In the 1990s, endovascular stent-graft treatment emerged as a new and less invasive method to treat ab- dominal aortic aneurysm

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Contents

38.1 Introduction . . . . 375

38.2 Descending Thoracic Aortic Aneurysms . . . . 375

38.3 Dissection . . . . 376

38.3.1 Timing of Treatment . . . . 377

38.3.2 Length of Coverage . . . . 377

38.4 Trauma . . . . 377

38.1 Introduction

Cardiovascular disease is the leading cause of death in most Western societies and is increasing steadily in many developing countries. Longer life expectancy, hypertension and the proliferation of modern noninvasive imaging modalities have contributed to the growing awareness of acute and chronic aortic syndromes. De- spite recent developments in epidemiology, diagnostic and therapeutic modalities, there is still a lot of pro- gress to be made to understand the spectrum of aortic syndromes and to define an optimal approach to man- aging aortic diseases.

In the 1990s, endovascular stent-graft treatment emerged as a new and less invasive method to treat abdominal aortic aneurysm. It soon led to the use of stent grafts in the treatment of thoracic aortic diseases, but their exact role remains approximate.

Although only midterm study results are now avail- able, they indicate a better outcome compared with conventional surgery, especially in elderly patients with significant comorbidities such as pulmonary and renal insufficiency, coronary heart disease, hypertension and diabetes mellitus, where morbidity and mortality rates after an open surgical repair are as high as 50%. How- ever, despite the good published results, endoluminal stent grafts are not risk-free: endoleaks, prosthesis dis- locations, neurological complications, acute or late rup-

ture of the aorta and side branch occlusions are described leading to therapy failure. Owing to the actual restrained number of patients treated by endovascular repair, the blur in the indications and the different types of devices used, it is nearly impossible to identify if the complications are device-, procedure- or patient- related and the exact place of this new therapy.

Nevertheless, we will attempt, in this chapter, to dis- cuss the ongoing studies and the need for future studies to better understand and treat the various thoracic aortic pathologies.

38.2 Descending Thoracic Aortic Aneurysms

Aneurysms of the thoracic aorta represent a potentially life-threatening situation. Surgical resection and inter- position with a vascular prosthesis have long been considered the standard treatment despite the substantial risks of the procedure. The use of an endovascular stent graft to treat thoracic aortic aneurysms emerged a de- cade ago propelled by the desire to reduce surgical risks and induce remodeling of the diseased aorta by initiat- ing a natural healing process after exclusion and de- pressurization of the aneurismal sac.

So far, all prospective studies and registers have shown that the stent-graft technique has better immediate results compared with classic open surgery, with lower 30-day morbidity±mortality and paraplegia rates.

In midterm studies, the complication rates are, however, not negligible and habitually consist of secondary leaks which can mostly be treated intravascularly [1, 2, 3].

Compared with stent-graft abdominal aortic aneurysm repair, complications of thoracic treatment differ con- siderably. Abdominal complications mostly relate to changes in aneurysmal volume after successful exclusion, which result in device distortions, kinks or modu- lar disconnections. At the thoracic level, as only one tu- bular device is needed in most patients, the risks of type III leaks, kinks, disconnections or thromboses are

Current Multicentric Studies and Those to Plan

for the Descending Thoracic Aortic Diseases

Herv Rousseau, Jean Philippe Bolduc, Francis Joffre

38

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either eliminated or greatly reduced. Furthermore, the diameter reduction after complete aneurysm exclusion is probably less than in the abdomen combined with the use of an oversized device (at least 10% more than the normal aortic diameter) and this reduces risks even more. Nevertheless, the most frequent thoracic complications are type8I endoleaks that occur at aortic and graft junctions allowing the aneurysmal sac to remain pressurized. They are more frequent because degenera- tive thoracic aortic disease is usually more diffuse than abdominal disease; thus, progression of the malady at attachment sites is more likely. To avoid this problem, we recommend the placement of longer stent grafts covering healthy aorta up to the visceral arteries. Type II endoleaks, except fromthe left subclavian artery, are rare. If two or more grafts are used, type III endoleaks can arise at junctions, requiring insertion of another stent-graft segment. This complication is greatly reduced when we systematically overlap a long segment of the grafts. Finally, pseudoaneurysms and intimal per- forations at distal implementation sites have been re- ported secondary to stent-graft erosions [4]. Complete long-standing studies are still needed to determine the incidence of these complications and their long-term ef- fects.

