Classification and aetiology

The descending thoracic aorta is the most common location for aneurysms. Descending thoracic aneurysms are classified on the basis of whether they involve the upper half, the lower half or the entire descending aorta, with the thorax divided at the sixth intercostal space (types A, B and C).⁴

The majority of thoracic aneurysms are non-specific or degenerative, and approximately one-quarter are caused by chronic thoracic aortic dissections. Less frequently, aneurysms are related to Marfan syndrome, Ehler–Danlos syndrome, mycotic aneurysms⁵ and connective tissue disorders, e.g. ankylosing spondylitis, rheumatoid arthritis, Reiter’s disease and systemic lupus erythematosus. False aneurysms may follow trauma or complication of a previous coarctation repair. Aneurysms of the descending aorta may extend into the abdomen and are referred to as thoraco-abdominal aortic aneurysms.

The majority of aneurysms are fusiform in morphology. Saccular aneurysms are less common and are usually the result of infection or previous trauma. Thoracic aneurysms have a strong association with coronary artery disease and abdominal aortic aneurysm.

Incidence and clinical presentation

Aneurysms of the descending thoracic aorta are a disease of increasing age and the male to female ratio is 3:1. TAAs are estimated to occur in 10 per 100 000 patient-years.⁶ Thoracic aneurysms are most frequently discovered incidentally on routine chest radiography or CT scans. Clinical presentations include substernal, back or shoulder pain, superior vena cava syndrome, dysphagia, dyspnoea, stridor and hoarseness (due to laryngeal nerve compression), and rupture. The survival of patients with untreated TAAs is bleak and is estimated to be 13–39% at 5 years.⁷ Eighty per cent of thoracic aneurysms detected at autopsy have ruptured. The risk of rupture increases with the size of the aneurysm, but data regarding estimated annual rupture rates and aortic diameter are sparse and less robust than for the abdominal aorta. Some data suggest that the rupture rate of thoracic aneurysms is 2.7 per 100 000 patient-years as compared to 9.2 per 100 000 patient-years for abdominal aneurysms.

Indications for treatment

The main indications for intervention are symptoms and size. There is controversy regarding the size criteria for treatment of TAAs. Juvonen et al. reported that the 2-year rupture rate for TAAs was 23% in aneurysms less than 7 cm,⁸ whereas Elefteriades observed a 30% 5-year rupture rate when the aorta exceeded 6 cm.⁹ Practically, most clinicians regard 5.5–6 cm as an indication for possible repair in an asymptomatic patient, the threshold obviously being balanced by the surgical risk. Diameter thresholds should be reduced in patients with defined connective tissue disorders as rupture may occur at smaller aortic diameters.

Technique of surgical repair

The mainstay of traditional surgical repair of TAAs includes a left thoracotomy for access, aortic clamping and inlay grafting, with intercostal reimplantation. The procedures are technically challenging due to the need to maintain visceral and spinal cord perfusion during the procedure. Several surgical adjuncts are available to achieve these aims, including the use of a Gott shunt, left heart bypass and distal aortic perfusion, selective intercostal shunting and routine cerebrospinal fluid (CSF) drainage.

Endovascular repair of thoracic aneurysms

Detailed preoperative imaging is required to assess the proximal and distal landing zones for the endograft and to plan an access route. Ideally, this should be performed using centre-line image reconstruction using vascular-specific software to ensure optimal graft deployment planning.

A segment of normal aorta above and below the lesion to be treated is required (landing zone), so that a seal can be achieved between the endograft and the normal aortic wall. The landing zone length should be at least 15 mm on the lesser curve, although 20 mm is optimal. With respect to the aortic arch, the landing zone length should be measured on the inner curve of the aortic arch and not the outer curve. If the landing zone is considered to be of inadequate length, surgical bypass may be performed to debranch the aortic arch or abdominal aorta so that an effective sealing zone is created. This is often referred to as a hybrid endovascular procedure and examples of surgical bypasses would include ascending aorta to innominate and left common carotid (Fig. 14.1), left to right carotid bypass (Fig. 14.2) and left carotid–subclavian bypass.

