Volume 8 Issue 3 - Australasian Society for Ultrasound in Medicine

CASE REPORTASUM Ultrasound Bulletin 2005 August; 8 (3): 28–30A 3D approach to antenatal diagnosis ofvasa previa with 2D ultrasound imagingHS Wong AC , K Burns B , L Strand B , S Parker BADepartment of Obstetrics and Gynaecology, Wellington School of Medicine and Health Sciences,University of Otago, Wellington, New ZealandBWomen’s Health Service, Capital and Coast District Health Board, New ZealandCCorrespondence to HS Wong email hsoowong @wnmeds.ac.nzAbstractAntenatal diagnosis of vasa previa has been reported with the use of 2D and colour Doppler imaging,and more recently with 3D and colour Doppler imaging. A case of vasa previa diagnosed with2D gray-scale and colour Doppler ultrasound imaging on combined transabdominal and transvaginalscans is described. The findings are compared with that in a case of placenta previa where blood vesselsare also found overlying the internal cervical os. The utilisation of three perpendicular planes on2D ultrasound imaging for a 3D impression of the area around the internal cervical os for differentiationof the two conditions is presented.IntroductionVasa previa is known to be associated with a poor perinataloutcome and a high fetal mortality of 50–60% where thereare intact membranes and 70–100% with ruptured membranes1,2 .Antenatal diagnosis allows the planning of elective deliverybefore onset of labour or rupture of membranes that maylead to tearing of fetal vessels and fetal exsanguination 3 .The use of 2D and colour Doppler ultrasound for prenataldiagnosis and, more recently, 3D ultrasound has beenreported. However, in many ultrasound units, 2D ultrasoundimaging is still the backbone for obstetric imaging becauseof its wide availability and simplicity of image acquisition.A case of vasa previa is presented and the diagnosticapproach utilising 2D gray-scale and colour Doppler imagingin three different perpendicular planes is described. Thesonographic findings are compared to a case of placentaprevia where a vessel could be identified overlying theinternal cervical os in association with placental tissue.Case reportThe patient, 42 years old, gravida 7 para 3, Samoan witha history of two previous caesarean deliveries, presentedfor an ultrasound scan at 17 weeks which showed that theplacenta was posterior, low-lying and covering the internalcervical os. Ultrasound examination was repeated at 36weeks gestation. The patient was scanned transabdominallyin sweeping sagittal and transverse sections targeting atthe internal cervical os. The main bulk of the placenta wasnoted to lie posteriorly, reaching the internal cervical os.However, there was a succenturiate lobe in the anterior wallof the uterus away from the internal os and vessels could beseen overlying the internal cervical os connecting the twolobes of placenta in both sagittal (Figure 1a) and transversesections (Figure 1b). These vessels were also demonstratedon transvaginal scan (Figure 1c). Placenta previa, succenturiatelobe and vasa previa were diagnosed. The patientwas scheduled for elective caesarean section. The baby wasdelivered in good condition. The placenta was examinedand the presence of succenturiate lobe and vasa previa wasconfirmed.DiscussionAntenatal diagnosis for vasa previa has been reported withthe use of 2D gray-scale imaging on transabdominal scan 4and colour Doppler imaging on transvaginal scan 5 . Morerecently, the use of 3D and colour Doppler ultrasound imagingin diagnosis of vasa previa has also been described 6 .We applied 2D gray-scale and colour Doppler ultrasoundscanning, sweeping in sagittal and transverse planestransabdominally and in sagittal and coronal sections transvaginallyto inspect the area around the internal cervical osin three perpendicular planes (Figure 1d) and obtained sectionssimilar to what could be obtained with 3D ultrasoundequipment 6 .We find that this approach can also depict those cases inwhich there are vessels running in the placental substanceoverlying the internal cervical os (as in placenta previa)rather than naked fetal vessels running in membranes only(as in vasa previa). In the former, the placental substancecan be seen as a layer between the vessel and the internalcervical os on sagittal section (Figure 2a) and along theside of the vessel on transverse section (Figure 2b and 2c).Particularly noteworthy is the sonolucent area around thefetal vessels at the internal os level on transverse sectionwith true vasa previa indicating the absence of any supportingplacental substance (Figure 1b), whereas in placentaprevia there is no sonolucent area around the vessels at thislevel (Figure 2c).Conclusion2D images are easy to acquire and study and, up to thepresent moment, 2D imaging is still more widely availableand commonly utilised than 3D. We find that sweeping in28 ASUM Ultrasound Bulletin 2005 August; 8 (3)