How is the diagnosis of a congenital diaphragmatic hernia (CDH) made sonographically, and what features and measurements are important?
CDH is suspected if:
- • The fetal stomach is not seen in the fetal abdomen and is evident at the level of the heart on a transverse image of the fetal thorax
- • The stomach is not at the heart level, but the heart is shifted to one side (usually the right) with bowel in the chest on the other side (usually the left). Most CDH are left-sided, which are 5 to 10 times more common than right-sided hernias.
- • CDH is associated with a number of other fetal anomalies, and so the ultrasound examination is also directed to detect any of these. Cardiac abnormalities are the most common association, and if fetal echocardiography is not performed as part of the evaluation of the fetus with anomalies, it is performed if a CDH is found.
Sagittal scans of the fetus can usually depict the defect in the diaphragm, usually posteriorly on the left (a Bochdalek hernia). With color flow Doppler US, the presence of bowel and liver in the chest can be demonstrated with the superior mesenteric artery (SMA), SMA branches, hepatic veins, portal veins directed from or into the chest. An important aspect of CDH evaluation is to determine whether or not part of the liver is intrathoracic in location. Color flow Doppler US is again helpful to show one (or more) of the hepatic veins turning cephalad and above the plane of the diaphragm. Gray scale imaging can show the intrathoracic portion of the liver in continuity with the intra-abdominal portion. Real-time imaging can be helpful to demonstrate peristalsis in the intrathoracic bowel loops.
If a part of the fetal liver is in the thorax, the prognosis is generally worse because presence of an intrathoracic portion of liver is a sign of a large hernia. To quantify the effects of the hernia on the fetal lungs, a metric has been developed that uses measurements of the contralateral lung at the level of the four-chamber heart view. There are several approaches to making this measurement, but the method in widest use is to measure the largest transverse diameter (in mm) of the contralateral lung at the base and parallel with the coronal plane of the fetal chest. An anteroposterior diameter is then measured perpendicular to this (also in mm). The resulting values are multiplied, and then divided by the HC (in mm). This value is known as the lung to head ratio (LHR). Some studies have shown the LHR to be predictive of outcome with high survival rates when the LHR is >1.4. In an attempt to normalize the LHR for fetal lung growth, the LHR of the affected fetus is compared with the LHR of a normal fetus at the same age. Tables of LHR values for both right and left lungs are available, and the observed to expected (O/E) LHR ratio is calculated, which is an index of lung hypoplasia. Studies have shown that O/E LHR ratios <15% have a 100% fetal mortality rate. If the O/E LHR ratio is >45%, the survival rate is above 75%. In between these values, survival rates increase with increasing O/E LHR ratio, with the 50% survival proportion correlated with an O/E LHR ratio of ≈38%.