This study demonstrates that late gestation fetal MRI–derived TLV may provide useful information for the counseling of patients who have a fetus with an isolated CDH. In this study, a clear association was observed between lower TLVs at 32 to 34 weeks gestation and the need for ECMO support and an increased postnatal mortality. These findings may have clinical benefit, especially in those patients with no previous workup or a workup from an outside facility without the capability of obtaining prognostic measurements such as LHR or PPLV. Although ultrasound-based LHR has been a standard means for assessing the severity of a CDH, studies on LHR have not been performed in late gestation, and the validity of LHR in late gestation has not been established. Lung-to-head circumference ratio may be less helpful in predicting outcomes when obtained in patients presenting later in gestation [2
]. Furthermore, there has been controversy over the accuracy of LHR in assessing the degree of pulmonary hypoplasia and its reliability in serving as a predictive biometric parameter for patient outcomes [6
Fetal MRI has developed an increasing role in the prenatal evaluation for isolated CDHs. The initial description of the use of fetal MRI–derived lung volumes in CDH was reported by Mahieu-Caputo et al [15
]. Subsequently, Barnewolt et al [14
] coined the term percent predicted lung volume
(PPLV) and correlated the PPLV with ECMO use and postnatal mortality. The PPLV was calculated by obtaining the predicted lung volume (subtracting the mediastinal volume from the total thoracic volume) and then dividing the total calculated lung volume (right and left) by the predicted lung volume. The study demonstrated that patients with a PPLV of less than 15% had an increased need for ECMO support and a higher mortality. Although this is an elegant way to assess pulmonary hypoplasia, it relies on multiple measurements of mediastinal structures, total thoracic lung volume, as well as individual lung volume, whereas other studies have focused specifically on TLV to predict postnatal outcomes [11
]. Kilian et al [11
] and Busing et al [18
] have reported a significant association between TLV and postnatal survival. Busing et al [18
] further concluded that the absolute TLV was an accurate predictor of ECMO use and postnatal survival.
The results of the present study support the findings of these previous studies. In this study, we used an absolute TLV without any further calculations to correct to normative data or adjust for gestational age. The ROC curves demonstrated that the sensitivity and specificity of this absolute TLV is acceptable for allowing prognostic accuracy in determining survival and ECMO use. The TLV was shown to have a significant association between lower TLVs and increased mortality. Survivors had a significantly higher TLV than nonsurvivors, and non-ECMO patients also had a significantly higher TLV than ECMO patients. To better discriminate patient survival based on TLV, we also stratified patients based on TLV ROC curve analysis, which identified the most accurate cutoff TLV at 21 mL. Patients with a TLV of less than 20 mL had a 65% mortality rate, whereas patients with a TLV of greater than 40 mL had a 90% survival. This same trend was also seen with the need for ECMO. This would support the concept that the TLV may reflect the severity of pulmonary hypoplasia and in turn play a role in CDH outcomes and mortality. The final variable that was studied was the correlation between the length of stay and the TLV. The results showed a significant association between the TLV and length of stay (P = .022). When these patients were once again stratified, the group with the TLV greater than 40 mL had the shortest hospital stay with an average of 7 to 8 weeks.
The value of this data and its analysis is the ability to appropriately counsel expectant mothers at our institution in regard to the anticipated duration of the postnatal hospital course and postnatal outcomes. As has been observed with other predictors of survival in CDH, the correlation of TLV with specific rates of survival, need for ECMO, and duration of hospital stay may be institution specific, but general trends should translate to other institutions. The utility of late gestational TLV MRI in our institution is that as a referral center, we often see patients later in gestation without the benefit of testing earlier in gestation (ie, LHR at 24–26 weeks). The use of fetal MRI–derived TLV in late gestation is a tool on which we rely not only in assessing the severity of a CDH and then appropriately counseling new patients but also in confirming the degree of severity of patients who were evaluated earlier in gestation. It has been our clinical impression that late gestation assessment of fetal lung volume has an advantage over earlier assessment because it reflects the degree of lung growth that should have occurred during the third trimester. Congenital diaphragmatic hernia prevents the normal lung growth that should occur during the third trimester; and the more severe the herniation, the less lung growth occurs.
Although this study supports the current body of literature, the results should be examined with an understanding that these are institution-specific findings with potential inherent biases. There are 3 full-time radiologists working in Fetal Imaging who review all fetal MRIs and a single technologist who generates the lung volumes for these studies. This allows for a level of consistency in the generation of TLV. In addition, this is a retrospective study of a quaternary referral patient population, which allows for observational biases, and the sample size is relatively small. Our findings in late gestation fetal MRI–derived TLV should be validated in other centers with fetal MRI capabilities or a multicenter collaboration where larger numbers can be obtained in a prospective fashion. This prospective manner would also allow for direct comparisons of the predictive values of TLV with LHR and PPLV.
In conclusion, the results of this study demonstrate a significant association between late gestation fetal MRI–derived TLV and patient outcomes in cases of isolated CDH. These results may allow for personalized predictive prenatal counseling for expectant mothers whose child has an isolated CDH. In particular, TLV may be used for more accurate counseling in patients who present later in gestation from an outside facility or who have had no previous imaging and allow patients to accurately plan for the duration of the postnatal hospitalization.