Cover Story: Vasa previa: Are most perinatal deaths preventable?
Vasa previa: Are most perinatal deaths preventable?
By Yinka Oyelese, MBBS, MRCOG, Kerry M. Lewis, MD, and Joseph V. Collea, MD
A patient with vasa previa need not lose her baby if the condition is diagnosed before membranes rupture and elective C/S is performed. A protocol that uses TVS with color Doppler on all women at risk has the potential to eliminate vasa previa as a cause of perinatal mortality.
The sudden unanticipated death of a healthy fetus at term is certainly one of the most heartbreaking tragedies a womanand her obstetricianwill ever face. Fortunately, thanks to improved prenatal care, such events are now exceedingly rare in the United States. Despite advances in antepartum fetal assessment, however, a ruptured vasa previa is one of the few conditions that still threatens a fetus at term.1 But the deaths of up to nine of every 10 fetuses with vasa previa could be prevented if ob/gyns were to use ultrasound to diagnose the condition prenatally and deliver these babies by elective cesarean at 36 to 37 weeks.
First described in 1831, vasa previa is a condition in which the fetal blood vessels for some distance do not run through the umbilical cord where they belong, but instead run freely and unsupported through the membranes, over the cervix, and under the presenting part (Figures 1 and 2).2 In Type 1 vasa previa there is velamentous insertion of the cord, and in Type 2 the vessels run through the membranes between two lobes of a bi-lobed or succenturiate (accessory) lobed placenta (Figure 2).3
When the membranes rupturespontaneously or artificiallythese vessels often rupture as well, leading to rapid fetal blood loss.1 Because the fetal blood volume at term is only roughly 350 mL, the loss of relatively small volumes of blood can have disastrous consequences.1 What complicates matters further is the fact that clinicians often mistakenly assume that the blood loss is maternal and attribute it to a "bloody show," leading to delayed action, which increases the fetus' risk of dying.
The condition is rare, with an estimated prevalence of 1 in 2,500 pregnancies.1 Without prenatal diagnosis, it carries a perinatal mortality ranging from 70% to 90%. Assuming a prevalence of 1:2,500 pregnancies and a perinatal mortality rate of 50%, we estimate that approximately 1:5,000 babies will die from ruptured vasa previa. In 1979 one investigator stated that "An active obstetric service can expect to have one perinatal death each year due to vasa previa."4 The next year, another researcher summed up a world literature review of vasa previa by saying: "Regrettably, one must therefore conclude that in spite of advances in perinatal care, the prognosis for the fetus in vasa previa will remain very poor for the foreseeable future."5
Our goal here is to review the risk factors for vasa previa and provide a guide for managing this condition based on our own protocol. The cornerstone of our approach is screening and early diagnosis through transvaginal sonography (TVS) with color Doppler, followed by hospital admission in the third trimester and elective cesarean delivery at 36 to 37 weeks' gestation.
Assessing risk factors and making the diagnosis
A second-trimester low-lying placenta is a risk factor for vasa previa regardless of whether the placenta remains low-lying at term or not.6,7 Researchers who examined 100 placentas from in vitro fertilization (IVF) pregnancies found velamentous cord insertion in 14 of these, which is higher than the prevalence in the general population, even after correcting for the higher prevalence of velamentous insertion in multiple pregnancies.8 Other investigators recently found a much higher occurrence of vasa previa in IVF pregnancies than in spontaneously conceived pregnancies.9 Table 1 lists other risk factors.
Second-trimester low-lying placentas
Bi-lobed, succenturiate-lobed placentas
Pregnancies resulting from in vitro fertilization
Bleeding in pregnancy
Before the widespread use of U/S, vasa previa was often diagnosed only after the fetus died.1 The condition was suspected when vaginal bleeding occurred at the time of membrane rupture (Table 2). A sinusoidal fetal heart tracing associated with bleeding after rupture of the membranes is virtually pathognomonic for vasa previa (Figure 3).10 Rarely, (and often fortuitously), one can diagnose the condition in time by palpating pulsatile vessels in intact membranes through a dilated cervix, and perform cesarean delivery.1 One team of researchers, by performing routine amnioscopy on 500 women prior to amniotomy, was able to diagnose one case of vasa previa. 11 Because amnioscopy is not routinely performed, however, this method of diagnosis is mainly of historical interest. Furthermore, the methods we've described will diagnose the condition only during established labor. Since a significant proportion of women's membranes rupture before labor or in early labor, clearly ob/gyns must make the diagnosis before the onset of labor. Although it's also possible to diagnose vasa previa antepartum via magnetic resonance imaging (MRI), its expense and lack of ready accessibility limit its value. 12
Significant bleeding at the time of membrane rupture
Sinusoidal fetal heart tracing, especially associated with vaginal bleeding (Figure 3)
Fetal heart rate abnormalities associated with vaginal bleeding
Palpable vessels on vaginal examination
U/S is pivotal in making an early diagnosis
Ultrasound, first used to diagnose vasa previa in 1987, is probably the most powerful tool in reducing its associated high perinatal mortality rate.13 Thanks to the advent of TVS and color Doppler, it's now possible to confidently diagnose vasa previa prenatally. Following the first reported second-trimester diagnosis of vasa previa in 1996,14 many investigators have confirmed the value of U/S.3,6,7,15-18
Superiority of TVS. TVS has advantages over transabdominal sonography: The higher-frequency transducers and the proximity of the probe to the cervix yield superior images of the cervix and the region overlying it.
