and tally using ultrasound.81,82 Routine obstetric ultrasound should includeusing methotrexate postoperatively.65-67 However, the numbers of an assessment of the placental site and number of placental lobes andreported cases are very few, and hemorrhagic and infectious complica- an evaluation of the placental cord insertion site. In all cases in whichtions have usually resulted. Consequently, a definitive surgical approach a multilobed or succenturiate placenta or a low-lying placenta or vela-to this serious obstetric complication is strongly recommended. mentous cord insertion is identified using transabdominal ultrasound, a detailed examination of the lower uterine segment and cervix shouldNeonatal Complications be performed using TVUS. Gray-scale ultrasound can identify placen-It has been suggested that repetitive bleeding from placenta previa is tal cord insertion in most cases, but color or power Doppler makes theassociated with fetal growth impairment,68 although this has been dis- process easier and should be used (Figs. 37-4 and 37-5).81-84 There haveputed.26 Pregnancies complicated by placenta previa have also been been several studies evaluating this approach for prenatal detection ofreported to be associated with higher rates of fetal anomalies,69 neuro- vasa previa,81,82,84,85 all of which showed high specificity and sensitivitydevelopmental delay,70 and sudden infant death syndrome (SIDS).71 of detection with little impact on the length of scan time. More impor-The reasons for these findings are unknown. tantly, in cases in which vasa previa was detected prenatally, there were As might be expected, placenta previa and previa accreta are a cause no fetal deaths from the condition. A recent retrospective, multicenterof preterm birth due to the need for iatrogenic preterm delivery. study showed newborn survival rates of 97% in prenatally detectedAccreta has also been reported to have a negative influence on fetal cases of vasa previa and a fetal loss rate of 56% in cases not identifiedgrowth (odds ratio, 5.05 compared with controls).72 before the commencement of labor.86 Newer imaging modalities such
  6. 6. 730 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa B A D C FIGURE 37-4 Vasa previa identified at 18 weeks’ gestation on routine ultrasound studies. A, Transabdominal power Doppler identifies the umbilical cord possibly overlying the cervix. B, A fetal arterial waveform using power and pulse-wave Doppler. C, Vasa previa in gray scale on transvaginal ultrasound. D, Confirmation of the vasa previa using color Doppler transvaginally. because the vessels may be compressed by the presenting part and thus difficult to visualize. Management There is no uniformity of opinion as to the optimal management strategy for pregnancies with a prenatally diagnosed vasa previa, par- ticularly in regard to the timing of elective delivery. It has been sug- gested that patients be hospitalized at 30 to 32 weeks and delivered at 35 to 36 weeks’ gestation without confirmation of lung maturity by amniocentesis.16 This approach is based on the 10% risk of membrane rupture before labor and the high associated fetal mortality rate. Our approach has been to assess cervical length weekly from at least 30 weeks using TVUS. If the cervix is 2.5 cm in length or greater, out-of-FIGURE 37-5 Vasa previa. The placenta from the case identified in hospital management continues. The patient is administered beta-Figure 37-4. The patient was delivered by elective cesarean section at methasone just before 34 weeks’ gestation and is delivered by cesarean34 weeks. Arrow shows velamentous cord insertion. section between 34 and 35 weeks, without additional testing for fetal lung maturity.as MRI87 and three-dimensional ultrasound have been described in theevaluation of vasa previa.87-89 MRI is of little practical use in routine Prevention of Adverse Outcomescases. Although transvaginal three-dimensional power Doppler pro- A significant reduction in fetal mortality should be possible with avides an excellent means of visualizing the entire lower uterine segment diligent search as previously described. Public and professional aware-for the evaluation of vasa previa, similar information may be obtained ness has been heightened by such organizations as The Internationalby careful use of a two-dimensional vaginal probe. Such imaging com- Vasa Previa Foundation (http://www.vasaprevia.com [accessed Febru-bined with maternal positional change, the use of the Trendelenburg ary 8, 2008]). However, a high index of suspicion by the attendingposition, and gentle manual elevation of the fetal presenting part aid physician and a meticulous approach to diagnosis provide the bestin visualizing the fetal vessels. The latter technique is particularly useful opportunity for a favorable outcome.
