Aortic dissection occurs primarily in one of two groups of patients. The first group consists mainly of men aged 40 to 60, in whom hypertension and atherosclerosis is prominent. The second group has a systemic or localized abnormality of connective tissue affecting the aorta (e.g., Marfan's syndrome).
Aortic dissection is usually a catastrophic illness (unless diagnosed and treated successfully prior to rupture) characterized by dissection of blood along the laminar planes of the media of the aorta, with the formation of a blood-filled channel within the wall of the aorta that often ruptures out through the adventitia, causing massive hemorrhage. Aortic dissection is usually not associated with marked dilation of the aorta, as is the case with atherosclerotic and syphilitic aneurysms.
Morphologically, there is usually, but not always, an intimal tear extending into but not through the media of the ascending aorta. The intimal tear in this case was 1 cm in length and present 1.5 cm proximal to the great vessels, 7 cm above the aortic valve. This area, which is usually the origin of dissection, is usually transverse or oblique, 1 to 5 cm in length, with sharp but jagged edges. The dissection can potentially extend proximally toward the heart as well as distally along the aorta to variable distances. The dissection in this case did both, that is it dissected both proximally toward the heart and distally toward the great vessels, and thus was classified as a DeBakey type II aortic dissection (DeBakey type I involves only the proximal ascending aorta and DeBakey type III involves only the descending aorta and usually begins distal to the subclavian artery).
The most common cause of death is rupture of the dissection into any of the three body cavities, including the peritoneal, pleural, or pericardial. The pleural and pericardial cavities were involved in this case. Other vascular complications, however, can occur, including extension of the dissection into the great arteries of the neck or into the coronary arteries causing critical vascular obstruction. Extension into the carotid arteries was most likely the cause of this patient's transient neurological symptoms and signs. The most common clinical manifestations of aortic dissection include cardiac tamponade (the final event in this case), aortic insufficiency, and myocardial infarction.
The dissected blood, which is actually a dissecting intramural hematoma, occurs characteristically between the middle and outer two-thirds of the media. In most patients with aortic dissection certain characteristic histologic changes can be identified in the aortic wall causing weakening (although some patients with dissection have minimal histologic abnormality). Uncertainty exists, however, as to the specificity of these lesions since they can be found incidentally in patients without aortic dissection (increasing in severity with age and with the presence of hypertension). These degenerative lesions range from mild fragmentation of elastic tissue to overt cystic medial degeneration, consisting of elastic tissue fragmentation and disruption and focal separation of the elastic and fibromuscular elements of the tunica media by small cystic spaces filled with amorphous material resembling ground substance of connective tissue. Eventually, there is large-scale loss of elastic laminae.
The actual triggering event for intimal tear and subsequent dissection is unknown. The exact role of hypertension, the major risk factor for aortic dissection, is not known either. Once the tear occurs, however, the increased blood pressure tends to advance the progression of dissection. The lack of classic severe chest pain in this case obscured the diagnosis of aortic dissection. A DeBakey type II dissection, which is proximal, is unlikely to produce the classic chest pain of distal dissection, which is also the most common type of aortic dissection.
Diagnostic techniques for aortic dissection include transesophageal echocardiography, aortic angiography, computed tomography, and magnetic resonance imaging. There are advantages and disadvantages to each of these procedures. However, clinical usefulness may hinge upon which one is most readily available emergently. Despite recent advances in noninvasive diagnosis, a correct antemortem diagnosis of aortic dissection is made in less than half of cases. Despite diagnosis and therapy, aortic dissection has a significant mortality rate--25 to 35%.