Contents
Cord vessels present in the membranes covering the internal cervical os.
Classification
- Type I VP (with velamentous insertion (VCI) of the umbilical cord):
- Velamentous insertion: Cord inserts into membranes through which unprotected vessels then run until they end in the placenta
- Type II VP (with bilobed/succenturiate placenta):
- Exposed vessels run through the membranes between lobes of a bi‐lobed placenta
Aetiology
Risk factors:
- Low‐lying placenta, placenta previa
- Abnormal placental morphology: Bilobed/succenturiate lobe placentas, placenta membranacea
- Multiple pregnancies
- Velamentous insertions
Pathophysiology
Vasa praevia, whether Type I or Type II, is thought to arise from a placenta that covers the cervix in the early part of pregnancy. As the pregnancy progresses, because the region over the cervix is poorly vascularised, the placenta grows preferentially toward the upper part or fundus of the uterus. At the same time, the placental tissue overlying the cervix undergoes atrophy, leaving exposed vessels running through the membranes.
The state is characterised by the fact that the blood vessels contained in the fetal membranes are loose, uncoated by placental tissues or Wharton’s jelly
Complications
Perinatal mortality rate of approximately 60% if not prenatally detected and appropriately managed (3% in those diagnosed prenatally)
Perinatal complications:
- Perinatal rupture of the vessels carrying fetal blood (major complication)
- Compression of the vasa previa by the presenting part (less common):
- Fetal distress, hypoxia
Postnatal complications:
- Related to prematurity (d/t early C-section with no confirmation of lung maturity):
- Hyaline membrane disease
- Bronchopulmonary dysplasia
- Transient tachypnea
- Respiratory distress syndrome
- Partial exsanguination
- Complications related to anemia, hypovolemic shock
- Complications of transfusions
Diagnosis
Ultrasound (USG):
GOLD STANDARD
- Blood vessels visible above internal cervical orifice
Transvaginal colour doplerometry:
Differentiate from the cervix–uterine varicose veins by comparing the blood flow in these veins, to the fetus and the mother’s blood flow
- Vasa praevia: Blood flow same as the umbilical cord blood flow
Magnetic resonance imaging (MRI):
Some advantages over ultrasound due to its accurate description of the vessels, the location, direction, placenta, its structure and the point of attachment, segments and their distribution. However, it is an expensive, relatively not easily accessible.
Differential diagnosis:
“Linear structures” in front of the inner os in grey-scale in USG may also represent the following. The differential diagnosis of those is easily established by color Doppler. Pulsed Doppler will demonstrate a fetal umbilical or venous waveform if it is a vasa previa. Sometimes marginal placental sinus may present with flow, but it will be a maternal heart frequency.
- Marginal placental sinus
- Chorioamniotic separation
- Simple folds of the membranes
Management
Caesarean section:
Delivery by C/S should be performed prior to rupture of membranes to avoid rupture of the exposed fetal vessels and fetal exsanguination.
