**2. Pathophysiology**

The pathophysiology of AD involves the breakdown of the intima and/or the media. The initiating event is an intimal tear. Less commonly rupture of the vasa vasorum may be the initiating event. Initial tear is commonly at the site of greatest hydraulic stress which are the right lateral wall of the ascending aorta in about 50–65% of the cases and the proximal segment of the descending aorta (20–30%) [3]. Subsequently, intramural extension of the bleeding both longitudinally and circumferentially causes the separation of aortic wall layers creating a true lumen and a false lumen. A further intimal tear may create a communication between the false lumen and true lumen. The dissection can extend in antegrade or retrograde

**Figure 2.** *Acute aortic syndrome.*

directions from the site of origin leading to complications including acute aortic insufficiency, cardiac tamponade, and organ ischemia and with disruption of the adventitial layer may lead to aortic rupture.

Intramural hematoma (IMH) is characterized by bleeding confined to the medial layer with no intimal tear visualized by current imaging studies.

Rarely ulceration of an atherosclerotic lesion penetrating to the medial layer may give rise to a penetrating aortic ulcer with similar consequences as of AD.

These three conditions, namely, AD, IMH, and penetrating aortic ulcer, are categorized under the broader category of acute aortic syndrome (AAS) (**Figure 2**) [4].
