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Nuchal Translucency Measurement on the Fetus in a Difficult Position

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  • 1. Tips and Techniques Tricks for Obtaining a Nuchal Translucency Measurement on the Fetus in a Difficult Position Bryann Bromley, MD, Thomas D. Shipp, MD, Mary Ann Mitchell, RDMS, Beryl R. Benacerraf, MD S creening for aneuploidy should be offered to all pregnant women.1 The demand for first-trimester sonography has been increasing rapidly as many women choose first-trimester risk assessment that involves the sonographic measurement of the fetal nuchal translucency (NT). Although most studies are eas- ily accomplished using a standard transabdominal approach within a 20-minute window, not infrequently, the fetus may be positioned such that an adequate mea- surement of the NT is not possible. For example, Wax et al2 reported that they were able to obtain an NT mea- Abbreviations surement in only 80% of eligible women at the time of the NT, nuchal translucency first visit, despite allowing up to 30 to 40 minutes before abandoning the examination. Repeat scanning of the patients who returned resulted in an 86.6% success rate.2 The fetal position was consistently a factor in not being able to obtain an NT measurement, accounting for 67.3% of such cases.2 Other factors that prevent optimal mea- Received February 23, 2010, from the Departments of Radiology and Obstetrics and Gynecology, Brigham surement of the NT include a high maternal body mass and Women’s Hospital (B.B., T.D.S., B.R.B.), and index, as imaging in these patients generally requires Departments of Radiology (B.R.B.) and Obstetrics and Gynecology (B.B., B.R.B.), Massachusetts General more time and more return visits, ultimately resulting in Hospital, Harvard Medical School, Boston, a higher failure rate.3 Massachusetts USA; and Diagnostic Ultrasound Associates, PC, Boston, Massachusetts USA (B.B., We have developed 2 scanning techniques that T.D.S., M.A.M., B.R.B.). Revision requested March 15, enhance the chances that an adequate NT measurement 2010. Revised manuscript accepted for publication will be obtained within the 20-minute time window. March 17, 2010. Guest Editor: Alfred Z. Abuhamad, MD. These techniques are based on approaching the uterus Address correspondence to Bryann Bromley, (and fetus) at right angles from the initial orientation, MD, Diagnostic Ultrasound Associates, PC, 333 Longwood Ave, Suite 400, Boston, MA 02115 USA. thus increasing the chances that the fetus will be imaged E-mail: bbsono@aol.com in the midsagittal view. © 2010 by the American Institute of Ultrasound in Medicine • J Ultrasound Med 2010; 29:1261–1264 • 0278-4297/10/$3.50
  • 2. Nuchal Translucency Measurement on the Fetus in a Difficult Position Scanning the Patient Sitting Upright surement when the midsagittal view is unachiev- Measuring the NT requires a perfect midsagittal able in the standard supine maternal position view of the cervical and thoracic portions of the (Figure 1). This is particularly helpful in obese fetus with visualization of the nasal tip as well as patients because the scan window when the the third and fourth ventricles of the fetal brain. patient is upright rises above the panniculus. The goal of the scan is to obtain this midsagittal view no matter what the fetal position. In cases in The Simultaneous Transvaginal and which the fetal or uterine position precludes a Transabdominal Technique midsagittal view when the mother is in the stan- The second technique involves scanning both dard supine position, sitting her up at 75° to 90° transvaginally and transabdominally simultane- results in a change in orientation of the uterus, ously. Although scanning transvaginally alone thus allowing the ultrasound beam to enter the will sometimes result in a midsagittal view, often uterus from the fundus. The transducer is placed the fetus has spun around, and the NT is still not just under the umbilicus and angled caudad to measurable transvaginally. While scanning trans- image the fetus at 90° from the original position abdominally, we use the endovaginal probe to when the patient was supine. This change is stabilize the uterus or to push the uterus toward often enough to obtain an adequate NT mea- the anterior abdominal wall (this works especial- Figure 1. A, Upright positioning of the patient and transducer when using the “sitting-up” technique. B, Fetus in the coronal plane, in an awkward position for the NT measurement when the patient is supine. C, Same fetus when the patient is sitting up and the ultrasound beam enters the uterus from the fundus. Note that the fetus is now seen in the midsagittal plane. D, Fetus in the mid- sagittal position showing the position for the NT measurement and caliper placement. A B C D 1262 J Ultrasound Med 2010; 29:1261–1264
  • 3. Bromley et al ly well if the uterus is retroverted). Depending on midsagittal view of the fetal head and neck is not the fetal movement and position, the sonologist always obtainable with the patient in the stan- or sonographer can toggle back and forth dard supine position. Additionally, a recently between the transabdominal and endovaginal emptied bladder may contribute to a less than images, displaying views that are oriented at optimal fetal position. We have found that when right angles to each other. Using a simultaneous standard techniques of turning the patient from transabdominal and endovaginal approach, a side to side fail, placing the patient in a sitting midsagittal orientation of the fetus should be position will provide a new view into the uterus, achievable in most cases (Figure 2). at 90° from the initial attempt. If this maneuver does not suffice, the sonographer can then try the Conclusions transvaginal approach. If the appropriate fetal The success of fetal sonographic NT measure- image is still not obtained, the patient should be ment depends on our ability to view the fetus rescanned transabdominally while the vaginal from multiple directions because the perfect probe is still in place and can be used to manipu- Figure 2. A, Positioning of the patient and transducer for the simultaneous transvaginal and transabdominal technique. Note that by angling the vaginal probe down, the uterus can be pushed up toward the abdominal probe. B, Transabdominal image of a first- trimester fetus referred for NT measurement. Note that the fetus is very deep within the maternal pelvis because of a high maternal body mass index. C, Transvaginal imaging of the fetus provides a much clearer view. However, the fetus is not in a midsagittal view; therefore, the NT cannot be measured. D, Transabdominal scan of the same fetus where the vaginal probe is in situ and is being used to push the entire uterus closer to the abdominal wall, thus resulting in a much clearer image. Note that the NT is easily seen and measured, and the nasal bone is also identified. A B C D J Ultrasound Med 2010; 29:1261–1264 1263
  • 4. Nuchal Translucency Measurement on the Fetus in a Difficult Position late the orientation of the uterus. We use these imaging techniques in fetuses where the standard transabdominal approach is unsuccessful in obtaining a perfect midsagittal view. Previously, these patients would have required another eval- uation at a later time. Since introducing this scan- ning protocol into our practice, we have been able to obtain an NT measurement successfully with- out a need for repeat scanning in the last several hundred patients. This brief communication is simply meant to introduce these techniques to help the practitioner obtain an NT measurement when the fetus is in a difficult position. We did not address how often these techniques were required to obtain an NT measurement. A prospective study is needed to determine the frequency with which each of these techniques is used in a large clinical practice. References 1. ACOG Committee on Practice Bulletins. ACOG Practice Bulletin No. 77: screening for fetal chromosomal abnor- malities. Obstet Gynecol 2007; 109:217–227. 2. Wax JR, Pinette MG, Cartin A, Blackstone J. The value of repeated evaluation after initial failed nuchal translucency measurement. J Ultrasound Med 2007; 26:825–828. 3. Thornburg M, Mulconry M, Post A, Carpenter A, Grace D, Pressman EK. Fetal nuchal translucency thickness evalua- tion in the overweight and obese gravida. Ultrasound Obstet Gynecol 2009; 33:665–669. 1264 J Ultrasound Med 2010; 29:1261–1264