Limitations of Current Management of Acute and Chronic Type B Dissections

Review

Submitted on Mon, 03/16/2009 - 10:05
Authors

*Brian J. Manning, MD, FRCSI and §Krassi Ivancev, MD, PhD

author affiliations:

From *Malmo University Hospital, Malmo Sweden and §University College London Hospital, London, England. Disclosure: The authors report no financial relationships or conflicts of interest regarding the content herein. Manuscript submitted October 15, 2008, provisional acceptance given November 17, 2008, accepted December 11, 2008. Correspondence: Brian J. Manning, MD, Endovascular Center, Entrance 44, Plan 4, 20502 Malmo, Sweden. E-mail: brianjmanning@gmail.com __________________________________ Natural history and evidence for intervention The limitations of current management of acute and chronic Type B dissections are demonstrated by contemporary studies, which report an in-hospital mortality rate of up to 25%1-4 and a 3-year survival rate of 77.6% among those discharged from the hospital following medical, surgical or endovascular treatment.5 Although autopsy studies have shown that the majority of dissection-related deaths occur within the first 2 weeks,6 survival free of aorta-related events at 5 years has been reported as 82% for those managed medically, 68% for those requiring intervention in the acute setting and 60% overall survival at 5 years among a cohort of 189 patients initially presenting with acute Type B dissections.7 Undoubtedly, the advent of endovascular techniques has contributed to improved early survival. Open surgery in the acute setting has been associated with a mortality rate of 2–69%4,8–10 and paraplegia in up to 36%11 of patients who present with complications such as rupture, end-organ ischemia, uncontrollable pain or hypertension. It is in this group that endovascular stent-graft treatment has made the most impact, with in-hospital mortality ranging from 0–25% in recent studies, and a paraplegia rate of less than 10% in most reports.12–16 Although some report comparable results from open surgery, such experience is exceptional.17,18 The case for intervention in asymptomatic acute dissections is far from convincing. Encouraging short-term results and unpublished registry data, suggesting a mortality rate of Perioperative morbidity and mortality Access-related problems Access-related complications, though not uncommon in the current literature, are not always included in results. The incidence varies from 14.313 to 25%,4 reported as 12.9% in a large series of endovascular procedures performed for all thoracic aortic pathology.22 These problems are not limited to femoral or external iliac dissections or disruptions, which are relatively easy to repair. Rupture at the aorto-iliac level is a real risk, particularly in women with smaller, heavily calcified vessels. This may occur at the time of device introduction, possibly only becoming apparent upon withdrawal of the delivery system. Two such cases occurred in the Malmo series of 52 patients treated for acute complicated Type B dissection (unpublished data), and both required emergency aorto-iliac bypass. Aortic rupture and retrograde dissection Perioperative aortic rupture and retrograde aortic dissection, the most common lethal aorta-related complications, have been reported with an incidence varying from 0%15,23,24 to 11%25–14%.12 Although these complications are described more frequently in studies of acute dissection, they are also reported in studies of endovascular treatment of chronic Type B dissection,16,26,27 and in one series were more common in this group.26 While rupture is usually a terminal event, retrograde dissection is the most commonly cited reason for conversion to open surgery in contemporary studies.12,14 The dissected aorta is fragile, particularly in the acute setting, and is susceptible to damage from stiff guidewires and delivery systems as well as rigid stent grafts, which require varying amounts of force to deliver across the aortic arch, depending on patient anatomy as well as individual device flexibility. The particular vulnerability of the dissected aorta to these complications is apparent in the results from the Talent Thoracic Retrospective Registry. Even though dissection was the indication for treatment in only 39.4% of patients, the only cases of intraoperative aortic rupture occurred in this cohort.28 Apart from the obvious precautions, such as avoiding excessive device manipulation and balloon dilatation, other factors including adequacy of the proximal landing zone, the degree of oversizing and the relevance of proximal bare-metal stents to these complications, as well as the choice of endograft, are all potentially important and may predispose the patient to early rupture or retrograde dissection. Unfortunately, varying patient selection and exclusion criteria, along with inadequate and incomplete methodological descriptions in the current literature, do not allow firm conclusions to be drawn as to the optimal treatment strategy. Of those studies in which a treatment strategy is outlined, most advocate exclusion of the primary entry tear as the main treatment goal.12,15,25 Szeto13 advocates the Petticoat technique,29 with favorable perioperative results, extending the stent-graft coverage after treatment of the primary tear if persistent false-lumen perfusion is demonstrated. Yuan et al describe simultaneous multi-tear exclusion using non-overlapping stents,24 as mentioned previously. Where described, stent-graft oversizing is reported at 10%,25 10–15%24 or 10–20%,12 and is taken to be at the level of the normal aorta adjacent to the proximal entry tear, although this is not defined in most studies. The current view is that for dissections, as opposed to true aneurysms, minimal if any oversizing is necessary. Stent-graft migration is rarely reported in contemporary studies. The presence of bare-metal stents in the aortic arch has been postulated as relevant to device-related complications. Device selection and optimal length of aortic coverage Most larger studies have described