It begins as an indentation of the posterior wall. The coarctation may be preductal , where the narrowing is proximal to the ductus arteriosus or ligamentum arteriosum. The coarctation may be postductal , where the narrowing is distal to the ductus arteriosus or ligamentum arteriosum. Collateral circulation connects the proximal and the distal aspects of the vessels over time. This collateral circulation will develop mainly from the subclavian, scapular, internal thoracic, and intercostal arteries.
Collateral circulation is divided into anterior and posterior systems.
In the anterior system, the internal mammary arteries and the epigastric arteries join to form collaterals which supply the abdominal wall and the lower extremities. In the posterior system, the parascapular arteries connect with the intercostal arteries to form collaterals which supply the distal aortic compartment and primarily the abdominal viscera. The left subclavian may form collaterals through linkage to the IMA's and intercostals to give blood supply distally. The right subclavian will join with the vertebral, spinal, cervical, and scapular branches, and will eventually provide blood supply to the intercostal circuit. The coarctation of the aorta may also involve the whole contour of the aorta which is known as hypoplasia segmentaria .