Layers of the dissected artery
Aortic dissection is the main cause of death in acute aortic diseases, more frequent than ruptured abdominal aortic aneurysms and traumas .
The false lumen enlarges suddenly and can lead to complete or partial compression of the true lumen; therefore, affecting the emergence of the major aortic branches leading to reduce blood supply to brain, abdominal organs (bowels, kidneys, and/or liver) and/or limbs, which is known as malperfusion syndrome . The sudden enlargement might cause pain, which is very common and often described as severe with a ripping or tearing quality [1, 4].
The two mechanisms of ostial obstruction to blood flow are: (1) dynamic obstruction and (2) static obstruction.
Dynamic obstruction is characterized by prolapse of the false lumen’s flap toward the ostium of a major branch during cardiac systole. When the flap touches the ostium, the blood flow for the aortic branch occludes temporally, causing blood flow restriction to the organ. This mechanism is responsible for about 80 % of the total malperfusion syndromes .
Static obstruction occurs when the flap occludes continuously and completely the ostium of the aortic branch, leading to the formation of secondary thrombus inside the branch and distal ischemia (Fig. 16.2).
Ostial flow impairment mechanism of the branches of the aortic dissection which leads to malperfusion. (a) Dynamic obstruction—ostial obstruction with dynamic flow restriction only during cardiac systole. There is no fixed obstruction, as with during cardiac diastole, when the blade (flap) does not obstruct the ostium of the branch; (b) Static obstruction—the blade (flap) obstructs the ostium of the fixed mode branch and leads to secondary thrombus formation in the branch ostium. Subtitles (T true, F false, black arrow blood flow during systole, asterisk thrombus)
Other complication of aortic dissection is the weakening of the arterial wall that can form an aneurysm. In the worst case scenario, disruption of the arterial wall can occur leading to bleeding. It can occur in the pericardial, thoracic, or abdominal cavities . In cases of excessive or persistent bleeding, cardiac tamponade or hemorrhagic shock and death are the final outcomes .
Acute Versus Chronic
Anatomical presentation of the dissection according to the origin of the intimal tear plays a major role in the prognosis and in the choice of the treatment. There are two main classifications for anatomic aortic dissection: DeBakey and Stanford. The most regular used anatomical classification is the Stanford Classification , categorizing dissection into two types: type A, when the injury in located at the ascending aorta and type B when the lesion is located at the descending aorta , as shown in the Fig. 16.3. The Stanford type A dissection comprises about 62.5 % of all cases [1, 2]. This classification is important because a Stanford type A is related with a worse natural history, and the treatment is surgery while the Stanford type B has a better prognosis and can be treated with isolated clinical treatment [1, 2].
Anatomical classification of acute aortic dissection (Stanford a and b). (a) The origin of the dissection is in the ascending aorta. (b) The origin of the dissection is in the descending aorta. Figure copyright by: “Thiago Roberto da Silva”
Hypertension: Hypertension is a major risk factor and plays a crucial role in both increasing the stress on the aortic wall and the subsequent intimal rupture .
Abnormalities of the aortic wall: Diseases such as aortic coarctation (congenital narrowing of aortic segments), aneurysms, and hereditary conditions such as Marfan syndrome and Ehler–Danlos syndrome and chromosomal abnormalities, like Turner and Noonan syndrome predispose the aortic wall to intima rupture [1, 8].
Cardiac disorders : Bicuspid aortic valve is a risk factor for acute type A dissection, due to turbulence from the left ventricle blood jet which stresses against the wall of the ascending aorta .
Cocaine use: Present in less than 1 % of dissections, this simphaticomimetic drug leads to systemic hypertension, vasoconstriction, and increased cardiac output.
Iatrogenic dissection: aortic dissection can occur during endovascular procedures (diagnostic and/or therapeutic) through the manipulation of catheters, guide wires, and endovascular devices .
Atherosclerosis is not a significant risk factor, present in only 31 % of people affected by acute aortic dissection .
Pain has a sudden onset —intense and continuous. It is located in the anterior face of the chest, especially in the type A, or in the back in most cases of type B. However, in some cases, the pain is located in the abdominal area, which raises the suspicion of impairment of mesenteric perfusion. Some patients describe the pain as “stabbing” or feeling “ripped from within” [1, 10].
Cerebral ischemia occurs due to impairment of blood flow to the supra-aortic trunks, particularly the carotid or vertebral arteries. The incidence of stroke is 6 % in type A dissections, with a mortality rate of 9.4–35.3 % within the first 24 h [1, 2, 11], mostly in patients with advanced age, systemic hypertension, and atherosclerosis . The most common clinical manifestation is syncope. Other associated symptoms are changes in sensitivity, strength, and motor function in the contralateral brain hemisphere affected [1, 11].
In addition, arterial expansion may cause extrinsic compression of nerves, causing symptoms such as: (1) pain and paresthesia in limbs; (2) Horner syndrome (supraclavicular involvement of the sympathetic ganglion leading to miosis, ptosis, facial anhidrosis, and enophthalmos); or (3) hoarseness, due to left recurrent laryngeal nerve compression that surrounds the aortic arc [1, 12].
In cases affecting the descending aorta, it may have the involvement of the ostia of intercostal arteries, causing spinal cord ischemia in about 2–10 % of cases .
In type A dissections , rupture can occur in the ascending aorta, most part of which is located within the pericardial sac. Therefore, bleeding inside the pericardial sac (hemopericardium) leads to cardiac tamponade. The patient may present with Beck’s triad: (1) muffled heart sounds, (2) hypotension, and (3) jugular vein engorgement. In these patients, pericardiocentesis is not recommended due to increasing risk of more bleeding and hemorrhagic shock .
The coronary arteries may also be involved causing myocardial ischemia. The electrocardiographic findings are consistent with acute coronary syndrome and can lead the doctor to an inadequate administration of thrombolytic agents or anticoagulants, which worsens the prognosis .
Malperfusion Syndrome and Limb Ischemia
The impairment of the blood flow to the ostia of the major branches of the abdominal aorta can lead to visceral and lower limbs ischemia, occurring in 31 % of patients .
Lower limbs: The most common site of involvement is the left common iliac artery. The clinical presentation is a classical acute arterial ischemia of the limb, with acute pain, absence of pulses, cold and pale skin, and paresthesia and paresis of the foot. Lower limb ischemia is an isolated factor that triples the rate of mortality .
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This is usually the first exam to be requested, but it is nonspecific. Widening of the superior mediastinum is the most common finding and is associated with calcification of the aortic arch  (Fig. 16.4).
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