  7. 7. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 731 of fetal nutrition. The process may be self-limited or ongoing with further dissection of the decidua. Dissection can lead to external bleed-Abruptio Placentae ing if it reaches the placental edge and tracks down between the fetal membranes; circumferential dissection leading to near-total separationDefinition and Epidemiology of the placenta can occur, particularly with concealed abruption. TheAbruptio placentae is the premature separation of a normally sited underlying event in many cases of abruption is thought to be vaso-placenta before birth, after 20 weeks’ gestation. It is a particularly haz- spasm of abnormal maternal arterioles. Some cases may result fromardous condition associated with significant maternal and fetal mor- venous hemorrhage into areas of the decidua that have become necroticbidity and mortality. About 1% of all pregnancies are complicated by secondary to thrombosis. Long-standing predisposition to abruptionclinically recognized abruption.90-93 The degree of abruption ranges may be inferred from the finding that women destined to suffer abrup-across a broad clinical spectrum, from minor degrees of placental tion have low levels of pregnancy-associated plasma protein A (PAPP-separation, with little effect on maternal or fetal outcome, to major A).105 Evidence of preexisting placental pathology in women withabruption associated with fetal death and maternal morbidity. Abrup- abruption includes poor trophoblastic invasion,106 inadequate remod-tion sufficient to cause fetal death occurs in about 1 of every 420 eling of the uterine circulation as reflected by abnormal uterine arterydeliveries.94 If placentas are routinely examined after delivery, evidence Doppler flow,107 and the well-established associations among pre-of abruption may be found in almost 4% of cases, most of which were eclampsia, IUGR, and abruption—all of which may be regarded asunrecognized and of no apparent clinical consequence. There has been primary placental disorders. Abruption may also occur secondary toan increase of almost 25% in the rate of clinically detected abruption acute shearing forces affecting the placenta-decidua interface, such asin the United States in recent decades, with a disproportionate increase those that occur with trauma—particularly rapid deceleration injuriesseen among African-American women.95 (motor vehicle accidents) and the sudden decompression of an over- The incidence of abruption peaks between 24 and 26 weeks’ gesta- distended uterus that occurs with membrane rupture in polyhydram-tion.96 Approximately 10% of all preterm births occur because of nios or delivery of a multiple gestation.abruption,90 and the infant outcomes are associated with increased As the abruption process continues, loss of placental functionrates of perinatal asphyxia,97 intraventricular hemorrhage, periven- results in fetal hypoxia and may end in fetal death. The acute hemor-tricular leukomalacia,98 and cerebral palsy99 when compared with ges- rhage activates the coagulation cascade, and, with ongoing bleeding,tational age–matched controls. Perinatal mortality in pregnancies disseminated intravascular coagulation (DIC) may result. Continuedcomplicated by abruption may be declining overall,93,100 but the rate bleeding, with maternal hypovolemia and poor tissue perfusion, aggra-continues to be higher than in gestational age–matched controls vates the DIC and results in a downward spiral into hemorrhagicwithout abruption.96 Placental separation is strongly associated with shock. Bleeding into the myometrial tissue can lead to a Couvelairepreterm premature rupture of the membranes (pPROM), in both a uterus which becomes atonic and increases the risk of uterine hemor-causal and a consequential manner.101 Most pregnancies complicated rhage after delivery.by abruption result in the delivery of an infant weighing less than the10th percentile for gestational age,102-104 suggesting a common pathway Risk Factors and Associations for Abruptionlinking abruption to placental dysfunction and intrauterine growth The most important risk factor for abruption is a history of abruptionretardation (IUGR). in a prior pregnancy.108 One meta-analysis showed an increase of up to 20-fold in the risk of abruption if a prior pregnancy had been simi- larly affected.109 With two prior pregnancies complicated by abruption,Pathogenesis the risk of recurrence is 25%.94Abruption results from bleeding between the decidua and placenta Maternal hypertension is also a significant risk factor for abruption.