Be highly suspicious of vasa previa on gray-scale sonography whenever echolucent linear or tubular structures are present over the cervix (Figure 4). Granted, a cord presentation may appear similar. But you can differentiate the two by observing that the vasa previa remains in the same position with repeated sonographic examinations, whereas a cord presentation will move along with the mother as she changes her position. In particular, Trendelenburg positioning will move an umbilical cord presentation away from the cervix, and a cord presentation may not be evident on repeated sonographic examinations.
Data from Doppler. Color Doppler or power Doppler will demonstrate flow through the structures, confirming that they are vessels (Figure 5). Pulsed Doppler, on the other hand, will demonstrate a fetal vessel waveform and a fetal pulse rate (Figure 6), confirming that the vessel is fetal in origin and avoiding false-positive diagnoses. This is particularly important since maternal vessels can also occupy the cervical region.
Screening for vasa previa
Routine nontargeted U/S frequently fails to diagnose vasa previa. In a retrospective review of 82 cases of velamentous cord insertion and vasa previa diagnosed at the time of delivery, investigators found that none of the cases had been detected on routine nontargeted sonography.19 No deliberate attempt was made to screen for vasa previa at the time of the sonographic examination.
Using sonography with color Doppler, other researchers attempted to consistently identify the umbilical cord's insertion into the placenta in 587 women between 18 and 20 weeks' gestation.18 They were able to locate the placental cord insertion in 586 of these patients (99.8%), diagnosing two cases of vasa previa in the process. The exam took 20 seconds, on average, and was achieved within 1 minute in 95% of cases. These researchers also examined placentas at delivery to confirm the sonographic diagnosis. The sonographic identification of velamentous cord insertion when investigators are specifically seeking it out has a sensitivity of 100%, a specificity of 99.8%, and a negative predictive value of 100%.18
More recently, Sepulveda and colleagues performed routine sonographic screening on 832 women at the time of their second-trimester sonogram and correctly identified the cord insertion in 825 (99%) of these women without prolonged scanning time.20 These results show that you can incorporate attempts at locating the umbilical cord insertion into the routine sonographic examination without significantly increasing cost or scanning time. This technique will identify all Type 1 vasa previas.
Other researchers routinely examined the region overlying the cervix using transabdominal U/S and TVS with color Doppler.21 Over an 8-year period, they examined nearly 94,000 women and diagnosed 15 cases of vasa previa. The average gestational age (GA) at diagnosis was 26+6.3 weeks. Eight of these women had a second-trimester low-lying placenta, while six had bi-lobed or succenturiate lobed placentas. The 15 women were delivered by elective C/S. Only one infant, the second of a set of twins, died from complications of prematurity. Other researchers, using U/S and color Doppler to screen more than 33,000 women over an 8-year period, found 10 cases of vasa previa.3 It is now our policy at Georgetown to perform TVS with color Doppler on all women at risk of vasa previa. Using this protocol, we have diagnosed three cases of vasa previa over the past year.
Tests for fetal blood
Because vasa previa is often associated with bleeding in pregnancy, it's important to determine whether the blood is of fetal or maternal origin whenever a woman bleeds in the third trimester. Among existing tests for identifying fetal hemoglobin are the Apt Test, the Londersloot Test, and the Ogita test (Table 3).22 Hemoglobin electrophoresis and the Kleihauer-Bettke test are highly sensitive and specific. However, the latter two tests take at least 1 hour, which may be too long if vessels have ruptured. In comparing the various tests, some researchers found the Ogita test, which yields results in a few minutes, the quickest and easiest to perform.22 But other investigators, in surveying 100 community hospitals and all the medical schools in the US, determined that because these tests were rarely performed, they did not appear to be the standard of care.23 Furthermore, recent Joint Commission on the Accreditation of Healthcare Organizations (JCAHO) regulations regarding bedside testing may make it impossible for physicians to perform these tests at the bedside. Nevertheless, when vaginal bleeding of unknown cause occurs, if the fetal status is reassuring, it may be prudent to send a sample of the vaginal blood to the hospital laboratory for a Kleihauer Bettke test.
