The Coarctation of the Aorta (CoA) is a narrowing of the aorta distal to the origin of the left subclavian artery with or without small or hypoplasia of various portions of the aortic arch and/or isthmus.
CoA constitutes about 8% of all CHDs and the prenatal detection rates for CoA are only 20–35%, among the lowest of all critical CHDs, this means almost 65% to 80% of newborns with isolated coarctation are sent home as “healthy” babies. This low prenatal detection rate is related to the fact that the actual narrowing of the isthmus may not develop until the ductus arteriosus closes postnatally.
Prenatal diagnosis of the CoA is crucial as it can facilitate the selection of the centre for the conduction of delivery of the child and allow the appropriate monitoring and management/surgery if required for the child.
In the absence of the diagnosis and with most centres practising early discharge of baby, it is likely the diagnosis gets missed in the absence of any sign or symptom. In this scenario, the baby may present with mild manifestations, LV dysfunction, or in the worst-case ductal shock.
Pathophysiology of Fetal circulation in Coarctation of Aorta
In a fetus, the aortic isthmus receives approximately 8% of the combined ventricular output. If this flow decreases any further, the growth of the aortic isthmus and distal aortic arch get compromised. That can result in significant undergrowth and thus a narrowing of the isthmus.
Approach to prediction and detection of Coarctation antenatally
With so much morbidity and mortality possible, it becomes consequential to have a sensitive screening tool to suspect CoA. With suspicion of CoA, the baby should be delivered at a higher centre, where the baby should be monitored till the closure/narrowing of the ductus or possibility of CoA is ruled out. So the prenatal parameters are required to be specific as well.
The following parameters are associated with an increased incidence of CoA.
- Ventricular Disproportion
- AV valve z score
- Great artery Z scores and ratios
- Isthmus/Ductal Ratio
- Arch Z scores
- Carotid-subclavian artery index (CSAI)
- Aortic/Ductal angle
- Arch Shelf and Arch Doppler pattern
Ventricular Disproportion
As the term suggests, Ventricle Disproportion means a noticeable discrepancy between the sizes of the ventricles. The routine cardiac screening mandates the display of a 4-chamber view and involves identifying the ventricles and atria. In a normal fetus, the ratio of the Right Ventricle (RV) and Left Ventricle (LV) is near one throughout, in most cases.
In practice, disproportion was noticeable without measurement when there was a 20 % difference in ventricular widths. A ventricular disproportion is a more specific marker in the second trimester, before 25-28 weeks of gestational age, and less in the third trimester. In the third trimester, there can be a degree of physiological disproportion. Thus false positive rate reaches very high, especially after 34 weeks when it is up to 80%. (Stos et al., 2007).
A study of RV and LV sizes and ratios by Rinat Gabbay-Benziv et al (2015) shows that the range of RV to LV ratio increases over time. But it is essential to notice that the mean remains around 1.
- 16 to 28 weeks Mean of 1.03 (range 0.86-1.23)
- 28-33 weeks mean of 1.033(range 0.85-1.26)
- 34-38 weeks mean 1.06 ( range 0.855-1.3)
As per classical teachings an RV to LV ratio of greater than 1.15 (LV to RV of <0.87 or 0.9) is considered abnormal and this holds value as a screening tool for RV enlargement. With these parameters, the positive predictive value is only 33% (Brown et al.). This is still valuable for screening of RV enlargement because it is easy to do, very reproducible and has less inter-observer variability.
The recommendation based on parameter studies and predictive value would be-
- It is a good practice to measure LV and RV width in all fetuses during cardiac screening.
- Any discrepancy between the right and left ventricle size should be noticed.
- The Z scores of the ventricle should also be determined in case of ventricle disproportion, and if the RV z score is more than +2 then RV should be considered dilated.
- If the ratio is more than 1.2 in the second trimester and more than 1.3 in the third trimester then Ventricle disproportion should be considered significant.
- In either case, the causes of dilated RV should be assessed along with other markers of coarctation suspicion.
- Ideally, all cases of Ventricular disproportion should have Fetal echo evaluation.
Ventricle and AV valve Z scores
These parameters and an adjunct to ventricular disproportion, to confirm whether the RV size is large or the ratio is high due to redistribution of flow volumes. Some studies have shown that Z scores may have better predictive value than ratio. The disadvantage of fetal z scores is that they vary significantly with the reference studies and need to be determined from the charts or calculators.
