Coarctation of the Aorta.

Given that the aorta is formed mostly from the branchial arch 4 on the left side and continuity on the right side is interrupted, it is no surprise that some process of narrowing may occur at the same spot on the left side. The arteries to the head and neck from left branchial arch 3 are usually absorbed into the aortic arch. On the right side merging remnants of right arch 3 and 4 form the innominate artery.

The abnormality of coarctation is expressed in two locations:

1. Preductal: The majority of these present in infancy but the postductal type (perhaps as many as a third) may also be found at this age. Other abnormalities may occur with the preductal coarctation and the associated shunts may have hastened their detection.
2. Postductal: The majority of adult presentation are of this type, often unsuspected until hypertension alerts the physician. Given the lack if clinical features before their detection, the logical will deduce that coexisting major cardiac abnormalities are much much rarer in this group, (except for a high incidence of bicuspid aortic valve).

Since the lower limbs still require perfusion, coarctation of the aorta involves collateral flow through the subclavian vessels, thoracic branches of the axillary artery, internal mammary arteries and upper intercostal arteries back to the more distal part of the descending aorta. As anyone who has blown-up a tubular party-balloon can attest, any increase in diameter of a vessel will also increase its length. Tortuousity of the dilated intercostal vessel results in notching of the posterior borders of the upper ribs, but not 1st and 2nd ribs, since those intercostals arise from the thyrocervical. The notching is only rarely seen in cases below 5 years of age. Asymmetry of rib notching may indicate an additional anomalous origin of left or right subclavian artery. The renal response to relative underperfusion results in hypertension in the upper vessels. There is an association with a bicuspid aortic valve (look for calcification), but aortic stenosis is most frequently found in Turner's syndrome.

A flat aortic arch may be a plain-film indication of an abnormal left subclavian artery, in which case the rib notching may be more obvious on the normal side. Incidentally, the collateral circulation in response to a proximal subclavian artery block is interesting, including thyro-cervical and vertebral arteries (even the circle of Willis in subclavian steal syndrome).

As with any stenosis, the associated turbulence may give rise to a post-stenotic dilatation in the descending aorta that may be seen on the Chest X-ray.