In the adult type of coarctation of the aorta the lumen is constricted by a shelf of fibromuscular tissue that protrudes from the dorsal wall of the aorta at a point near the site of insertion of the ligamentum arteriosum. The intima covering this constricting shelf has been studied using conventional light microscopy and scanning electron microscopy of surgical specimens of coarctation removed at ages varying from 2 to 46 years. Immediately proximal to the stenosis, the intima was found to run in fine longitudinal folds that radiated into the orifice. On the distal side of the shelf the intima was grossly irregular with deep longitudinal and transverse folds that gave it a convoluted appearance resembling the surface of the brain. In adults and older children the intima immediately distal to the orifice was covered with a laminated layer, distinct from the elastica, which often gave positive staining reactions for fibrin. Intimal thickness increased irregularly with age, but large increases of 500 μm or greater were found only in patients over the age of 15 years. These findings are consistent with the view that the constriction has both a fibroelastic component that is congenital and a fibrous component that is acquired and progressive. It is suggested that this progressive element is caused by deposition of fibrin on an abnormal intimal surface in a region of turbulent blood flow. Such a mechanism would explain the occasional recurrence of coarctation of the aorta after surgery. This may be due to failure to remove the abnormal area of endothelium completely and to persistent abnormalities of blood flow at the site of anastomosis.
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