Aggressive management includes liberal use of U/S
Once you've made the diagnosis, closely monitor the patient. It's imperative to perform a full sonographic survey for fetal structural anomalies and growth, since vasa previa may be associated with fetal malformations, intrauterine growth restriction, and poor pregnancy outcome.1 Hospitalize your patient in the third trimester, chiefly so she's close to facilities for emergent delivery, should the membranes rupture. In the absence of bleeding, strict bed rest isn't necessary. But do encourage her to reduce her activity level and strongly discourage sexual activity. Institute regular antenatal testing, and for patients at less than 34 weeks, administer steroids to promote fetal lung maturation.
Perform elective C/S at 36 to 37 weeks' gestation. Although this is earlier than the GA of 39 weeks usually recommended for elective C/S delivery, it's justified because of the considerable risk of fetal death or severe morbidity in the event the fetal vessels rupture. Should vessels rupture, immediately deliver by C/S. Aggressively managing the infant with blood transfusion may result in a good outcome.24 Be aware, though, that babies delivered after vessel rupture often have extremely low Apgar scores, require blood transfusions, and have a stormy neonatal course, often with serious long-term sequelae.
Although clinicians have traditionally linked vasa previa with poor perinatal outcomes, that view is unduly pessimistic. In fact, the perinatal outcome is good to excellent when ob/gyns are able to diagnose the condition prenatally and perform elective C/S before the membranes rupture. A high index of suspicion is crucial to a good outcome. Most of the time you'll be able to diagnose this condition prenatally by determining the placental cord insertion and scanning the area overlying the cervix as part of routine obstetric sonography. Such an examination is achievable in most obstetric sonographic units without placing excessive demands on time, personnel, or equipment.
Patients in the following risk categories deserve targeted sonographic scrutiny of the region overlying the cervix with color Doppler to identify or rule out vasa previa: women with second-trimester low-lying placentas, multiple pregnancies, IVF pregnancies, those with vaginal bleeding in pregnancy, and especially women with bi-lobed or succenturiate-lobed placentas. Most deaths associated with vasa previa can be prevented by adhering to the protocols we've described here.
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8. Englert Y, Imbert MC, Van Rosendael E, et al. Morphological anomalies in the placentae of IVF pregnancies: preliminary report of a multicentric study. Hum Reprod. 1987;2:155-157.
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Dr. Oyelese is a Fellow, Division of Maternal-Fetal Medicine, University of Medicine and Dentistry of New Jersey/Robert Wood Johnson Medical School, New Brunswick, N.J.; Dr. Lewis is Assistant Professor, Division of Maternal-Fetal Medicine, and Director, Prenatal Diagnostic and Ultrasound Center, Georgetown University Medical Center, Washington, D.C.; and Dr. Collea is Professor and Chief, Division of Maternal-Fetal Medicine, Georgetown University Medical Center, Washington, D.C.
- Undiagnosed prenatally, vasa previa carries very high perinatal mortality. A high index of suspicion is crucial to making the diagnosis.
- You can diagnose vasa previa prenatally with confidence using transvaginal sonography with color Doppler.
- Screen the following women, all of whom are at risk for vasa previa: women with second-trimester low-lying placentas, multiple pregnancies, IVF pregnancies, patients with third-trimester vaginal bleeding, and those with bi-lobed or succenturiate-lobed placentas.
- If a patient has third-trimester vaginal bleeding or bleeding in labor, test her vaginal blood for fetal blood cells.
- Admit women with vasa previa in the third trimester, and deliver them by elective C/S at 36 to 37 weeks' gestation.
The Vasa Previa Foundation
The membership of the Vasa Previa Foundation ( www.vasaprevia.org ), established in 2001, includes women and families who have been affected by pregnancies with vasa previa. Its mission is to raise awareness about the condition, to disseminate information, and to facilitate research aimed at furthering understanding of vasa previa with the aim of minimizing perinatal mortality. The foundation maintains a worldwide database of cases of vasa previa.
Yinka Oyelese, Kerry Lewis, Joseph Collea. Cover Story: Vasa previa: Are most perinatal deaths preventable? Contemporary Ob/Gyn Nov. 1, 2003;48:43-56.