The question of intentional exclusion of the left subclavian artery is still unanswered. In patients with a very short neck between the left subclavian artery and the aneurysmrequiring coverage of the former, different treatment attitudes have been described; left subclavian transposition or bypass either systematically before stent-graft insertion or only if the patient has ischemic neurological or left armsymptoms after occlusion. Left subclavian artery coverage is routinely done without complication in many centers [5, 6]. Nevertheless, it should be kept in mind that it is crucial to evaluate the vertebral arteries before occluding the left subclavian artery to prevent ischemic symptoms in cases of steno- tic vertebral arteries or absence of collateral pathways between the two as observed in up to 6% of cases.

As devices improve, better results should be observed in the future. Therefore, requests to place endo- grafts in patients with small lesions, in which the risk of rupture is extremely low, should be more frequent. It will be important to resist these demands until further data prove otherwise. So, as far as we are concerned, we recommend that endograft use should be limited to patients who truly exhibit surgical indications.

38.3 Dissection

Despite the frequency of acute aortic dissection, there are few large series published on the outcomes of dissections and most are long retrospective multicenter studies confounded by inconsistent methods of treat-

ment and data collection. The IRAD study, a prospective multicenter registry has now been created to ad- dress some of these concerns. This study [7, 8] provides better understanding of the clinical profile and outcomes of patients with acute type B aortic dissection, helping clinicians in early risk stratification and decision-making. Unfortunately, there is an inherent selec- tion bias because the study results are mainly based on data fromtertiary referral centers that may not neces- sarily be extrapolated to the general population. Even though the IRAD study is a step forward, to better evaluate survival predictors, prospective studies are still needed mainly because the actual registry does not re- group homogeneous patients with similar risk factors whose outcomes could be rigorously compared nor does it take into consideration factors such as nonfatal morbidity, quality of life and cost effectiveness.

Actual consensus exists regarding the need for emergency surgical treatment of patients with acute Stanford type A aortic dissection. The optimal treatment strategy for Stanford type B dissection remains controversial [9±

12]. Most groups today reserve the surgical replacement of the descending aorta for patients with aortic rupture, organ ischemia, refractory pain, uncontrolled hypertension, false lumen dilatation or other life-threatening conditions. Other teams have advocated early surgery for young and good operative candidates irrespective of the presence of complications [13], arguing that if the surgery is successful, these individuals would be at lower risk of late dissection-related aortic complications.

Finally, f percutaneous interventional techniques, i.e., fenestration and stent-graft repair to correct ischemic complications related to thoraco-abdominal malperfusion, have become a valuable adjunct to both medical and surgical therapy, but their role is still debated.

For type B dissections with complications, percutaneous stent-graft placement seems to be superior to surgery on short-termfollow-up [14±19]. Recently, it was shown that percutaneous stent-graft treatment has an early mortality rate of 16% among patients with acute Stanford type B aortic dissections associated with life-threatening complications [16]. If treated surgically, i.e., an emergency thoracotomy, these patients would be facing an early mortality risk of 40%. The rate was said to be 60±70% if treated medically [11, 14, 15] The effectiveness of stent-graft treatment in patients with complicated acute type B aortic dissections must however still be confirmed by long-term prospective randomized trials. Such a study was started in early 2003 but regret- tably had to be stopped after the intentional retrieval of the Gore device after cases of nitinol wire fractures.

In cases of acute type B dissection without complications, medical treatment was long the only accepted treatment until stent grafts were used successfully [17], complicating the decision-making process. The IN- STEAD study was started in Europe in 2002 to compare medical and stent-graft treatment in patients with un-

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complicated acute type B aortic dissections. The aim of this multicenter randomized controlled clinical trial is to evaluate the 1-year outcomes, including complication rates and quality of life, of patients with type B dissection treated either percutaneously or medically. Early results can be expected in 2005.

Other concerns include the timing of the intervention and the length of coverage necessary to exclude the false lumen.

38.3.1 Timing of Treatment

Stent-graft placement could become, in the near future, the standard treatment for most cases of complicated or uncomplicated aortic dissection mostly because the operative mortality rate approaches 70% if we wait for complications to occur. Another argument in favor of early endovascular treatment is the evolution of aortic morphology with time following dissection. In acute type B dissections, an isolated tear is more frequent and usually no thrombus is present in the false lumen, while in chronic dissections multiple entry and exit points are seen along the aorta associated with thrombus formation enlarging the vessel diameter. Therefore, delaying treatment could increase implantation failure rate or make the intervention no longer possible [20±

22].

