Acute dissection is the most common fatal aortic catastrophe. Although abdominal aortic aneurysms occur more frequently than do thoracic aortic dissections, they less often present with rupture and, when they do so, are less often fatal than are ruptured thoracic aortic dissections.
The incidence of aortic dissection is estimated at 10 to 15 per 100,000 adults in the United States annually. Accordingly, while thoracic aortic dissections are uncommon, their malignant course makes them an important cause of cardiovascular morbidity and mortality.
The underlying cause of aortic dissection is medial degeneration, which may be secondary to inherited connective tissue diseases such as Marfan syndrome, Ehlers-Danlos syndrome, Loeys-Dietz syndrome, or any of a family of thoracic aortic aneurysm and dissection syndromes.
More commonly, medial degeneration occurs secondary to the ravages of hypertension over time. Tobacco use accelerates the process. Dissections also occur more commonly among patients with aortic dilation as increasing aortic diameter increases wall tension and the mechanical stress placed on the aortic tissues.
Rarely, dissections occur during pregnancy, most often among individuals with connective tissue disorders. Dissections tend to occur somewhat earlier in men (peak incidence in their 50s to 60s) than in women (peak incidence in their 60s to 70s).
Thoracic dissections are classified anatomically as Stanford type A if the ascending aorta is involved and Stanford type B if the dissection is confined to the descending thoracic or thoracoabdominal aorta. The associated natural history of type A and type B dissections is markedly different: If treated nonoperatively, the mortality rate during the index hospitalization for type A dissections may be as high as 80%, while that for management guidelines for acute aortic dissection and the International Registry of Acute Aortic Dissection surgical treatment is 10% to 25%.
The preferred treatment algorithm is clear: Acute type A dissections are treated surgically on an urgent or emergent basis except under unusual circumstances (such as advanced age or comorbid conditions such as acute stroke).
The preferred treatment of type B dissections is more controversial, particularly today in the era of endovascular stent grafts. Historically, the mortality rate associated with medical treatment of type B dissections (approximately 10%) has been clearly less than that for open surgical repair (approximately 30%). Accordingly, the preferred treatment for a patient with type B dissection is aggressive blood pressure control.
But when, then, is intervention indicated? Our understanding of the clinical presentation and outcome of acute aortic dissection has been greatly enhanced through the efforts of the International Registry of Acute Aortic Dissection (IRAD).
The IRAD database was established in 1996 by a group of interested investigators, including cardiovascular surgeon Thoralf M. Sundt III, M.D. and Jae K. Oh, M.D., a cardiologist at Mayo Clinic, who recognized that there was a paucity of information about this disease. The database initially included 12 large referral centers in 6 countries and has now grown to include 24 centers in 12 countries.
A recent publication from this group reviewed the impact of refractory pain and persistent hypertension on outcome and, by inference, their role as indications for surgical intervention among patients with type B dissection:
"These data support the notion that recurrent pain and refractory hypertension should encourage a more aggressive interventional approach to patients with type B dissection," says Dr. Sundt. The data are further suggestive, although not definitive proof, of a role for endovascular stent grafts in this subset of patients. It must be clearly stated, however, that endovascular stent grafts are currently approved by the US Food and Drug Administration for the treatment of aneurysmal disease, but not dissection.
Of equal interest in this analysis was the outcome of the patients with uncomplicated type B dissection who did not experience refractory pain or hypertension. The mortality rate among medically treated patients was 1.5%. This is of particular note, as interest has risen in the possible role of endovascular stent grafting among patients with uncomplicated dissection in the hope of preventing late complications. Clearly, any intervention, be it surgical or endovascular, must be accomplished among these patients without incremental increase in this risk.
Finally, it is important to remember that aortic dissection is a chronic condition. "Once a patient has experienced aortic dissection, whether type A or type B, he or she should be followed carefully with aggressive control of blood pressure and serial imaging studies to observe for aneurysmal dilation of the injured aorta," says Dr. Oh.
All too often, patients with acute dissection are lost to follow-up after an initial surgical repair or successful nonoperative hospitalization at the time of the acute event. As many as 30% of patients with dissection of the descending thoracic and thoracoabdominal aorta ultimately demonstrate expansion of the aorta sufficient to warrant consideration of surgical intervention.