If Z scores are out of range, a fetal echo should be offered to all of them.
Vascular Disproportion
Aorta Pulmonary artery ratio
Like ventricles, the vascular structures that follow the ventricle are in proportion to each other. Their absolute diameters (z-scores) and ratios are used to predict changes in the flows in the respective circuit reflecting the change in anatomy.
These measurements are taken in the three-vessel view (3VV; Yoo et al.), which is obtained by tilting the transducer cephalad from a four-chamber view. A 3VV shows a round cross-section of the ascending aorta & mid superior vena cava and an elongated long-axis of the main pulmonary artery.
It has been shown that the mean and the median of the PA/ Ao ratio is around 1.15 with a range of 0.87-1.58 (95% CI)(Wong et al), for the second trimester. Due to large variation in the normal range and even larger variation in the third trimester this parameter is less commonly used for coarctation screening.
In a series by M. Slodki et al, in 3VV, a ratio of more than 2 is suggestive of type A interrupted Aortic Arch (IAA) and a ratio above 2.6 is highly suggestive of Type B IAA, in the third trimester
Isthmus Ductal Ratio (ID ratio)
The three vessels and trachea view (3VT) are different from the 3VV view in that it shows even more superior mediastinum and leftward towards the meeting point of the ductus arteriosus and the isthmus. 3VT view again obtained by tilting transducer cephalad from a four-chamber view to demonstrate lv outflow first, then the RV outflow and then with slight left rotation tilting further along the axis of the main pulmonary artery.
As shown in the arch diagram, the aortic isthmus is a part of the aorta between the distal end of the origin of the left subclavian artery and the ductus. Isthmus narrowing occurs in approximately 90% of cases of coarctation of the aorta.
ID ratio is of significant interest because it is a relatively duration-independent parameter and it has significantly better specificity and positive predictive value. In the three vessels trachea view (3VT view), the internal diameter of the aortic isthmus was measured immediately proximal to the insertion of the ductus arteriosus and ductus at the distal-most point near insertion to descending aorta, inner margin to inner margin.
Matsui et al. showed that the isthmus to ductal ratio enabled good separation between their two groups. They used the 95% reference range of 0.74 to 1.23 for normal fetuses and an isthmus/ ductal ratio of < 0.74 for all fetuses who required surgery or surveillance after birth.
Study Gómez-Montes showed mean isthmus to the ductal ratio in normal fetuses of >0.7 and fetuses with post-natal coarctation it is <0.6 before 28 weeks and 0.5 in >28 weeks.
A retrospective study by Toole et al stated that an isthmus to the ductal ratio of <0.64 on any single fetal echocardiogram was nearly 90% sensitive and that <0.5 was 100% specific for postnatal coarctation.
Overall isthmus to ductal ratio is a good screening marker. When the ratio is above 0.74 without another additional marker of fetal coarctation, then the possibility of fetal coarctation is low. When the isthmus to ductal ratio is less than 0.74, the possibility of coarctation should be kept in the diagnosis. If the ratio is less than 0.6 in the first and second trimesters and less than 0.5 in the third trimester, the chance of CoA is very high. The parents in this situation should be counselled accordingly and delivery at a centre equipped with a pediatric cardiac unit should be recommended.
Carotid-subclavian artery index (CSAI)
CSAI is defined as, the ratio of the aortic arch diameter at the left subclavian artery(LSA), to the distance between the left carotid artery (LCA) and the left subclavian artery.
In a study by Shanthi Sivanandam et al., the mean CSAI was 0.7 in fetuses with coarctation compared with 1.1 without coarctation (p <0.0001)
In another study by Familiari et al., CSAI of < 0.78 has 92.3% sensitivity and 96.8% specificity.
In a study by Ali Dodge-Khatami et al., the mean distance from the LCA to the LSA in the normal neonate was 2.37 mm and in the neonates with coarctation was 7.32 mm, whereas in infants, the mean distance was 2.67 and 7.27 mm respectively. In their study the CSAI value of <0.67 have a sensitivity of 97.7% and specificity of 92.3% in neonates for coarctation and 94.7% and 100% respectively for infants with coarctation.