(Fig. 37-6). The hemorrhage dissects the decidua apart, with loss of Chronic hypertension is associated with a fivefold increase in risk,the corresponding placental area for gaseous exchange and provision which rises to eightfold with superimposed preeclampsia.96 Preeclamp- sia alone is also strongly linked to abruption and to the severity of abruption.110 It seems plausible that preeclampsia and abruption share many underlying pathologic mechanisms. Perhaps the most readily preventable risk factor for abruption is cigarette smoking. Cigarette smokers are up to 2.5 times more likely to have an abruption than nonsmokers,111-115 and they have twice the perinatal mortality of nonsmokers.116 There is a dose-response rela- tionship between the number of cigarettes smoked and the risk of abruption.115,117,118 Even women who stop smoking before pregnancy are at increased risk. Substance abuse is closely linked to abruption— any agent that causes vasospasm or transient severe hypertension may be causative.119,120 In the United States, as many as 10% of pregnant women who are cocaine and crack cocaine users will experience pla- cental abruption.121 Multiparity is also positively correlated with a small increase in the risk of abruption.1,102 The apparent association between maternal age and abruption is not significant when parity is taken into account. There has been substantial recent interest in the possible associationFIGURE 37-6 Abruptio placentae. A large retroplacental abruption between thrombophilic disorders and abruption. Some retrospectiveat 30 weeks’ gestation is shown. studies of abruption have found increased rates of thrombophilia.122,123
  8. 8. 732 CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa PreviaHowever, both retrospective124 and prospective case-control studies125 hemorrhage. Fetal compromise is a common finding, and if more thanof women with the factor V Leiden mutation showed no increase in 50% of the placenta is involved, fetal death is likely. Massive concealedabruption risk. It is established that hyperhomocysteinemia is associ- abruption often manifests with severe pain, a hard uterus, and a deadated with abruption,123 although, in the absence of hyperhomocystein- fetus; such a picture may occur in association with severe preeclampsiaemia, the specific MTHFR gene mutations themselves do not appear or the recent use of a vasoactive drug such as cocaine.139 If abruptionto be associated with an increased risk.126,127 occurs in a posteriorly located placenta, severe back pain may be the Motor vehicle accidents are the most common traumatic event only symptom; it may be worsened by abdominal palpation that pushesassociated with abruption, and clinical evidence of abruption may not the fetus against the placenta. Abruption may precipitate pretermbe apparent for 24 hours or longer after the trauma. Women with labor, and it should always be considered in the differential diagnosisvaginal bleeding or contractions after a motor vehicle accident should for a patient in apparent idiopathic preterm labor.be observed for at least 24 hours; those who are asymptomatic can Although ultrasonography is an integral part of the diagnosticsafely be discharged after 6 hours of monitoring.128,129 approach to late pregnancy bleeding, its utility is primarily for the Membrane rupture may precede or follow chronic retroplacental exclusion of placenta previa as the cause of hemorrhage. At least 50%bleeding or an acute abruption, and women with ruptured membranes of abruptions produce no findings on ultrasound.140-142 What is visual-should be monitored carefully for this possibility.109,130 Those with ized by ultrasound depends on the site, scale, and timing of bleeding.early pregnancy bleeding who have a subchorionic hematoma visible In early acute abruptions, blood and clot retained within the uteruson ultrasound are also at increased risk for both pPROM and appear as hyperechoic or isoechoic collections relative to placentalabruption.131 echogenicity.143 In cases that remain undelivered, the hematomas Screening tests performed for other indications may identify groups resolve over several weeks, becoming hypoechoic and then sonolucent,of women who are at increased risk for abruption but may have no usually by 2 weeks after the event.143 Intrauterine clot may “jiggle”other high-risk factors. These tests include maternal first- and second- when bounced by the transducer—the “jello” sign. An acute abruptiontrimester serum screening for aneuploidy. Women with PAPP-A levels with obvious vaginal bleeding, in which little or no blood is retainedbelow the 5th percentile at the time of first-trimester serum screening within the uterus, may have no specific sonographic findings. There-for trisomy 21 have an increased risk of abruption,105 but low levels of fore, the absence of ultrasound findings never excludes an abruption.human chorionic gonadotropin (hCG) in the first trimester are not Cardiotocography is an integral part of the evaluation for late preg-similarly associated.132 In one study of routine