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.