38.3.2 Length of Coverage

An unanswered technical question concerns the length of aortic coverage necessary to achieve dissection healing. The key is to cover the proximal entry site to reduce pressure in the false lumen and consequently shrink the total aortic diameter and improve flow in the true lumen expanding the later, resolving ischemic complications or malperfusion syndromes. Given our results and those of others, it seems that complete thrombosis of the false lumen is necessary to reduce the overall aortic diameter and protect against subse- quent aneurismal dilatation and rupture [23, 24]. So, fromthese results combined with the fact that the risks of neurological paraplegic complications are particularly low in dissections treated by stent grafts, one can suggest covering a long part of the descending aorta above the diaphragmat the time of initial implantation to exclude all entry points feeding the false lumen. Adjunc- tive measures to achieve complete thrombosis of the false lumen such as use of coils or glue have also been described. Again, long-termcontrolled trials are needed to categorically guide our future therapeutic strategies.

38.4 Trauma

Despite advances in surgical and reanimation techniques, surgery is still associated with significant morbidity and mortality rates ranging between 8 and 15%

depending on whether circulatory assistance to main- tain satisfactory perfusion of the distal aorta is used or not [25]. The postoperative paraplegia rate without circulatory assistance can be as high as 19% and increases significantly when the aorta is clamped for more than 30 min [26]. With circulatory assistance, the rate is about 2% [25] However, the systemic anticoagulation required for the extracorporeal circulation is often un- desirable in traumatic patients with multiple fractures and/or parenchymal or cerebral lesions.

In the last 10 years, several studies showed, for stable and nonbleeding lesions, that surgical mortality after aortic injury can be significantly reduced when surgical repair is deliberately delayed [27±29]. These studies support the fact that free rupture of a contained acute traumatic tear of the thoracic aorta is unlikely to occur under proper blood pressure control. Therefore, it ap- pears safe to allow patients who suffered a major trauma to be stabilized, undergo other emergent operations if needed and then have elective repair of the aortic tear. Although this attitude is justified by objective data, it is not entirely risk-free because as many as 4% of patients awaiting surgery might die of a ruptured aorta usually within 1 week of the traumatic injury [30].

More recently, the advent of the endovascular stent- graft technology has provided a less invasive alternative to thoracic aortic injury treatment. This substitute to open thoracic aortic replacement is attractive for several reasons but one of its main advantages is the possibility to avoid heparin use when necessary, decreasing hemor- rhagic complications related to associated lesions if present.

Although some authors reserve endovascular treatment for patients for whom standard surgery is contra- indicated [31], one might raise the issue of extending the indication to all patients with traumatic injury of the thoracic aorta. Our current experience, as that of others, has shown encouraging results of the endovascular technique compared with those for conventional surgery [31±41]. The benefits of aortic endoprosthesis in terms of morbidity and mortality by far outweigh those of classic surgery by thoracotomy. Our compara- tive study with similar lesions and severity scores (ISS) confirms that stent-graft therapy is an advantageous alternative to conventional open surgery. The mortality and the paraplegia rates were 21 and 7%, respectively, for the 35 patients surgically treated compared with 0%

for the 29 patients treated with a stent graft [41]. With a mean follow-up of 46 months, we did not observe any aneurysmexpansion or rupture. Complete healing of the aortic wall without any residual pseudoaneurysm

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and total shrinking of the aorta over the stent graft were seen in all cases.

However, controversy remains regarding the best method of management. Studies must be carried out to determine the precise place of endovascular treatment in the management of acute rupture of the thoracic aorta. An ideal study would compare the outcomes of patients of similar health status subjected to conventional surgical intervention, to stent-graft placement or to medical treatment. Unfortunately, such a prospective study is not feasible for ethical reasons; patients incap- able of undergoing conventional surgery for any reason should of course not be operated. Additionally, since a small number of patients receive treatment in each cen- ter, even a multicenter randomized study comparing the two treatment methods is illusive. A prospective registry evaluating patients considered unfit for surgical intervention because of comorbidities treated with or without stent-graft placement would best assess the ef- fect of the endovascular strategy compared with that of medical treatment. In order to do so, we suggest the creation of an international registry similar to the one for aortic dissections to compile the results of endovascular treatment and consequently help to define its indications.

As a whole, we can actually consider that endovascular stent-graft treatment of the aorta is a less invasive strategy for most of the thoracic aortic diseases, particularly in patients with comorbidities; however, large prospective studies for the complete evaluation of this new therapeutic option are still needed.

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