Discussion
The purpose of this case report is to draw the attention of the obstetrical practitioner to the importance of detailed transvaginal ultrasound examination including colour doppler to prevent accidents caused by unusual vasa praevia rupture.
The guidelines of the International Society of Ultrasound and Gynecology (ISUOG) for the second trimester examination mention the position of the placenta [2]. If the lower placental edge reaches or overlaps the internal os, a follow-up examination in the third trimester is recommended. The guideline mentions that detailed examination for vasa praevia may not need to be performed if the position and morphology of the placenta are normal.
Prince reported a case of vasa praevia with anomalous umbilical cord formation [3]. In this case, vasa praevia was not diagnosed prenatally. Routine screening by transabdominal ultrasound in the second and third trimester showed an anterior placenta not covering the cervical os. Further examination for vasa praevia was not performed. Fortunately, vaginal delivery was possible without serious complications for the neonate. This case is similar to our case in having an unexpected course of foetal vessels which resulted in vasa praevia. In such cases, transvaginal ultrasound without colour doppler may not be able to detect vasa praevia. The accuracy of transvaginal ultrasound in the diagnosis of vasa praevia was previously reported to be high when performed in combination with colour Doppler [4]. Initially, obstetricians usually perform transvaginal ultrasound without colour doppler. If the placenta and/or umbilical cord vessels are not detected near the internal cervical os by ultrasound without colour doppler, further examinations including colour doppler may not be performed. In our case, ultrasound with colour doppler was performed in order to exclude for abnormal insertion of the umbilical cord before introduction of a transcervical foley catheter for cervical ripening. A few foetal vessels indicating vasa praevia were only detected after we performed ultrasound with colour doppler, an incidental and fortunate diagnosis.
After diagnosis of vasa praevia, caesarean section before rupture of foetal vessels is thought to be required for good prognosis of the neonate. If foetal vessels are running on the anterior lower segment of the uterus, obstetricians should be careful to avoid intraoperative rupture of foetal vessels. Aoki et al. reported a case of vasa praevia in which the cord vessels were running on the anterior lower uterine segment [5]. They made a horizontal incision on the uterine fundus to avoid rupture of the cord. In future pregnancies after horizontal incision on the uterine fundus, the risk of uterine rupture or placenta accreta are higher relative to that of an incision in the lower uterine segment. However, in their case they concluded that incision in the lower uterine segment would have had a higher risk of rupture of cord vessels and loss of massive foetal blood. We also predicted that the cord vessels were running on the anterior lower uterine segment in our case. Both the placental position and the site of umbilical cord insertion were above the low transverse incision scar. A few foetal vessels were meandering and running over the internal cervical os. We thought that safe delivery via low transverse incision was feasible by putting the foetal vessels at the lateral part of the incision scar. The precise diagnosis of vasa praevia and assessment of the course of the cord vessels is essential for a safe delivery. A change in the site of uterine incision may be required. If the selected incision is not a normal low transverse incision, an increased risk in future pregnancies should be considered.
Both the patient and her father had moyamoya disease. Although genetic testing was not performed, familial disease was suspected. RNF213, the gene related to angiogenesis and vascular inflammation was identified as a susceptibility gene for moyamoya disease [6]. Recent research on moyamoya disease has shown that RNF213 variants are associated not only with moyamoya disease but also with intracranial atherosclerosis and systemic vascular diseases, such as peripheral pulmonary artery stenosis and renal artery stenosis [7]. Vasculopathy of umbilical and placental vessels has not been reported. The main cause as to why vasa praevia occurred in our case is not clear. However, a specific gene such as RNF213 may be related to anomalous formation of umbilical cord vessels which cause vasa praevia.