Vasa Previa

Vasa previa is a rare condition in which fetal vessels, unprotected by Wharton’s Jelly of the umbilical cord run through the amniotic membrane and traverse the cervical os or run within 2 cm of it. Traditionally it is classified into 2 types; type 1 is associated with a single placenta and velamentous cord insertion; and type 2 where fetal vessels connect separate lobes of the placenta (e.g.Succenturiate or accessory lobe) and those vessels course through the lower segment. Recently, a 3rd type of vasa previa has been proposed. This type of vasa previa results from a resolving placenta previa where there is a very prominent fetal vessel which runs at or just beyond the edge of the placenta and is located in the lower segment. (1)

The reported incidence of vasa previa is around 1 in 1300 to 1 in 2500 deliveries; however, higher incidences (1 in 202) have been reported in pregnancies conceived following assisted reproductive techniques.

The exact pathogenesis is unknown. Based on its classification, it may result from velamentous cord insertion in which the fetal vessel course through the membranes before inserting into the placenta, overlying the lower part of the lower segment or cervix, or from a resolution of placenta previa or low-lying placenta. It may also occur in presence of succenturiate or multiple lobed placenta. 

A high suspicion of vasa previa should be raised in pregnancies with the following risk factors: as one or more of these factors is present in 80-90% of cases of vasa previa (3, 13).
 
- Velamentous or marginal cord insertion 
- Placenta previa or low-lying placenta 
- Succenturiate or bilobed placenta
- In vitro fertilization
- Multiple gestations

Vasa previa may result in catastrophic perinatal outcome. Vasa previa should be clinically suspected in a patient presenting with vaginal bleeding and abnormal fetal heart tracing (sinusoidal or bradycardia) after rupture of the membranes. When vasa previa is detected in the intrapartum period, there is a high risk of fetal exsanguination, stillbirth, and neonatal death and morbidity in survivors (2, 6).

Prenatal detection begins with careful evaluation of the placenta to ensure that it contains only one piece (no succenturiate lobe) and that the umbilical cord inserts directly into the placenta (4). In pregnancies where a placenta previa has resolved, the margin of the placenta should carefully be evaluated for vessels. 

In all pregnancies, the lower segment should be carefully evaluated to ensure that there are no gray scale circular structures (bubbles) or linear structures(lines) which are actually vessels when evaluated with colour Doppler flow.  If a vasa previa is suspected transvaginal scanning using gray scale, Colour Doppler and pulsed wave Doppler should be applied to confirm the suspicion of aberrant vessels, and their relationship to the internal os. 

Some authors suggest a sweep of the lower segment with colour Doppler in all pregnancies (Yinka Oyelesi, ISUOG Meeting, London 2022)

Prenatal diagnosis is confirmed based on the demonstration of the passage of fetal vessels across or in proximity (usually less than 2 cm) the internal cervical os by real-time ultrasonography. A combination of transabdominal and transvaginal Doppler velocimetry ultrasonography provides the best diagnostic accuracy with low false positive rates.  

A detailed ultrasound evaluation with a detection rate of 96% with 100% specificity was reported by Sutera and colleagues. (3) 

One potential pitfall in diagnosis should be highlighted. Compression of the fetal vessels overlying the cervix can be encountered when the fetal head is well-applied to the internal os.. This can result in a false negative diagnosis as Doppler flow may be absent.  Manual elevation of the fetal head has been suggested by Kagan et al to displace the head from the cervix and promote vascular flow, resulting a discrete Doppler flow signal (13). Conversely, Oyelese et al have demonstrated the opposite; that applied suprapubic pressure to displace the fetal head from the cervix can result in obstruction of the aberrant vessels  resulting in a negative Doppler study and  a missed diagnosis, particularly in the third trimester, Both authors stress the importance of careful evaluation on the lower segment  to rule out vasa previa preferentially  in the second trimester when the fetal head is not generally low, as opposition  of the fetal head onto the cervix in the third trimester can result in false negative diagnosis. (13, 14)

- Cord presentation: can be distinguished from vasa previa by the presence of Wharton/s jelly surrounding the cord loop and movement of the cord away from the cervical os with change in maternal position or fetal presenting part. 
 
