The classic clinical presentation of aortic dissection is the acute onset of severe tearing or ripping substernal chest pain with radiation to the back. This occurs in up to 70% of patients. Aortic valvular murmur due to creation of aortic valve insufficiency occurs in up to 65% of patients. Asymmetric pulses, as well as absent femoral pulses (25%), may occur in the upper extremities if the aortic arch is involved. Unfortunately, 15% to 20% of
patients have no chest pain, making the diagnosis difficult. These patients usually present with symptoms related to secondary involvement of aortic branch vessels. Such presentations include myocardial infarction and congestive heart failure, abdominal pain due to mesenteric ischemia, stroke, confusion, coma, or syncope with involvement of the cranial or spinal arteries (25%) (1).

Aortic dissection is a tear of the aortic wall intima, followed by separation of the tunica media, thereby creating two channels for passage of blood. The true lumen is surrounded by intima, and the false lumen is surrounded by media. This is seen in approximately 70% of cases. The tear in the intima allows blood to enter the aortic wall and extend longitudinally along the wall, thus separating the media and forming true and false lumens. The entry point most commonly arises in the ascending aorta within several centimeters of the aortic root or in the descending aorta between the origin of the left subclavian artery and ligamentum arteriosum. Iatrogenically induced aortic dissections arise at sites of aortic cannulation, bypass grafting and cross-clamping, or during catheterization.

There are two widely used systems for the classification of aortic dissections: the Stanford classification and the DeBakey classification.

The methods used for the diagnosis of aortic dissection include catheter aortography (the traditional gold standard) and bedside transesophageal echocardiography. More commonly, noninvasive techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), are used both to establish the diagnosis of dissection and to evaluate the extent of dissection in the aorta and branch vessels.

Chest radiographs are the most commonly used radiologic examination in all acute thoracic conditions. Although findings can indicate the presence of aortic dissection (Table 18.2), a normal chest radiograph does not exclude the presence of an aortic dissection (Fig. 18.1).

CT is probably the most commonly used radiologic examination to diagnose and evaluate aortic dissection. Often, the extent is sufficiently well presented as to avert the need for catheter angiography before surgery and to plan for aortic stent grafting. Fast helical CT scanners accurately and consistently display the aorta and branch vessels (Fig. 18.1), and advanced multiplanar and three-dimensional reconstructions are important to do this. The CT findings of aortic dissection are listed in Table 18.3. The most characteristic finding is the intimal flap. The typical configuration of the flap is a linear filling defect within the aorta. The intimal flap may also have an atypical configuration: dissection of the entire intima creates a circumferential intimal flap, filiform (extremely narrow) true lumen, a three-channel aorta (Mercedes-Benz sign), and several false channels (Fig. 18.2). The dissection flap is more likely to be curved in an acute dissection and flat in a chronic dissection. CT findings that indicate a ruptured type B dissection include irregularity of the aortic wall, extravasation of vascular contrast material, mediastinal or pericardial hematoma, and hemothorax (1) (Fig. 18.3).

Once the diagnosis of aortic dissection is made, the key information needed by the surgeon is the extent of the dissection and whether branch vessels originate from the true lumen or false lumen. Key features that can be used to distinguish between the true and false lumens are listed in Table 18.4 (Fig. 18.4) (6,7).

A limitation of CT is the use of intravenous contrast, for which some patients are known to be allergic and which must be avoided in other patients with acute renal failure secondary to dissection or preexisting renal insufficiency. In such settings, MRI is recommended (Table 18.5).

MRI features are similar to those seen on CT. The intensity of signaling of the false lumen is variable and dependent on the blood flow, as well as the age and composition of thrombus. Standard MRI techniques demonstrate pleural and pericardial effusions, mediastinal hemorrhage, and aortic wall thickening (Fig. 18.5).

At aortography dissection appears as an intimal flap or multiple lumens, with opacification of double channels. Linear radial lucency may be seen secondary to the torn or separated intima, with entry and reentry points. Additional findings include a true lumen compressed by the false lumen, a thick aortic wall greater than 5 mm, ulcer-like contrast projections beyond the true lumen, abnormal catheter position away from the lateral aortic border, and arterial branch occlusion.

This method can image the entire thoracic aorta, with the exception of the distal transverse arch that is partially obscured by the trachea. Transesophageal echocardiography has the advantage of being performed at the bedside in acutely ill or unstable patients. At transesophageal echocardiography, dissection appears as a mobile linear echo within the aortic lumen. Transesophageal echocardiography also provides information regarding ventricular and aortic valve function, both important in surgical decision making.

Aortic dissection that involves branch vessels and compromises blood flow is treated based on the mechanism of occlusion. When the dissection flap extends into the lumen of the branch vessel and statically narrows it, treatment is usually angioplasty, with or without the deployment of an intravascular stent. When the dissection flap does not enter the branch vessel but dynamically occludes the vessel by prolapse of the intimal flap into the vessel origin, this obstructs the true lumen above the branch vessel origin and is usually treated with balloon fenestration of the dissection flap, intraaortic endoluminal stent in the true lumen, or both. Additionally, the compressed true lumen may be enlarged, by using an uncovered intraaortic stent (5,8).

