Screening the fetal heart

Two papers in this month’s Journal discuss the prenatal detection of congenital heart disease (CHD), albeit from very different approaches. Rasiah et al.1 reviewed the literature for publications about screening for CHD in the first trimester of pregnancy. The authors took a structured approach to the analysis of published data on the topic, in order to pool similar material. This type of analysis not only has the advantage of allowing evaluation of a larger patient group than is contained in any single study, but also demonstrates that the technique under scrutiny works in the hands of more than just a few experts in the field. The papers eventually selected from the literature search were published between 1993 and 2004, representing a study period of 1988 to 2003, which is of significance with respect to both technology and experience. All but two of the studies were targeted at high-risk fetuses. Inclusion criteria were strict and only patients scanned with the aim of cardiac evaluation in the fetus prior to 13 + 6 weeks of gestation were included. There were 1243 patients reported in the 10 studies finally selected, although four studies contributed almost a 1000 of the patients. The authors defined adequate quality of follow-up as postmortem examination or postnatal echocardiography in all fetuses examined. I accept the criticism of mid-trimester fetal echocardiography as ‘inadequate’ follow-up in our study2, but over 20 years’ experience and detailed follow-up data in more than 20 000 of our original series of fetuses have demonstrated that very little is missed by a targeted mid-trimester fetal echocardiogram in our hands. False negatives tend to be minor malformations of little or no consequence, or alternatively late-developing lesions, which, although important, are numerically rare. Improved sensitivity rates were shown in later studies, which is not surprising, as ultrasound technology has continued to advance during the time period studied. In addition, techniques and operator expertise almost invariably improve with time and practice. Rather surprisingly, on the other hand, improved sensitivity rates were shown in the transabdominal rather than in the reported transvaginal studies. This may be because half of the transvaginal group were from publications of more than 10 years ago, and this result might be different were the studies more recent. Or this may be accounted for by the fact that, in general, the transabdominal studies were mainly performed by pediatric cardiologists, with perhaps ‘sharper eyes’ for cardiac defects, whereas the transvaginal studies were mainly conducted by obstetricians. There is no doubt in my mind that transvaginal ultrasound image quality can be much better when directly compared with transabdominal scans in the same patient but, in my opinion, even an experienced fetal cardiologist will never develop sufficient expertise at transvaginal scanning to become good at it. Even in a busy referral unit for nuchal translucency (NT), at the most he/she may do 2–6 high-risk 11–14-week scans a week. Compare this to the obstetrician or sonographer who is doing, perhaps, 10–20 nuchal scans per day transvaginally and therefore has the opportunity to learn and maintain the skills required for this approach. The ideal set-up would be for the pediatric cardiologist to use the skill of the transvaginal sonographer to demonstrate the heart in the 10% or so of patients where transabdominal views fail to provide adequate information for diagnosis. Because of the relatively small numbers, Rasiah et al. were unable to determine from the published material they analyzed whether the accuracy of cardiac diagnosis was greater if the fetus was closer to 14 weeks than 11. However, from personal experience, every day of fetal growth after 12 weeks makes an appreciable difference to the quality and clarity of the ultrasound image, and thus the security of diagnosis. In summary, then, using modern technology, with practice and experience, an accurate diagnosis of CHD can be made by the expert in the first trimester, particularly in the high-risk fetus. Because of the high rate of association of CHD with increased NT – at least 25% of fetuses with major CHD have an increased measurement3, with an increasing incidence the greater the nuchal measurement – this may be a more important high-risk group than any other traditionally considered at increased risk and referred for fetal echocardiography. However, an extremely important caveat must be borne in mind. We have learned from detailed follow-up of secondtrimester scans that a few cases of major CHD develop or progress late in gestation and that even with hindsight, on review, the earlier scan was either completely normal or showed only minor abnormalities. This has been found to apply particularly to aortic4 or pulmonary stenosis and coarctation of the aorta. Dramatic progression of severity between 20 weeks’ gestation and term is unusual; my estimate would be of the order of 1/5000 high-risk scans. At the present time, however, we do not yet know how frequently CHD progresses between 12 weeks and 20 weeks. We have had personal experience of a typical case of pulmonary atresia with intact septum at 20 weeks, which showed a normally sized right ventricle with normal flow into it, and into the pulmonary artery, at 12 weeks, and I have seen a similar case of aortic atresia presented at a meeting (Heling K-S, Chaoui R. unpubl. observ.). In another of our cases, mild ventricular disproportion was