We very much enjoyed the recent article in the Society for Maternal-Fetal Medicine Consult series regarding diagnosis and management of vasa previa but would like to point out several key omissions.
The authors state that “the diagnosis of vasa previa is confirmed if an arterial vessel is visualized over the cervix. …” This criterion is also given in a recent article by Silver but is not accurate. Vasa previa is diagnosed if there are any unprotected fetal vessels in the membranes over or near the cervix, with either an arterial waveform or venous flow.
Fetal veins are thin walled and hence potentially more prone to rupture; prior to the advent of sonographic diagnosis, one of us (V.C.) treated a case in which near exsanguination occurred within minutes of the rupture of membranes because of the rupture of venous vasa previa. When venous flow is seen, the sonographer must confirm that the flow is indeed within a fetal vessel by tracking the vessel back to the placenta with color Doppler or color power angioscanning.
Vasa previa is the most important sonographic diagnosis for the fetus: more than half of the 1 in 2500 babies with vasa previa, and half of the 1 in 250 babies from in vitro fertilization with vasa previa, die if the diagnosis is not made. We can think of no other condition in which an accurate prenatal diagnosis leads to such a dramatic difference between death and survival. The authors state that “routine ultrasound examination of the placenta and lower uterine segment permits detection in the majority of cases.”
The diagnosis may be missed if fetal parts obscure the cervix or if incorrect color Doppler settings are used. Fetal vessels running transversely or obliquely across the cervix or lower uterine segment are easiest to miss but will be appreciated by angling the transducer from the side.
Whereas the authors discuss a threshold of a 2 cm distance between the vessels and the internal os, we are aware of several cases of fatal fetal vessel rupture with fetal vessels as far as 4 cm from the internal os. The fetal biparietal diameter is 9.5 cm at term; hence, any vessel within a radius of 5 cm from the internal os is potentially in jeopardy. Because we do not have a specific distance that places vessels at low risk for rupture, we would urge extreme caution in dealing with any velamentous vessels in the lower segment.
Finally, the images in the article are not ideal. Vasa previa refers to fetal vessels running through the membranes over the cervix, unprotected by cord or placental tissue. The images presented show a fetal vessel overlying a thickened uniformly echogenic structure that may be placenta or myometrium. The echolucent structure to the upper right of both images does not appear to be the bladder. No clear landmarks are shown. For accurate diagnosis, and to minimize the false-positive diagnosis of vasa previa, it is crucial that the cervix should be clearly demonstrated and the vessels should not be encased by cord or overlying the placental tissue. Above is one such image ( Figure ).