Surgical treatment of type A dissection consist of replacing the ascending aorta, aortic root, and aortic valve, reconstructing the aortic root to restore aortic valve competence, and directing blood flow to the true lumen. The coronary arteries are reimplanted into the graft. Mortality rates for surgical treatment range from 10% to 35% (9). Type B dissections are usually treated medically with aggressive antihypertensive control and pain control. For type B dissections treated surgically, a graft is usually placed in the proximal descending thoracic aorta. Early postoperative complications of dissection include myocardial infarction, stroke, respiratory insufficiency, pulmonary embolism, aortic rupture, pseudoaneurysm, and graft infection. Late complications are noted 10 years or more after surgery in 15% to 30% of patients and require reoperation for dilatation of the dissected region to avoid rupture. Reoperation may be required for progressive reduction of myocardial perfusion due to aortic insufficiency (1).

After the diagnosis or repair of an aortic dissection, follow-up imaging is usually performed annually. Complications that may be noted include an aneurysm of the false lumen, with continuous dilatation of the aorta that may result in aortic rupture. An aneurysm of the true lumen may develop, particularly in older hypertensive patients with advanced atherosclerosis. Obstruction or aneurysm of aortic branch vessels may also occur. Postoperatively, pseudoaneurysms may develop at the anastomosis of the graft to the native aorta, further weakening the aorta and usually requiring additional surgery.

Initial presenting symptoms include chest pain (50% to 74%), intrascapular back pain (44% to 84%), and neurologic or vascular complications such as syncope, transient ischemic attack, hoarse voice, paraplegia, mesenteric ischemia, and acute renal failure (13). Patients are usually older than those with classic aortic dissection, with a mean age of 66 years versus 55 years for classic aortic dissection. There is an equal male-to-female ratio for intramural hematoma, compared with the 3:1 ratio for classic aortic dissection. Risk factors for intramural hematoma include hypertension or previous trauma.

Intramural hematoma can be diagnosed by CT, MRI, and transesophageal echocardiography. At aortography, it may not be detected due to the inability to opacify a false lumen with contrast in the absence of an entrance tear. This often occurs when the intramural hematoma is thin and does not deform the true lumen.

On nonintravenous contrast-enhanced CT, a continuous high attenuation crescent along the aortic wall representing hematoma is usually found without mass effect on the true lumen (Fig. 18.6). As in classic dissection, internally displaced intimal calcification may be seen. At intravenous contrast-enhanced CT, the crescentic area along the aortic wall appears as low attenuation thrombus. There is no intimal flap. The intramural hematoma usually maintains a constant circumferential relationship with the aortic wall. Associated features include pericardial effusion, hemothorax, pleural effusion, and hemomediastinum. Distinguishing an intramural hematoma from classic aortic dissection with a thrombosed false lumen is still problematic.

At MRI, focal crescentic wall thickening, without mass effect on the aortic lumen and an absence of intimal flap, is seen. In reference to true spin echo (SE) sequences, the signal characteristics of the intramural hematoma depend on the age of the hematoma. If acute, it appears high signal on T2-weighted images and isointense or hypointense to muscle on T1-weighted images due to oxyhemoglobin. When subacute, it is high signal on both T1- and T2-weighted images due to methemoglobin. When chronic, it appears low signal on T1- and T2-weighted images due to organization of the blood (14). At transesophageal echocardiography, intramural hematoma appears as localized circular or crescentic thickening of the aortic wall, greater than or equal to 5 mm, and again, no intimal tear is seen.

Treatment consists of intensive care unit monitoring, aggressive medical treatment with antihypertensive therapy, and frequent serial follow-up noninvasive imaging studies. Surgery is performed in patients with coexistent aneurysmal dilatation or when the intramural hematoma progresses on serial studies. Evaluation of intramural hematoma on serial noninvasive imaging has shown that intramural hematomas may decrease in size and partially or completely resolve, particularly if the aortic diameter is less than 5 cm (12,15). Sometimes ulcer-like projections into the intramural hematoma develop, which can progress to saccular aneurysm. Fusiform aneurysms without ulcer-like projections and even overt aortic dissection can develop (12). Risk factors for overt aortic dissection

include intramural hematoma involving the ascending aorta that had a maximum thickness of 16 mm, compression of the true lumen, and the presence of pericardial or pleural effusion (16).

Atheromatous ulcers develop in patients with advanced atherosclerosis. At this stage, lesions are usually asymptomatic and confined to the intimal layer. As the lesion progresses and a deep atheromatous ulcer penetrates through the elastic lamina into the media, an intramural hematoma is formed. The hematoma can extend through the adventitia, forming a false aneurysm. Rarely, it may progress to complete transmural aortic rupture (19).