- Cervical varicosities and vessels: pulsed Doppler distinguish maternal blood flow from fetal or maternal venous plexus. If a venous structure is suspected, maternal valsalva will produce changes in the venous waveform while a fetal vessel will not demonstrate changes. 
 
- Amniotic band or chorioamniotic separation: free-floating linear lines noted close to cervical os, representing chorioamniotic separation, may be falsely suspected to represent vasa previa. The distinguishing factor will be the absence of blood flow on Doppler interrogation. Other thin, membrane-like strands close to the os but crossing the amniotic sac and attaching to fetal body parts may be indicative of amniotic bands, rather than vasa previa and absence of blood flow on colour Doppler can discriminate these conditions. 

Current literature has shown that prenatal diagnosis of vasa previa has increased the survival rate up to 97-100% with perinatal mortality decreasing to less than 10% compared to pregnancies with missed diagnosis (fetal survival rate less than 50 %). 

Although universal screening for vasa previa has been advocated by some authors, currently there are no clincial practice guidelines promoting universal screening for this condition;  however, careful assessment of the umbilical cord insertion and lower uterine segment with the use of colour Doppler in pregnancies in all pregnancies but especially in those with recognized risk factors  in the mid-trimester  can aid early recognition of this condition (8). Zhang et al has reported that 75% of cases of Vasa previa were associated with velamentous cord insertion and 25% with bilobed and/or low-lying placenta detected at the 20-22 weeks ultrasound. (12)

Prenatal diagnosis has significantly improved fetal survival. In a minority of cases, vasa previa may resolve spontaneously. 
Due to the lack of protection of fetal vessels, they are prone to compression with the descent of the fetal presenting part leading to fetal asphyxia 
Spontaneous or iatrogenic rupture of the fetal membranes will result in fetal bleeding and exsanguination within minutes. 
 

There is no consensus on optimal management protocols in parentally diagnosed cases of vasa previa. The current recommendations are based on small case series, expert opinions, and clinical judgment as there are no clinical trials available. A Delphi consensus is underway. 

The Society of Obstetricians and Gynaecologists of Canada (SOGC) guideline on management suggests that patients be hospitalized at 30-32 weeks of gestation and delivered at 35-36 weeks of gestation by Cesarean section (5). Similar recommendations are suggested by The Royal College of Obstetricians and Gynecologists in UK with individualized prophylactic hospitalization at 30-32 weeks of gestation based on risk factors such as multiple pregnancies, antenatal bleeding, and threatened preterm labour (6). Administration of antenatal corticosteroids to promote fetal lung maturity is recommended although optimal timing is variable based on clinical circumstances.
Cervical length assessment by transvaginal ultrasonography may be useful to predict the risk of preterm delivery and to guide clinical decision-making regarding timing of steroid administration and choice of inpatient versus outpatient  management. (7)  

In cases of vasa previa which improve over the course of the pregnancy, there is no consensus about how far away from the internal os the vessels need to be to ensure a safe vaginal delivery. Some authors suggest 2 cm is safe, however, these authors also report a catastrophic outcome when 2 cm has been used (9,10).

Ultrasound prior to delivery may be useful to ensure that entry into the lower segment is not over the vessels in the lower segment which could inadvertently be transected on entry into the uterus. Some authors suggest that delivery of fetuses with vasa previa can be accomplished with a planned en caul delivery (11).   