the use of stent grafts manufactured by either Medtronic, Gore or Cook (as well as custom-made devices), and the majority have used devices from at least two of these manufacturers in their analyses. Unfortunately, even though these products differ in structure, composition, flexibility and profile, comparative device-related outcome analysis is generally not provided. The optimal length or degree of aortic coverage is controversial. While more extensive coverage maximizes the potential to exclude most re-entry tears and reduce complications in the long term, many advise minimizing the extent of aortic coverage in order to reduce the risk of paraplegia.20 Herold et al30 describe their intention to use only 1 stent for all cases if possible, limiting coverage of the thoracic aorta to 10 cm; however, this proved insufficient in 3 of 6 acute Type B dissections treated in their series and further, more extensive stent-graft coverage was required. Most authors report using more than 1 stent graft. Steingruber describes a mean aortic coverage length of 191 mm, with 1.6 stent grafts used on average. Sayer et al used a mean of 1.5 stent grafts per patient with acute dissection, and 2.12 stent grafts for those with chronic dissection, covering on average 182 mm and 204 mm of aorta in each group, respectively.26 Left subclavian artery (LSA) coverage varies considerably in the literature and is thought to be of relevance to the risk of stroke as well as paraplegia. Risk increases when the proximal seal zone is extended more proximally in the arch. In the Talent Thoracic Retrospective Registry analysis, stroke was significantly associated with occlusion of the LSA without previous revascularization and paraplegia was significantly associated with a greater than 20 cm length of covered aorta.28 The EUROSTAR experience also reported LSA coverage without prior revascularization as one of four risk factors significantly associated with spinal-cord ischemia, along with concomitant abdominal aortic surgery, renal failure, and the use of 3 or more stents.31 Against this strategy, it has been shown that false-lumen volume increases are significantly greater during follow up in patients who have been treated with less lengthy (Reinterventions and late mortality The need for reintervention as well as aorta-related complications during the course of follow up are good indicators of the efficacy of current endovascular techniques in the treatment of Type B dissection. To expect that endograft coverage of the primary entry tear will usually resolve what is a complex pathophysiological process is perhaps misguided. The difference from studies of aneurysmal disease is striking. Of 457 consecutive patients treated with Talent thoracic stent grafts, only 180 were treated for dissection, yet 9 of 11 aorta-related deaths during follow up had dissection as the primary presenting pathology, including 6 of 7 patients who died from aortic rupture.28 Of 16 patients requiring late conversion to open surgery during follow up, the indication for primary stent-graft treatment was dissection in 10 patients, and in all cases, the indication for intervention was either progressive aneurysmal expansion or impending rupture.32 Whether these results constitute failure of the initial treatment depends on the initial treatment goal. For some, correcting malperfusion and preventing rupture in the short term is enough in itself to justify the endovascular intervention. Superior short-term results compared with open surgery in most studies would support this approach, but only in those with acute complicated dissections. Of course, in the broader context, relatively few of these patients will succumb to complications of dissection. The longest follow-up period is from the Stanford group, reporting on 16 acute complicated Type B dissection (ACTBD) patients, with a mean follow up of 36 months. No late deaths are reported in this group of patients and follow up was complete.4 Most studies do not subdivide patients with Type B dissection beyond acute or chronic, and the relationship between dissection extent and subsequent outcomes has only recently been described. Rodriguez et al found false-lumen thrombosis at follow up to be significantly more common in acute rather than chronic dissections, and also in those with initial Type IIIa rather than Type IIIb dissections.33 In Malmo, of 52 patients with acute complicated Type B dissection treated with endovascular stent grafts, those with DeBakey Type IIIa dissection or intramural hematoma were compared with patients found to have the more extensive Type IIIb dissection at initial presentation. Although perioperative mortality and morbidity were similar in both groups, the need for reintervention in the course of follow up was strikingly different, at 0% versus 33%, respectively, with a high incidence of false-lumen thrombus at the time of the last computed tomographic follow up in the Type IIIa/IMH group (100% versus 33%, unpublished data). Two patients (both Type IIIb dissections) required renal artery stenting at 1 week and 6 months post stent-graft insertion because of renal artery ostial compression by the perfused false lumen. Our experience would indicate that current endovascular techniques have the potential to obliterate all false-lumen flow in those with more limited dissection, while persistent false-lumen perfusion, particularly distal to the stent graft, requires close follow up and potentially further intervention in those with more extensive dissections. Other reported late reinterventions include treatment of endograft migration and component separation, reoperation for retrograde Type A dissection,14 subclavian revascularization for upper-limb ischemia25 and endograft kinking associated with aortic elongation.12 Of aorta-related complications encountered during follow up, three problems predominate: Persistent perfusion of the false lumen, aneurysmal dilatation of the perfused false lumen (usually distal to the stent graft), and aneurysmal dilatation or endoleak of the proximal