The diameter thresholds are dictated by the available device sizes. A degree of oversizing is required with respect to the aortic diameter during device selection. In general, for the treatment of aneurysms, devices are oversized by 15–20%. The maximum and minimum diameters of the available stentgrafts are 46 and 22 mm. Therefore, the upper and lower limits of landing zone diameter are 40-42 and 18 mm respectively. If it is necessary to place more than one device because of the long length of aorta to be treated, the operator should also take into account the overlap required between adjacent devices, which is usually around 5 cm. Clearly, as with all endovascular aortic procedures, adequate access is required given the relatively large sizes of the endograft introducer sheaths.

The procedure is usually performed under general or regional anaesthesia, although stents can also be inserted using local anaesthesia alone. A diagnostic angiographic catheter is placed in the ascending aorta via the left brachial artery or the contralateral femoral artery. A femoral arteriotomy is often required (although percutaneous procedures are feasible and gaining in popularity), and an exchange length extra-stiff guidewire (e.g. Lunderquistguidewire, Cook, UK) is advanced so that the tip is placed in the low ascending aorta. The systolic blood pressure should be reduced to below 100 mmHg peak systolic pressure to prevent the windsock effect of the cardiac output displacing the endograft.

The endograft delivery system is advanced over the guidewire to the desired deployment site. Accurate positioning is achieved by serial aortography. When the correct position is achieved, the stent graft is released (Fig. 14.2). Further endografts are placed as necessary with at least 4–5 cm of overlap between endografts.

Management of the spinal cord during endovascular thoracic procedures

One of the most feared complications of thoracic aortic intervention is paraplegia. Placement of an endograft in the thoracic aorta causes coverage of many critical intercostal vessels (T6–L1), with the risk of spinal cord ischaemia (SCI). Some centres advocate prophylactic CSF drainage whilst others adopt an expectant policy. No independent precipitating factor of SCI has been identified, although coexisting infrarenal pathology or previous abdominal aortic repair may reduce the collateral network supplying the spinal cord.¹⁰ Clearly, if there is any sign of SCI the cord perfusion pressure must be increased by reducing CSF pressure (target 10–12 mmHg) through spinal drain insertion, and increasing mean arterial pressure (MAP; target MAP 85–100 mm Hg) with inotropes.¹¹ This selective approach appears safe for patients developing SCI after thoracic endovascular repair (TEVR), which has an acceptably low permanent neurological deficit, although overall survival of patients experiencing SCI after TEVR is diminished relative to non-SCI patients.¹²

Controversy has surrounded the management of the left subclavian artery during thoracic endografting. Early experience suggested that it was reasonably safe to cover the origin of the subclavian artery with respect to upper limb perfusion. However, more recent evidence has demonstrated that stroke and paraplegia rates are lower when subclavian perfusion is preserved, presumably due to the contribution of the vertebral artery to the anterior spinal artery.^13,14 Routine revascularisation of the left subclavian artery (via left carotid–subclavian bypass) is recommended in the authors’ practice for the majority of cases where coverage of the left subclavian origin is planned,¹⁵ and this approach is supported by the recent recommendations from the Society for Vascular Surgery (SVS).