uterine artery Doppler nancy bleeding. Abruption is commonly accompanied by uterine con-velocimetry performed at 11 to 14 weeks’ gestation as a screen for tractions that may not be appreciated clinically, particularly afterIUGR and preeclampsia, a pulsatility index higher than the 95th per- trauma.144 Fetal heart rate tracings may exhibit a variety of abnormalcentile or a PAPP-A value lower than the 10th percentile predicted 43% patterns, including variable and late decelerations, poor variability,of pregnancies with a subsequent abruption.133 In an earlier study of prolonged bradycardia, or a sinusoidal pattern; these are not specificuterine artery Doppler ultrasound, persistent notching of the wave- to abruption and reflect underlying evolving fetal asphyxia.form after 24 weeks was associated with increased risk for abruption Kleihauer-Betke testing is of no diagnostic value in abruption: Itas well as IUGR and preeclampsia.107 may be negative with proven abruption128,129,145 or positive when no Women with otherwise unexplained elevated serum levels (>2 mul- abruption has occurred. Its only value in this setting is to guide Rhtiples of the median [MOM]) of α-fetoprotein (AFP) on second-tri- immune globulin dosing in Rh-negative women who are thought tomester serum screening for trisomy 21 have long been thought to be have sustained an abruption.at increased risk for a wide range of adverse pregnancy outcomes, Chronic abruption-oligohydramnios sequence (CAOS) is a termincluding abruption.134,135 However, a recent case-matched, prospective that was coined to describe women who present with bleeding attrib-study found elevated AFP levels to be associated with an increased risk uted to abruption and go on to develop oligohydramnios withoutof abruption but no increase in the frequency of IUGR, preterm deliv- evidence of ruptured membranes.146 Twenty-four patients wereery, low birth weight, or fetal death.136 A recent attempt to establish a described, all of whom delivered preterm (average gestational age, 28critical cutoff value for elevated AFP and increased risk of abruption weeks). For the most part, the earlier the onset of bleeding, the earlierstressed the low specificity and high false-positive rate of the test.136 the delivery. More than half of the women went on to develop pPROM Elevated hCG values at the time of second-trimester serum screen- before delivery and after the development of oligohydramnios.ing have similarly been associated with adverse pregnancy outcome,including abruption.137 Previously, a value greater than 2.0 MOM wasthought to be significant in this context, but one recent case-controlled Managementstudy showed that the threshold should be set at 3.0 MOM. Even at The key to optimizing maternal and fetal outcome in abruptio placen-that level, a positive test had poor predictive value and was not associ- tae is the individualization of care. Precise management depends onated with increased risk of abruption.138 Abnormal inhibin values do the extent of maternal and/or fetal compromise and the gestationalnot appear to be predictive of abruption. age. Decision making should be rapid but methodical; delay in diag- nosis and inappropriate triage leads to significantly increased perinatal mortality.147 Twenty percent of all fetal deaths from abruption occurDiagnosis after presentation to the hospital, and 30% of those deaths occurThe diagnosis of placental abruption is made based on clinical find- within 2 hours after admission.ings. The classic presentation is that of vaginal bleeding, usually Initial assessment should focus on maternal hemodynamic statusaccompanied by abdominal (uterine) pain. Examination often reveals (remembering that blood pressure may be elevated in the setting ofuterine tenderness, and contractions may be present. About 10% of preeclampsia) and fetal well-being. Maternal vital signs should be mea-abruptions are concealed, with no vaginal bleeding. If bleeding is sured frequently, because they may change suddenly as the abruptionpresent, the amount is often a poor guide to the degree of separation, evolves. Electronic fetal monitoring should begin immediately and bebecause there is usually a mixed picture of apparent and concealed continuous throughout further assessment and management. A large-