1.    Kyosuke Kamijo, Tsutomu Miyamoto, Hirofumi Ando, Yasuhiro Tanaka, Norihiko Kikuchi, Manaka Shinagawa, Satoshi Yamada, Ryoichi Asaka, Chiho Fuseya, Satoshi Ohira & Tanri Shiozawa (2022) Clinical characteristics of a novel “Type 3” vasa previa: case series at a single center, The Journal of Maternal-Fetal & Neonatal Medicine, 35:25, 7730-7736, DOI: 10.1080/14767058.2021.1960975
2.    Hernandez-Andrade E, Huntley ES, Bartal MF, Soto-Torres EE, Tirosh D, Jaiman S, Johnson A. Doppler evaluation of normal and abnormal placenta. Ultrasound Obstet Gynecol. 2022 Jul;60(1):28-41. doi: 10.1002/uog.24816. PMID: 34806234.
3.    Sutera M, Garofalo A, Pilloni E, Parisi S, Alemanno MG, Menato G, Sciarrone A, Viora E. Vasa previa: when antenatal diagnosis can change fetal prognosis. J Perinat Med. 2021 May 4;49(7):915-922. doi: 10.1515/jpm-2020-0559. PMID: 33939903.
4.    Ranzini AC, Oyelese Y. How to screen for vasa previa. Ultrasound Obstet Gynecol. 2021 May;57(5):720-725. doi: 10.1002/uog.23520. PMID: 33085148.
5.    Gagnon R, Morin L, Bly S, Butt K, Cargil YM, Denis N, Hietala-Coyle MA, Lim KI, Ouellet A, Racicot MH, Salem S, Hudon L, Basso M, Bos H, Delisle MF, Farine D, Grabowska K, Menticoglou S, Mundle W, Murphy-Kaulbeck L, Ouellet A, Pressey T, Roggensack A; Diagnostic Imaging Committee; Maternal Fetal Medicine Committee. SOGC CLINICAL PRACTICE GUIDELINE: guidelines for the management of vasa previa. Int J Gynaecol Obstet. 2010 Jan;108(1):85-9. doi: 10.1016/j.ijgo.2009.09.011. PMID: 20050202.
6.    Jauniaux E, Alfirevic Z, Bhide AG, Burton GJ, Collins SL, Silver R; Royal College of Obstetricians and Gynaecologists. Vasa Praevia: Diagnosis and Management: Green-top Guideline No. 27b. BJOG. 2019 Jan;126(1):e49-e61. doi: 10.1111/1471-0528.15307. Epub 2018 Sep 27. PMID: 30260094.
7.    Society of Maternal-Fetal (SMFM) Publications Committee; Sinkey RG, Odibo AO, Dashe JS. #37: Diagnosis and management of vasa previa. Am J Obstet Gynecol. 2015 Nov;213(5):615-9. doi: 10.1016/j.ajog.2015.08.031. Epub 2015 Aug 18. PMID: 26292048.
8.    Oyelese, Y., Lees, C.C. and Jauniaux, E. (2023), The case for screening for vasa previa: time to implement a life-saving strategy. Ultrasound Obstet Gynecol, 61: 7-11. https://doi.org/10.1002/uog.26085

9.    Rebarber A, Dolin C, Fox NS, Klauser CK, Saltzman DH, Roman AS. Natural history of vasa previa across gestation using a screening protocol. J Ultrasound Med 2014; 33: 141– 147.
10.    Klahr R, Fox NS, Zafman K, Hill MB, Connolly CT, Rebarber A. Frequency of spontaneous resolution of vasa previa with advancing gestational age. Am J Obstet Gynecol 2019; 221: 646.e1– 7.
11.    Yinka Oyelese, Matt Iammatteo, Steve Domnitz, Martin R. Chavez,Vasa previa: avoiding incising the membranes at cesarean delivery,American Journal of Obstetrics and Gynecology, Volume 227, Issue 5, 2022,Pages 770-772,ISSN 0002-9378,https://doi.org/10.1016/j.ajog.2022.07.010.
12.    Zhang W, Geris S, Al-Emara N, Ramadan G, Sotiriadis A, Akolekar R. Perinatal outcome of pregnancies with prenatal diagnosis of vasa previa: systematic review and meta-analysis. Ultrasound Obstet Gynecol 2021; 57: 710–719
13.    Kagan KO, Hoopman M, Sonek J. Vasa previa: easy to miss. Ultrasound Obstet Gynecol 2018; 51: 283–284.
14.    Oyelese Y, Reforma L, Sewell McGough R, O’Brien B. -Manual elevation of the fetal head as a potential cause for missed vasa previa. Ultrasound Obstet Gynecol 2022; 60: 429–431.

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