Lung-to-Head Ratio. This sonographic ratio was developed to improve prediction of survival in fetuses with isolated leftsided CDH diagnosed before 25 weeks' gestation (Metkus, 1996). he lung-to-head ratio (LHR) is a measurement of the right lung area, taken at the level of the four-chamber view of the heart, divided by the head circumference (Fig. 10-23, p. 198). Investigators found that the survival rate was 100 percent if the LHR wasi> 1.35, and there were no survivors if it was <0.6. Nearly three fourths of pregnancies had values between 0.6 and 1.35, and prediction was diicult in this large group because the overall survival rate approximated 60 percent (Metkus, 1996).
As of 2017, trials underway have selected a threshold LHR of < 1.0 or an observed-to-expected LHR <25 percent for study inclusion. An observed LHR is obtained sonographically from the afected fetus, whereas rhe expected LHR is an established reference value from normal fetuses (Peralta, 2005). In a recent metaanalysis, the odds ratio for survival with an LHR < 1.0 was only 0.14 (Oluyomi-Obi, 2017). Similarly, with an observed-to-expected LHR <25 percent, survival rates ranged from 13 to 30 percent. In contrast, an observed-to-expected LHR > 35 percent was associated with survival rates ranging from 65 to 88 percent.
Magnetic Resonance Imaging. This has been used to estimate the total volume of lung tissue, both ipsilateral and contralateral to the diaphragmatic hernia, which may then be compared with a gestational age-matched reference. Mayer and coworkers (2011) performed a metaanalysis of 19 studies involving more than 600 pregnancies in which isolated CDH was evaluated with feral MR imaging. Factors signiicantly associated with neonatal survival included the side of the defect, total fetal lung volume, observed-to-expected lung volume, and fetal liver position.
Fetal MR imaging has also been used to quantiy the volume of herniated liver (Fig. 10-57, p. 219). Two reasons underlie the rationale for assessing liver volume. First, liver herniation is perhaps the strongest predictor of outcome in fetuses with isolated CDH. Second, liver volume might be a more reliable predictor because lungs are inherently more compressible than liver. Indeed, these MR parameters-lung volumes and degree of liver herniation-correlate well with postnatal survival rates and may be more useful predictors than sonographic parameters (Bebbington, 2014; Ruano, 2014; Worley, 2009).
Tracheal Occlusion. Early attempts to treat severe diaphragmatic herniation used open fetal surgery to reposition the liver into the abdomen, which unfortunately kinked the umbilical vein and led to fetal demise (Harrison, 1993). Knowledge that lungs normally produce luid and that fetuses with upper airway obstruction develop hyperplastic lungs formed the rationale for tracheal occlusion. The idea was to "plug the lung until it grows" (Hedrick, 1994). Initial eforts focused on occluding the trachea with an external clip (Harrison, 1993). Subsequently, a detachable silicone balloon was placed within the trachea endoscopically (Fig. 16-6).