So CSAI is a very sensitive and specific marker of prediction for coarctation in post-natal life.
Aortic Isthmus-Ductal Angle (AIDA)
AIDA is the anterior angle measured between descending aorta in the alignment of the isthmus and duct axis in sagittal view.
A retrospective study with prospective control by Quarello et al.(2008) described AIDA
ranges from 128.2° to 167 degrees and in fetuses with coarctation of the aorta from 82.2 to 125°. This means AIDA was significantly smaller in fetuses with coarctation.
A retrospective study by Toole et al (2015) found an angle of < 125° was 81.8% sensitive and 71.4% specific for having postnatal coarctation and if the cutoff decreased to <117° the specificity increases to 96% but the sensitivity drops to 24%.
The AIDA probably implies that the inner angle between the isthmus and duct is higher in coarctation and lesser in normal fetuses, and this could be a good parameter to research, as this angle can easily be measured in the 3VT view. Can simple mathematics of 180 minus outer angle is applied and an angle of more than 65° be used for the detection of the risk of coarctation, need to be determined.
We are not using this parameter regularly as we are getting significant Intra and inter-observer variability. Once the methodology can be standardized, we would like to integrate this parameter as well.
Arch Shelf and Arch Doppler
Coarctation is believed to be caused by a malformation of the aortic media that creates a prominent posterior infolding called the posterior shelf. The presence of posterior-shelf is associated with coarctation in post-natal echo. In the presence of optimal images in a lateral view, a posterior shelf is demonstrable if some cases, during the fetal echo. When a posterior shelf and/or flow disturbance at the isthmus is demonstrable, it was shown to have high predictability for the requirement of surgery after birth (Matsui et al, 2008). In a practical sense, it is the demonstration of the presence of ante-natal coarctation rather than predicting the development of post-natal coarctation.
Associated Lesions
Persistent Left Superior Vena Cava (PLSVC)
PLSVC has been associated with the occurrence of CoA in many prenatal series. There is no causal association has been determined so far and it is shown to be co-existing lesion.
In a meta-analysis by Gustapane et al. of PLSVC in antenatal USG of 13 studies with 501 fetuses with PLSVC, antenatal Coarctation of the aorta was associated with isolated PLSVC in 21.3% and 15.4% have no other sign such as chamber disproportion. A prospective study in the pediatric population by Ghada S E et al has shown the incidence of PLSVC in CHD is 7.8% and has the highest association with Coarctation at 27%.
Thus the detection of PLSVC should always alert the possibility of the presence or development of Coarctation of the Aorta, and evaluation should be done accordingly. Neonatal echocardiography should also be done to rule out post-natal CoA and other associated lesions.
Bicuspid Aortic Valve (BAV)
BAV is the most common CHD(prevalence of 1–2% of all neonates), whereas, only about 7% of patients with BAV have a concomitant CoA. On the other hand, BAV is found in 70–75% of patients with CoA. The incidence of BAV in fetal echo is very low, due to imaging limitations. The diagnosis of bicuspid aortic valve usually does not add to the increase in diagnostic accuracy of diagnosis of CoA, however, if a diagnosis of BAV is made in a fetus with altered ventricle ratio or ID ratio, it increases the possibility of postnatal CoA.
Final Comment:
The Coarctation of the Aorta presents in infancy as critical heart disease and a timely diagnosis can prevent morbidity and mortality. The challenge is that it is a changing spectrum over the antenatal and post-natal life and its progression is variable in different fetuses.
Thus stress should be given to identify signs which can be identified easily and reliably. Ventricle disproportion has good sensitivity to developing CoA and should be used as a screening tool for the referral for fetal echocardiography. I/D ratio is now documented regularly with the addition of 3VT view as a part of 2nd-trimester ultrasound. The CSAI and AIDA have shown excellent specificity in several studies and should be incorporated as a part of fetal echo, at least in cases of suspicion of CoA. Direct demonstration of an aortic shelf and abnormal doppler has an unmatched specificity and these signs show the presence of significant CoA antenatally.
Finally, in cases where the suspicion of CoA is raised, the caretaking neonatal unit should be informed and an early neonatal echo should be done. Sometimes repeated neonatal echo is required, especially when a large PDA during the neonatal echo.
Fetal Cardiac Care 