aorta, usually at the level of the left subclavian artery (Figure 1). The first and second of these are related, are the most common, and perhaps most difficult to predict and to manage. Significant aneurysmal dilatation or increase in false lumen diameter is usually reported without accurate definition, with rates varying from 7% to 14.3% at mean follow up of 4314 and 3412 months, respectively, in studies confined to acute Type B dissections. In probably the largest combined series, Rodriguez describes an increased false-lumen diameter in 7.5% of 106 patients, with a mean follow up of 15.6 months. False-lumen diameter occurred with equal frequency in acute and chronic groups, with 5 of 8 patients requiring reintervention for this aneurysmal progression.33 Sayer describes the need for distal stent-graft extension in 3 of 38 acute patients because of progressive aneurysmal expansion, and reported enlargement of the thoracic aorta in 20% of patients after 30 months of follow up. Perhaps surprisingly, persistent false-lumen perfusion does not always correlate with aneurysmal expansion. In reports of acute dissections, Verhoyle et al describe persistent false-lumen perfusion in 75% of patients after a mean follow up of 36 months, yet only 1 case of increased distal aortic diameter.4 Szeto reported persistent false-lumen perfusion at the level of the abdominal aorta in 67% of patients, yet no reintervention for dilatation at this level after 13 months of follow up.13 The reason for this seemingly unpredictable relationship between aneurysmal expansion and false-lumen perfusion is unclear, but may be related to the varying size and number of fenestrations distal to the stent graft. Several studies report the need for coil embolization of the LSA during the course of follow up, with endoleak (and presumed12 proximal false-lumen perfusion) being the usual indication cited.13,26,33 Although not specifically reported in other studies, we noted significant dilatation of the aorta proximal to the stent graft in 5 of 52 acute complicated cases (Figure 1), although the mean aortic diameter at this level was not significantly greater for the entire series at the time of the last follow up (unpublished observations). It is our view that delayed proximal aortic rupture, proximal Type I endoleak, proximal aneurysmal progression and late retrograde Type A dissection may all be linked to erosion of the proximal aorta or dissection membrane by the proximal edge of the stent graft. For this reason, we advocate a low threshold for extending the stent graft across the left subclavian artery in order to provide a longer proximal seal zone in a healthier part of the arch and also to allow better alignment of the stent graft along the longitudinal axis of the often angulated and tortuous aortic arch. Unlike acute dissections, where life-threatening malperfusion or rupture is effectively treated in most cases by stent-graft deployment, the indication for intervention in most cases of chronic dissection is aneurysmal dilatation. Does the available evidence support endovascular treatment for this indication? The most impressive results come from China. Yuan et al, in a series of 29 patients, 27 of whom were treated for chronic dissection with a mean follow-up period of 47 months, report no false-lumen enlargement after this time, 100% false-lumen thrombosis at the level of the stent graft, and more distally complete thrombosis of the false lumen in all but 17% of patients. Although they advocate a novel technique (outlined above), the indications for intervention are not clear, with a mean preoperative aortic diameter of 43.9 mm at the widest point and a mean false-lumen diameter of 22.1 mm.24 Rodriguez et al report chronic dilatation as the indication for treatment in their series, which includes 27 “chronic” patients. They report significant false-lumen decrease at all levels at a mean follow up of 15.6 months, with a 7.5% incidence of false-lumen dilatation, as discussed above.33 Despite varying false-lumen thrombosis and reintervention rates, most studies report positive results for chronic as well as acute dissection, with few if any recorded deaths from rupture.15,25,26 Conclusion Current management of acute and chronic type B dissection has evolved with the progression and application of stent-graft technology. Management has, in many centers, seen the replacement of open surgery with a less invasive technique as a first-line treatment for acute complicated dissections as well as for complicated chronic dissections. Significant perioperative mortality in most studies limits the adoption of endovascular interventions to those uncomplicated cases where medical management has, at worst, equivalent results in the short term. Its role in such cases remains to be established. Perioperatively, the incidence of aortic rupture and retrograde dissection may be reduced by more flexible stent grafts with lower profile delivery systems that are less likely to traumatize the access vessel or calcified, tortuous iliac arteries. Stroke and paraplegia are less common than in the era of open surgery, but remain a significant concern. The dilemma remains whether to extend the aortic coverage and increase the likelihood of false-lumen thrombosis or minimize aortic coverage to preserve spinal perfusion at the cost of potentially inadequate coverage of the fenestrated dissection membrane. The optimal strategy has yet to evolve. It is not clear if an endovascular cure is feasible for patients with more extensive dissections, particularly in chronic cases, where the false-lumen aneurysm is at risk of rupture. However, the limited studies to date have reported encouraging rates of false-lumen shrinkage, even in the presence of false-lumen perfusion. With careful surveillance and a readiness to reintervene, it would appear that aorta-related deaths and rupture are not common, supporting the role of endovascular management, particularly in those not fit for open repair, but perhaps also for all cases of chronic dissection. Further evidence is anticipated.