Coarctation of aorta.
Coarctation of aorta was first described by Morgagni in 1760 and still engages the attention of the workers in the field of cardiology as its pathogenesis is still not well understood. From the reported incidence, it appears to show significant geographic variation. Its incidence in the Western countries is around 4-5%. Series on incidence of congenital heart disease from South-East Asian countries show variable incidence. The incidence in Singapore as reported by Muir is 4.6%. In Japan, the reported incidence varied between 0.3%-1.5%. Ongley mentioned non-existence of coarctation of aorta in Thailand.
Bonnet, in 1903, first classified this lesion into infantile or foetal and adult types. The foetal type is characterised by narrowing of a long segment of aorta proximal to the ductus, persistence of the ductus and death in early infancy due to congestive cardiac failure. The adult type, on the other hand, consists of sharp narrowing of aorta in the juxtaductal area usually accompanied by closed ductus and is manifested in later life. This classification has not yet been replaced by any other satisfactory classification though several have been put forth.,  Edwards et al, in 1948, suggested the term tubular hypoplasia for foetal coarctation while reserving the term coarctation, for adult type. Some workers feel that patients are best classified into clinical groups without regard to the anatomical situation. It was, therefore, thought worthwhile to analyse the incidence in our material and to examine the validity of classification.
Forty two cases of coarctation were recorded in the cardiovascular pathology section of the King Edward VII Memorial Hospital during the period 1959-1981. Detailed anatomic features of these cases were noted. In 10 cases of foetal coarctation where the lesion was isolated and the specimens were available, the circumferences of the ascending and descending aortae, coarctate segment, pulmonary artery and the ductus were measured.
There were 34 cases of foetal coarctation,  cases of adult coarctation and  cases with a combination of foetal and adult types. These 42 cases of coarctation were seen in a series of 906 congenital heart diseases giving an incidence of 4.6%. The incidence of adult type of coarctation is very low, as compared to the foetal type.
Age and sex distribution
There is a male predominance with a ratio of 1.8:1. The age incidence is shown in [Table - 1]
Nearly 8O% cases of foetal coarctation did not survive the first year of life, with more than 50% succumbing in the neonatal period. However, a few managed to survive the adulthood. All the cases of coarctation were over the age of 5 years. Only 2 cases of adult coarctation manifested in the paediatric age group. The maximum survival was 33 years in foetal coarctation and 40 years in adult coarctation. There were only 2 cases where a combination of foetal and adult patterns was seen, one in early infancy and the other in early adulthood.
This is of various types:
Foetal coarctation [ Fig. 1]
The coarctate segment was proximal to the ductus in all the cases. In 70°Jo cases the narrowing was seen between the left subclavian artery and the entry of the ductus. In the remaining cases the hypoplastic segment was extending from the left common carotid to the entry of the ductus. The length of the hypoplastic segment varied from 6-10 mm in the former group and 15-24 mm in the latter group.
Quantitative measurements done on  hearts, which were not accompanied by any other anomaly, are given in [Table - 2]
The circumference of coarctate segment is compared with that of ascending aorta and expressed as a percentage narrowing. In 6 cases, the pulmonary artery was larger than the aorta and in 2 cases it was only slightly smaller than the aorta. In 6 cases, the ductus was larger than the coarctate segment. As compared to the ascending aorta, 30-50% narrowing was seen in the coarctate segment in 3 cases. In 6 cases the narrowing was 50-70%. In one case it was more than 70%. The hypoplastic segment was more or less of uniform diameter. A ridge was seen at the entrance of ductus in 35% of cases [ Fig. 2].
Surgical correction was done in three out of the  cases of adult coarctation. The resected coarctate segment in these cases showed localised narrowing with a lumen, 2-3 mm in size. In the non-operated cases also the narrowing was localised with a lumen varying from 2-4 mm. The coarctation was situated in the juxtaductal region in  cases. In the remaining case it was in the abdominal aorta below the origin of inferior mesenteric artery.
Combined foetal and adult coarctation There were  such cases in both of which the adult coarctate segment had been resected and the proximal hypoplastic segment was left behind.
Ductus is an essential part of the complex of foetal coarctation as the pattern of foetal circulation continues in these cases after birth. Of 34 cases of foetal coarctation, ductus was completely obliterated only in 3 cases which reached adulthood. In 10 cases, the ductus was undergoing obliteration, while in remaining cases it was widely patent. In all cases of adult coarctation, ductus was obliterated. In one of the cases of combined foetal and adult coarctation ductus was patent and was ligated.
