Do we really need a screening test for open spina bifida?

Abstract

In a recent issue of the Journal were published a series of articles1-4 and an Opinion5 regarding possible approaches to early diagnosis of open spina bifida (SB) by way of a screening examination of the fetal brain posterior fossa, specifically the cerebral fourth ventricle, as imaged as an intracranial translucency (IT) between the brain stem and choroid plexus. Each of the studies applied a slightly different imaging approach and varying biometry to the target area, with the aim of flagging up suspected cases or ruling out SB during the late first trimester of pregnancy. The WHO defines certain criteria for screening tests6. The disease must be of sufficient prevalence and severity and should have a fixed spectrum of symptoms, the screening method should be simple and acceptable, screening should be accurate, confirmation and follow-up should be available, the disease should be treatable and screening should show improved outcome and a positive cost/benefit ratio. First and foremost, therefore, a screening test is performed because the definitive diagnostic test is too invasive, too expensive, too difficult technically, or otherwise unacceptable or unavailable to the general population. Screening acts as a ‘sieve’ (hence its name): screen-positive cases are caught in the sieve and are referred for further examination, i.e. either a definitive diagnostic test or more invasive, expensive or technically challenging screening. Clearly, prenatal diagnosis of SB as an entity fulfills the criteria of a target lesion for a screening test. SB prevalence is approximately 1:2000 and each case represents a significant cost to society in long-term care; it is treatable, i.e. avoidable, by pregnancy termination, or early treatment in-utero may ameliorate its damage. However, intrauterine repair of SB is performed only during the second half of the second trimester, at 23–26 weeks of gestation7, 8. Thus, whether the lesion was diagnosed in the first or early second trimester is irrelevant. The advantage of early diagnosis is only in earlier provision of pregnancy termination, when this is the parents' wish. Does screening for fetal IT, as an indicator for SB, fulfill the criteria of screening tests? Is it easier, more accurate or more acceptable to patients than is direct ultrasound scanning of the fetal spine at 11–13 weeks' gestation for diagnosis of SB? The authors of the various papers that appeared in the Journal did not show that their scanning method increased screening performance, was easier to perform or was more acceptable to patients compared with direct visualization of the fetal spine. What advantage, then, does the method confer? According to the authors, five or seven specified reference points must be imaged to visualize satisfactorily the IT, and all authors reported low success rates for satisfactory imaging of IT as well as low sensitivity of the test. No comparison was made with transvaginal imaging of the spine. It would seem that if image quality is sufficient to allow for IT imaging and measurement, direct imaging of the fetal spine would be possible, whether by transabdominal or transvaginal route. At these gestational ages, with the rapid changes of fetal position, or with simple bimanual manipulation, the whole spine can be visualized and SB or meningomyelocele can be diagnosed. In our center we have been performing late first- and early second-trimester targeted fetal organ scanning by the transvaginal route for about 20 years. Examination of the fetal spine is of course an integral part of this examination. During this period we have scanned about 15 000 fetuses and diagnosed about 20 cases of open SB, while we have not missed any; since this anomaly is subject to national registration, and because of the medicolegal climate here, we would be informed promptly if an undiagnosed case were to be delivered, whether in our hospital or elsewhere. In their recent Opinion5 Profs Chaoui and Nicolaides claim that the transvaginal approach is not acceptable for screening of SB. However, in the pyramid of prenatal diagnosis put forward by Nicolaides9, the first-trimester screening test includes transvaginal ultrasound to evaluate cervical length, and transvaginal ultrasound has also long been applied to uterine artery pulsatility index measurement. An overwhelming majority of women agree to undergo transvaginal ultrasound examination. We maintain that if women agree to transvaginal sonography for cervical length or uterine artery Doppler measurement, they will surely agree to it to exclude open SB. The early diagnosis of open SB can be achieved confidently by transvaginal scanning of the fetal spine as part of the 11–13-week scan, after measurement of uterine artery Doppler flow and cervical length measurement. A separate screening test for a marker for open SB is not necessary. Y. Gadot*, S. M. Cohen*, S. Yagel*, * Department of Obstetrics and Gynecology, Hadassah–Hebrew University Medical Centers, Jerusalem, Israel