  9. 9. CHAPTER 37 Placenta Previa, Placenta Accreta, Abruptio Placentae, and Vasa Previa 733gauge intravenous line should be placed (two lines if the patient is If the patient has sustained a mild separation at a premature gesta-hemodynamically unstable). Initial laboratory studies should include tional age and is asymptomatic without evidence of bleeding, dischargea baseline complete blood count and platelet count, type and screen home may be considered as an alternative to prolonged hospitalization.and cross-match where appropriate, blood urea nitrogen and electro- Either way, a clear management plan for delivery should be developedlytes, coagulation studies, and a wall clot. These studies serve as useful based on subsequent events or the reaching of an arbitrary gestationalbaseline references. An indwelling bladder catheter should be placed to age (usually 37 weeks). The evaluation of patients undergoing expect-allow urinary output to be closely monitored. In unstable or critically ant management should include regular assessment of fetal growthill patients, management may be aided by placement of a central and tests of well-being, because these fetuses are at increased risk forvenous pressure line (preferably with a Cordis introducer) or an arte- IUGR.rial line. The involvement of the obstetric anesthetic team should be If an abruption occurs after 34 weeks’ gestation and maternalsought early. and fetal condition permit, vaginal delivery is preferred. Amniotomy After these steps are taken, attention should be directed at excluding should be performed, and, if needed, an oxytocin infusion shoulda placenta previa (by ultrasound examination) and deciding on the be started. Labor usually progresses rapidly, even without augmenta-timing and route of delivery. Maternal or fetal compromise mandates tion. However, if progress is slow or maternal or fetal status deterio-immediate delivery, usually by cesarean section unless the patient is in rates, cesarean section should be performed. If abruption has resultedan advanced stage of labor. If the event occurs after 34 weeks’ gestation, in fetal death, vaginal delivery is preferred unless there are otherdelivery should not be delayed, because the risks of conservative man- obstetric contraindications or the mother is hemodynamicallyagement outweigh any considerations of prematurity of the fetus. unstable.Between 20 and 34 weeks, if mother and fetus are stable, an attempt at Coagulopathy develops in about 10% of abruptions. It usually isconservative management may be considered.148,149 Betamethasone related to the severity of the event and is particularly likely to occur ifshould be administered to enhance fetal lung maturity in all such cases. there is fetal demise or massive hemorrhage. An aggressive approachThe patient should be monitored closely, because she continues to be should be used to maintain maternal blood volume and oxygen-at risk of an evolving process. The use of tocolytics is controversial; in carrying capacity, including the use of component therapy (fresh-most cases, they should not be used. Although studies addressing this frozen plasma and platelets). The coagulation tests most frequentlyissue have found no increase in adverse fetal or maternal events,27,141,148-150 used, and the component replacement therapy for women with DIC,no prospective trial has been performed. are summarized in Tables 37-3 and 37-4. TABLE 37-3 COAGULATION TESTS USED IN THE DIAGNOSIS OF ABRUPTIO PLACENTAE Test What It Measures Normal Value Value in Abruption Bleeding time Vascular integrity and platelet 1-5 min Usually normal; test is of little clinical use in function diagnosing abruption Whole blood clotting time Platelet function Clot formation: 4-8 min Clot formation abnormality indicates severe Fibrinolytic activity Clot retraction: <1 hr deficiency Clot lysis: none in 24 hr Abnormal retraction with thrombocytopenia Fibrinogen Fibrinogen level 400-650 mg/100 mL Usually decreased Platelet count Number of platelets >140,000/mm3 Usually decreased Fibrin degradation products Fibrin and fibrinogen <10 μg/mL Almost always elevated; most sensitive test degradation products Euglobulin clot lysis time Fibrinolytic activity None in 2 hr Difficult to interpret with low fibrinogen levels Prothrombin time Factors II, V, VII, X 10-12 sec Normal to prolonged Partial thromboplastin time Factors II, V, XIII, IX, X, XI 24-38 sec Normal to prolonged Thrombin time Factors I, II 16-20 sec Parallels fall in fibrinogen; good marker of Circulating split products abruption severity Heparin effect Red blood cell morphology Microangiopathic hemolysis Absence of distortion Presence of distortion or fragmentation is or fragmentation uncommon but indicates risk of renal cortical necrosis TABLE 37-4 BLOOD REPLACEMENT PRODUCTS Component Volume per Unit (mL)* Factors Present Effect of 1 Unit Fresh whole blood 500 RBCs; all procoagulants ↑ Hematocrit 3% Packed RBCs 200 RBCs only ↑ Hematocrit 3% Fresh-frozen plasma 200-400 All procoagulants; no platelets ↑ Fibrinogen 25 mg/dL Cryoprecipitate 20-50 Fibrinogen; factors VIII, XIII ↑ Fibrinogen 15-25% Platelet concentrate 35-60 Platelets; small amounts of fibrinogen; ↑ Platelet count approximately 8000/mm3 factors V, VIII RBCs, red blood cells. *Volume depends on individual blood bank.
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