Associated cardiac anomalies
Inconsequential lesions like bicuspid aortic valve and left S.V.C. were seen in  and  cases respectively. Major cardiac anomalies were seen in 22 cases of foetal coarctation and one case of adult coarctation [Table - 3] The commonest associated anomaly was secundum type atrial septal defect, noted in 7 cases. The first six defects listed in [Table 3 ] result in increased pulmonary flow. The next four considerably reduce the left ventricular output. Both groups reduced the flow through the aorta while increasing the flow through the pulmonary artery and ductus into the descending aorta.
Truncus arteriosus usually is associated with enlargement of the aortic arch; however, there are a few cases in which the aortic arch is hypoplastic with dominance of the derivative of 6th arch. Association of foetal coarctation with transposition, though rather difficult to explain, has been noted in most of the series.
One case of adult coarctation was associated with aortic valvular stenosis. In rest of the cases of adult coarctation as well as in cases with combined foetal and adult coarctation, there was no associated intracardiac defect.
These were seen in 4 cases of foetal coarctation. In 3 cases, imperforate anus and in the remaining one, tracheo-oesophageal fistula were present.
The incidence of 4.6% seen in our material is comparable to the incidence in Western countries. The incidence of foetal coarctation is high as compared to adult type. This is expected as mortality in foetal coarctation is very high at early age. Diagnosis as well as surgical correction at this stage is difficult in our country due to lack of facilities. Associated complicated cardiac anomalies also contribute to the fatal outcome. Adult coarctation which presents at a later age is easily corrected by surgery. The incidence of adult coarctation in this series therefore does not reflect the true incidence of this anomaly.
The present study shows that the two types of coarctation present a distinct clinico-pathologic picture and hence should not be lumped together. Only 20% cases of foetal coarctation survived first year of life. Associated intracardiac anomalies were seen in 22 cases with severe noncardiac anomalies in 4 cases. Ductus was patent in 93% cases. All the cases of adult coarctation presented after the age of 6 years and associated significant intracardiac defect was seen only in one case. Ductus was not patent in any of these cases.
Coexistence of foetal and adult coarctation was seen only in two cases out of 42. This is in contrast to 50'% incidence reported by Becker et al. Sinha et a1, in their clinical study, noted the frequency as 75%. In 35% of our cases of foetal coarctation, a small intimal ridge was seen at the entrance of the ductus. It was difficult to say whether this was obstructive or not as none of these were catheterized. There was no corresponding constriction on the external surface of the aorta which would suggest that the lesion represents incipient adult coarctation. Ho and Anderson found such a ridge in 50% of their cases. Lev et a1 have postulated that foetal coarctation will get converted into adult type if the infant manages to survive. This hypothesis has not yet been substantiated by temporal studies. The pathogenesis of the two also appears to be different though still not well understood.
The Skodaic hypothesis of coarctation propounded that coarctation is due to extension of obliterating ductal tissue in the aorta. Histologic studies by Edward et al and Clagett et al did not support this hypothesis. Studies of Wielenga and Dankmeijer as well as Ho and Anderson show extension of ductal tissue in the aorta causing an obstructive lesion. Ho and Anderson feel that ductus and descending aorta form a continuous channel in which aortic isthmus enters and that coarctation, adult as well as to a certain extent foetal is due to defective junction of the 4th and 6th aortic arches.
Rudolph et al have proposed haemodynamic explanation for the development of foetal coarctation on the basis of flow pattern in the aorta. In normal foetal circulation, only 16% blood passes through isthmus as against 60 through the descending aorta, which keeps the isthmus hypoplastic in foetal life. Cardiac anomalies which are associated in large number of cases of foetal coarctation are the ones which either lead to increased flow through the pulmonary artery or restrict the flow through the aorta, thus causing further reduction in isthmic flow. It is not found in association with anomalies which reduce pulmonary flow. The present series also show similar findings except that the commonest defect was A.S.D. rather than V.S.D. as reported in most of the series.
However, this hypothesis does not explain the cases of foetal coarctation which are unassociated with any intracardiac anomaly. In our series, one third of cases had foetal coarctation as an isolated anomaly.
Apart from cardiac anomalies extra cardiac anomalies are also common. Malm et al found them in 25% of cases. The commonest noncardiac anomaly found in their study was tracheo-oesophagus fistula; in our study imperforate anus was the common one.
Thus from our study it is evident that the frequency of coarctation is the same as seen in Western countries. There is need to distinguish the two types of coarctation though the present terminology may need a change. The exact pathogenesis is not clear and is likely to be different in the two types.