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EDITORIAL |
Hong Kong, China
Despite having a better survival prognosis than acute dissection of the ascending aorta (type A), acute type B aortic dissection sparing the ascending aorta is associated with substantial morbidity and mortality. Even for initially stable patients, the mortality rate is close to 9.6%, with the most common cause of death being sudden rupture or end-organ malperfusion.1 The timing of these complications is very variable and unpredictable. From the International Registry of Acute Aortic Dissection (IRAD), involving 498 patients, Tsai and colleagues2 showed that arch involvement (127/498, 25.5%) is not associated with higher follow-up mortality, provided blood pressure control is meticulous. Long-term survival in medically managed patients is estimated to be approximately 80% at 1 year, 72% at 3 years, and 58%–76% at 5 years.3,4 Overall, of those who remain stable and asymptomatic, 70% will not require any future surgical intervention, but continual surveillance to prevent late complications such as aneurysmal dilatation is important.5
The treatment of uncomplicated type B dissections is mainly medical with antihypertensives (beta blockers). The management of patients with complications of acute type B aortic dissection (including contained rupture, malperfusion of visceral organs or extremities, rapid increase of aortic diameter, refractory hypertension, and persistent severe pain) is more challenging and controversial. Conventional emergency open surgical intervention with replacement of the descending aorta carries significant perioperative morbidity and a mortality rate ranging from 21% to 50%.6,7 First described by Dake and colleagues,8 endovascular treatment is gaining popularity in the treatment of complicated type B aortic dissection and in selected cases of type A dissection with entry tears in the descending aorta. A meta-analysis of 39 international studies with 609 patients who underwent endovascular treatment for type B dissection (acute and chronic) showed a technical success rate in excess of 95%, with perioperative complication and mortality rates for acute dissection of 21% and 9.8%, respectively.9
Good quality preoperative computed tomography angiography is crucial in planning endovascular repair, and should clearly define the anatomy of the aorta in relation to the entry tear. There are many technical considerations and much expertise required. Firstly, there should be sufficient length (1.5–2 cm) of normal aorta proximal to the tear to establish an adequate seal (so-called proximal landing zone). The device should conform to the aorta, especially to the inner curve of the aortic arch. This may involve sacrificing the left subclavian artery, and level 3 evidence shows that revascularization is probably unnecessary if the patient does not have a dominant left vertebral artery, left internal mammary artery coronary graft, or left arm hemodialysis fistula. If the dissection involves the distal arch proximal to the left subclavian origin, hybrid extraanatomic bypasses, carotid-carotid bypass, or even aortic debranching with ascending to innominate/left carotid bypasses, are usually required. In addition, the stent-grafts should not be generously oversized as the patients are younger and the diminutive distal true lumen may be so narrow that kinking or infolding of the stent-graft may occur.10 The length of the stent-graft should provide enough stability in the flap to prevent significant distal reentry points, thereby causing thrombosis of the false lumen. Intravascular ultrasound may be useful to identify this perioperatively. Although the theoretical risk of paraplegia increases with the number of intercostals being covered, the length of the stent-grafts should not be so short as to compromise distal stability. This has led to the use of uncovered stents in the distal thoracic aorta (such as the Cook Zenith Dissection Z-Stent), in which the proximal part is covered and the distal end is bare. Furthermore, device access is a real problem, especially in the Asian (female) population, as the iliofemoral arteries are often very small and thin, and placing an emergency conduit onto a fragile proximal iliac artery can be challenging. Last but not least, technical support is important, as devices with the desired diameter and length need to be available as soon as possible.
To successfully treat mesenteric or renal malperfusion, a sound understanding of aortic dissection pathology and reentry hemodynamics is essential to distinguish between static and dynamic obstruction. Good quality computed tomography angiograms (especially those with electrocardiogram gating) and intravascular ultrasonography are most helpful. A self-expanding stent-graft may suffice for static obstruction. In contrast, the entry tear at the top must be sealed adequately, and the obstructed artery should be stented or stent-grafted through the true lumen for dynamic obstruction. Lower limb ischemia can be treated with extraanatomic femorofemoral or axillofemoral bypasses.
Despite a technically successful procedure, these patients continue to be at risk of further complications. Strokes and paraplegia are the most dreaded. Although reports suggest that covering the left subclavian origin is safe, patients are still at risk of developing a vertebrobasilar hypoperfusion stroke. Retrograde (type A) dissection may occur, and the cause is probably multifactorial. Various theories have been proposed, including retrograde aortic damage from the proximal bare stents or anchoring barbs, aggressive over-sizing, balloon dilatation, and deployment of the device in a (previously missed) dissected aorta.10 Retrograde dissection may happen sometimes after the endovascular procedure.
There are still a lot of unanswered questions and a lack of level 1 evidence to guide clinicians as to the best treatment modality for complicated acute type B dissection. Early data show that endovascular repair is more minimally invasive than open surgical repair, although the precise timing of treatment is unclear. Should we offer endovascular treatment to all patients, even those stable on antihypertensives, to prevent their 9.6% mortality? Should we treat all cases of contained rupture, knowing that disturbance of the intimal flap by the endovascular device may alter the hemodynamics within the true and false lumina and may convert this to a free rupture? A number of multicenter clinical trials are currently ongoing to try to answer these questions. The European INSTEAD (INvestigation of STEnt grafts in patients with type B Aortic Dissection) Trial aims to randomize 136 stable patients with acute dissection to either endovascular treatment with a Medtronic Talent stent-graft or best medical treatment alone.11 The STABLE (Study for Thoracic Aortic type B dissection using Endoluminal Repair) Trial is investigating the use of uncovered stents to support the true lumen, and comparing the Cook Zenith Dissection Z-Stent and the Zenith TX2 endograft. Whilst we wait for the results of these straight-endo (stent) graft trials, the option of branched endografts is being explored, and another chapter in the saga of endovascular treatment of acute dissection is about to begin.
REFERENCES
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