Etiology and Management of Vasa Previa
Oct 2011 – by C. Thompson, MD
Vasa Previa Review
Incidence/Etiology: The exact frequency of vasa previa is difficult to determine but it probably occurs in 1 in 1,500 to 5,000 pregnancies. It occurs with placentation and cord development as it is related to either a velamentous cord insertion (type I) or succenturiate placental lobe formation (type II). In either case vasa previa occurs when fetal vessels traverse the membranes, separate from the umbilical cord or placenta, and overlie the cervix before the presenting fetal parts. This extremely vulnerable condition then portends a potentially catastrophic event should the vessel rupture and result in fetal exsanguination. Therefore successful management relies on diagnosis, preparation, readiness and surgery.
Diagnosis: In general diagnosis is no longer difficult with grayscale ultrasonography and color Doppler. However having an index of suspicion to screen for and establish the diagnosis is important. There is debate whether this should be performed routinely in all pregnancies. Conditions which may be associated with vasa previa such as low lying placenta, multi-fetal gestation, IVF or accessory lobe of the placenta should warrant further investigation. However many ultrasongraphers routinely perform cervical views during routine OB ultrasound visits and suggest this may serve as a method to detect vasa previa. In addition identifying the placental cord insertion is a routine part of an anatomy scan and can aid in the diagnosis.
Management: Once the diagnosis is established successful management includes pelvic rest, possible hospitalization and preterm delivery. While no single recommendation is agreed upon the tenets are consistent. The ultimate goal is to avoid rupture of membranes and potential vascular tear and plan for surgical delivery of the infant as close to maturity as possible. While hospitalization does not guarantee a good outcome it does improve the odds of intact fetal survival compared to a ruptured vasa previa occurring outside of a hospital. For this reason experts have recommended hospitalization at 30 to 32 weeks until delivery. Some people have suggested close monitoring with fetal fibronectin (fFN) and cervical ultrasound length as a way to obviate the need for hospitalization, if the fFN is negative and the cervix is long and closed. The timing of delivery has also been the subject of discussion in order to maximize fetal well-being and pulmonary maturity and minimize the adverse consequences of preterm birth. This is balanced against the risk of spontaneous labor or spontaneous rupture of membranes. Aside from rupture of vasa previa there is a concern for cord compression and fetal compromise as well. Elective cesarean delivery around 35 weeks may optimize this risk/benefit scenario with a prior course of antenatal steroids. The addition of amniocentesis for fetal lung maturity may pose its own risks and does not appear to add value in this setting. The possibility exists to treat some cases of vasa previa due to a succenturiate lobe (type II) antenatally with laser ablation of the vessels, however this may not be routinely available to candidates.