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Letters to the Editor |
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Fred E. Avni, MD 
, For the GRRIF Group (Groupe Radiopédiatrique de Recherche en Imagerie Foetale [Pediatric Radiology Research Group on Fetal Imaging])
* Department of Pediatric Radiology, La Timone Children Hospital, 264 Rue Saint Pierre, 13385 Marseille Cedex 5, France
e-mail: guillaume.gorincour{at}ap-hm.fr
Department of Pediatric Radiology, Charles Nicolle Hospital, Rouen, France
Department of Imaging, Erasme Hospital, Brussels, Belgium
We read with great interest the article by Dr Cannie and colleagues (1), in the December 2006 issue of Radiology, on body and lung volumes in fetuses suspected of having pulmonary hypoplasia, and we appreciated the authors' usual rigorous methodology. However, their results raise questions that need to be addressed.
First, three fetuses were included in the "at risk" population because they had congenital cystic adenomatoid malformation, which we believe is misleading. The lungs do not experience the same risk of hypoplasia with this condition as with congenital diaphragmatic hernia (CDH) or early oligohydramnios (2), which is confirmed by the authors' observed-expected ratios. A bias is obviously introduced, since the sample size is still small.
Second, larger studies are needed to demonstrate that magnetic resonance (MR) imaging is superior to ultrasonography (US) in the prediction of fetal weight. We also believe in the future of fetal MR imaging, yet US still has some advantages—namely, that there have been 40 years of research in US evaluation. The results would have been more convincing if the authors had given the MR imaging and US fetal weight estimations with postnatal correlations.
Third, the prognostic value of MR imaging was strongly enhanced by morphologic US information (particularly the liver position). In table 1, if the liver position is not considered, among the four fetuses with ratios of less than 35%, two are alive and well. This definitely hampers the value of these results. Moreover, the definition of liver position was not clearly defined; this surely can be difficult to assess by means of US alone.
We conducted a 4-year prospective multicenter study on fetal relative lung volumes (3), independent of liver position and lung-to-head ratio. Seventy-seven fetuses with isolated left CDH underwent MR imaging with fetal lung volume measurements that were compared with normal values (4). Independent of any parameter, the measured-expected fetal lung volume ratio was significantly lower in newborns who died compared with those who survived (23.6 vs 36.1, P < .001). When the ratio was less than 25%, there was a significant decrease in postnatal survival (19% vs 40.3%, P = .008). Survival was significantly lower when one lung could not be seen at fetal MR imaging (17.9% vs 62.1%, P < .001).
We are convinced by the authors' hypothesis that lung volume will be predictable by means of MR imaging of fetal body volume. However, we think that prediction of outcome needs to be studied with larger and more homogeneous study groups. Until then, "simple" US and MR imaging parameters can help us better inform future parents facing this ever-challenging situation.
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Frederik De Keyzer, MD *,
Steven Dymarkowski, MD, PhD *,
Jan A. Deprest, MD, PhD
* Department of Radiology, University Hospital Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium
Department of Obstetrics and Gynaecology (Fetal Medicine Unit), University Hospital Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium
e-mail: Steven.Dymarkowski{at}uz.kuleuven.ac.be
We appreciate the comments by Dr Gorincour and colleagues. Although they are partly justified, in our opinion they miss the point of our study (1). Our aim was the use of a single biometric marker in the prediction of relative fetal lung volume. We have deliberately chosen a parameter available at the time of MR imaging. Since all data were collected during a single examination, we opted for the term "simple" to highlight the contrast with earlier algorithms (2,3). Next to liver volume and gestational age, fetal body volume showed the best correlation with total lung volume. This was no surprise since it has been shown that fetal body volume correlates well with birth weight, the latter being more accurately determined with MR imaging than with US (4,5). In conclusion, in our study we did not aim to compare two imaging modalities that are, to the opinion of our multidisciplinary fetal assessment group, supplementing one another (6).
We included any patient with potential decreased lung development, irrespective of its degree. In other words, our study was not set up to determine cutoff values or predict lethal lung hypoplasia for specific disorders. We proportionally had more fetuses with CDH, which is a complex and hard-to-determine disease. Moreover, predictive studies all experience huge differences in prenatal selection and postnatal management (7). Apart from that, we agree that the limiting factor for neonatal survival seems to be lung size. In the study by Dr Gorincour and colleagues (8), beside having larger numbers of fetuses, their findings did not add to what has already been published by others (9–11). However, the above does not prove the hypothesis that lung size is the sole predictor of survival. For instance, in the study by Dr Gorincour and colleagues (8), total lung volume could be determined in only 64% (49 of 77) of patients. Further clinical relevance remains limited, as mean gestational age was as late as 31 weeks, and a 25% cutoff level predicted a two in 10 rather than a four in 10 chance of survival.
We agree with Dr Gorincour and colleagues that no efforts should be spared to obtain large numbers with identical lung conditions. Our study contained only eight expectantly managed cases of CDH, measured early in the third trimester at the latest, at which point all options are still open. As expected, lung size was correlated with postnatal outcome but only in the presence of liver herniation. This so far biologically unexplained observation is in concordance with findings of a previous US study in 184 fetuses with isolated CDH assessed between 24 and 28 weeks gestation (12). Although the latter study is, predictive-wise, the most powerful study at present and the findings are reliable toward patient counseling, lung hypoplasia is a "moving target" that needs further research in order to accomplish the needed improvement.
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