Discussion
Vasa previa can be classified into two types. Type I vasa previa refers to velamentous insertion of the cord with resultant vasa previa, and Type II indicates interconnecting vessels between two lobes of placenta in a bipartite placenta or connecting vessel with a succenturiate lobe of the placenta[6]. The parachute placenta with vasa previa in our case was a little special. The previa vessel originated for the cord directly like the Type I. But that vessel doesn’t carry the major umbilical blood flow, as it was only a small branch of the three main umbilical vessels(one umbilical vein and two umbilical arteries). In this aspect, it was more like Type II vasa previa. Our case could be considered as a transitional type between Type I and Type II.
The American College of Obstetricians and Gynecologists recommends the use of color Doppler in patients who are at a high risk for vasa previa[7]. The reasons we considered that the second time of ultrasonic examination hadn’t detected the vasa previa may include the follows, but not limited to.
First, the most common sections for pelvic ultrasonography are standard horizontal, sagittal, coronal, and oblique sections originate from the standard ones[8]. However, from the MRI we know the previa vessel was from the patient’s right posterior to left anterior. If the previa vessel long axis was not just in the ultrasound section, it would not form a curve image but merely a tiny cross section. In that circumstance, the color Doppler could only see a round red or blue signal in small diameter. And this patient’s placental lower margin was also close to the cervical internal os, the previa vessel signal might disguise itself among the placenta blood flows.
Second, even if the transvaginal probe actually rotated and stayed at the previa vessel long axis direction of left-anterior to right-posterior, the ultrasonic definition of transvaginal gray scale might not be high enough to distinguish the vessel[9]. In that circumstance, the color Doppler could help. But color Doppler scanning should be turned on before the transvaginal probe rotation. Also the lower uterine segment hadn’t formed well because of low lying placenta. Thus, the previa vessel was not just over the cervical internal os, but posterior to the relatively thick cervix in transvaginal scanning, bringing more difficulty to identify it.
Third, as mentioned above, our case was between Type I and Type II, and the previa vessel only carried a small portion of the umbilical blood flow. Thus, the blood flow in that previa vessel even might not be continuous but intermittent. Thereby, if there was just no obvious blood flow at the time gap during ultrasonic scanning, the vasa previa could still not be identified[10].
MRI can be an alternative in identifying vasa previa, if the ultrasound scanning results are contradictory[11]. MRI has some advantages in this field[12]. First of all, it is tomography[13]. So the image definition for spatial structures and soft tissues will be able to detect a really thin vessel. Second, the image can be reconstructed in three dimensions[14]. Therefore, any direction of the previa vessels’ courses can be confirmed, not like the ultrasonography that needs the previa vessel’s long axis within the certain scanning section to see the vessel curve. Meanwhile, the disadvantage of MRI in this field is unawareness of the blood flow direction, unlike the ultrasonography with color Doppler[15].
In conclusion, vasa previa results in poor pregnancy outcomes if not diagnosed prenatally. Ultrasonography, especially transvaginal scan with color Doppler is commonly used as diagnostic method. But if there’s contradictory ultrasonography results, MRI may offer some assistance in diagnosis.