Acute dissection is the most common fatal aortic catastrophe, with the incidence estimated at 10 to 15 per 100,000 adults in the United States annually. 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. This disorder may be secondary to inherited connective tissue diseases such as Marfan syndrome, Ehlers-Danlos syndrome, or any of a family of thoracic aortic aneurysm and dissection syndromes. More commonly, medial degeneration occurs secondary to long-term complications of hypertension. 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 in women 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.
If treated medically, type A dissections may have a mortality rate as high as 80 percent during the index hospitalization, while the mortality rate for surgical treatment is 10 to 25 percent.
Surgery, on an urgent or emergent basis, is the treatment of choice for type A dissections. The preferred treatment of type B dissections is more controversial, particularly in the era of endovascular stent grafts.
Historically, the mortality rate associated with medical treatment of type B dissections (approximately 10 percent) has been clearly less than that for open surgical repair (approximately 30 percent). Accordingly, the preferred treatment for a patient with type B dissection is aggressive blood pressure control. But when, then, is intervention indicated?
The International Registry of Acute Aortic Dissections (IRAD) has greatly enhanced understanding of the clinical presentation and outcome of acute aortic dissection. IRAD was established in 1996, and cardiovascular surgeons and cardiologists at Mayo Clinic in Rochester, Minn., have been a part of the registry team. IRAD now includes 24 centers in 12 countries.
This group recently published data on the effect of refractory pain and persistent hypertension on outcome and, by inference, their role as indications for surgical intervention in patients with type B dissection. In a series of 365 patients with acute type B dissection without rupture or malperfusion:
These data support the notion that recurrent pain and refractory hypertension should encourage a more aggressive interventional approach to patients with type B dissection.
These data also suggest 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 Food and Drug Administration only for the treatment of aneurysmal disease, but not dissection.
Among the patients with uncomplicated type B dissection who did not experience refractory pain or hypertension, the mortality rate in medically treated patients was 1.5 percent. When considering the use of endovascular stent grafting to prevent late complications in patients with uncomplicated dissection, any intervention, be it surgical or endovascular, must be accomplished without incremental risk increases.
Finally, patients experiencing either type A or type B should be followed carefully with aggressive control of blood pressure and serial imaging studies to observe for aneurysmal dilation of the injured aorta. All too often, these patients do not receive appropriate follow-up after an initial surgical repair or successful nonoperative hospitalization at the time of the acute event.
As many as 30 percent of patients with dissection of the descending thoracic and thoracoabdominal aorta ultimately demonstrate expansion of the aorta sufficient to warrant consideration of surgical intervention.