The underlying lesion is cystic mesangial necrosis. There is localized fracture or necrosis of the elastic fibers of the middle layer of the artery and mucus-like and cystic formation of the stroma. Splits in the interstitial layer often occur in the ascending aorta, which experiences the greatest blood flow impact, while the distal part of the aortic arch has fewer and less severe lesions. The aortic wall splits into two layers with accumulation of blood and clots between them, where the aorta is significantly enlarged and pyknotic or cystic in shape. If the lesion involves the aortic annulus, the annulus enlarges and causes aortic valve insufficiency. The lesion may extend distally from the aortic root as far as the iliac and femoral arteries, and may also involve branches of the aorta, such as the innominate artery, common carotid artery, subclavian artery, and renal artery. The coronary arteries are generally unaffected, but the clot in the aortic root can have a compressive effect on the coronary artery opening. Most clots have transverse fissures in the origin of the intima, often located above the aortic valve, and the fissures can also be in two places, with the clot communicating with the aortic lumen. In a few cases, the endothelium is intact without fissures. In some cases, the outer membrane ruptures and causes hemorrhage. The rupture is all in the ascending aorta, and the bleeding can easily enter the pericardial cavity, or the mediastinum or the thoracic cavity if the rupture site is lower, or the mediastinum, thoracic cavity or retroperitoneal space if the rupture site is lower. DeBakey classifies aortic coarctation into three types: type I coarctation starts from the ascending aorta and extends to the descending aorta, type II coarctation is limited to the ascending aorta, and type III coarctation starts from the descending aorta and extends distally. In addition, Daily and Miller divided aortic coarctation into two types: Type A (including DeBakey type I and II) for those with ascending aortic involvement and Type B (i.e., DeBakey type III) for those with lesions in the distal opening of the left subclavian artery, with Type A accounting for about 2/3 of all cases and Type B for about 1/3.