Outcome of treatment

Results of open repair in centres of excellence are good, with 30-day mortality rates for all types of thoracic aneurysm below 12% and paraplegia rates below 4%.¹⁶ Community results that incorporate several centres are more realistic of patient-centred outcomes and demonstrate 30-day mortality rates approaching 20%.¹⁷ Mortality is predicted by renal insufficiency, age and emergency presentation.¹⁸ Outcome data for EVR of thoracic aneurysms are available from several sources. Leurs et al., on behalf of the EUROSTAR collaborators, reported data in 249 patients with 30-day mortality for elective total endovascular repair (TEVR) of 5.3% and paraplegia of 4%.¹ A cohort of patients with TAAs who underwent endografting with the Gore TAG device was compared retrospectively with the results of a cohort of 94 patients who underwent open surgical repair. The perioperative mortality (2.1% vs. 11.7%), paraplegia (3% vs. 14%) and freedom from major adverse event (48% vs. 20%) rates were all better in the endovascular group.¹⁸ Similarly, the European Talent Registry reported technical success of 98%, in-hospital mortality in 5% (4.1% and 7.9% for elective and emergency procedures respectively), paraplegia in 1.7% and stroke in 3.7%.¹⁹ Similar outcomes have been reported for the newest generation of endografts, despite the fact the patients in these later data series had more challenging anatomy compared with earlier series.¹³ Endovascular thoracic aneurysm repair has a spectrum of complications unique to the procedure. The most common is endoleak. There seems little doubt that the most common endoleak is an attachment site (or type 1) endoleak. These may occur immediately or during follow-up and should be treated with additional devices. Type 3 leaks (between adjacent devices or through disrupted grafts) may occur and should also be treated with additional devices.

Recommendations for practice

EVR of thoracic aneurysms has changed practice. In the authors’ opinion, endovascular procedures should be used as first-line therapy for most thoracic aneurysms. Exclusions to this policy would be patients with unfavourable anatomy, poor long-term durability and patients with documented connective tissue disease, in whom the results of endografting are not well defined, and the results of surgery in experienced centres are excellent.

The management of patients with Marfan syndrome has been the subject of much debate since the introduction of endovascular therapy. Patients with Marfan syndrome develop both thoracic aneurysms and dissections. However, the patients are usually younger and fitter than patients with non-specific aneurysms, have excellent outcomes from conventional surgery²⁰ and equivocal outcomes from EVR.^21,22 At present, there is no body of evidence to support the routine use of EVR in patients with Marfan syndrome.

Surgical management

As with all complex aortic diseases, detailed imaging and careful assessment of comorbid risk factors are essential. Due to the high mortality rates associated with repair of TAAAs, consideration must be given to the balance between predicted surgical risk and aneurysm diameter. Preoperative coronary revascularisation may be appropriate.

A left thoracotomy is usually required, although some type IV aneurysms may utilise an abdominal approach. The abdominal aorta is usually exposed by left medial visceral rotation. The diaphragm is partially divided with a circumferential incision preferred to preserve nerve supply. After the dissection has been completed the aneurysm is clamped at its proximal and distal extents, and an arteriotomy made in the lateral aneurysm wall. It is important that the arteriotomy is sited away from the visceral origins in the abdominal portion of the aneurysm. The proximal anastomosis is performed with a completely transected aorta to prevent aorto-oesophageal fistula. If possible, the proximal anastomosis is fashioned to include adjacent intercostal or visceral arteries, whilst any remaining intercostals are directly reimplanted into the graft or revascularised by separate jump grafts. The visceral arteries are then directly anastomosed into elliptical openings in the graft, utilising an inclusion technique. If possible, the coeliac, superior mesenteric and right renal arteries are taken on one patch; anastomosing the distal graft to the aortic bifurcation or iliac arteries completes the reconstruction (Figs. 14.4 and 14.5).

Adjunctive surgical techniques

During thoraco-abdominal aneurysm repair, prolonged visceral ischaemia is the main cause of postoperative renal dysfunction and can also contribute to multiple organ failure. To reduce the severity of organ hypoperfusion, partial left heart bypass with a centrifugal pump may be utilised to drain blood from the left atrium or upper pulmonary vein, and return it via the femoral artery or aorta. This facilitates renal and visceral perfusion whilst the proximal anastomosis is fashioned as long as the distal aortic clamp remains proximal to the visceral vessels.²⁶ During the abdominal part of the aneurysm repair, the coeliac trunk, superior mesenteric artery and both renal arteries can be selectively perfused with catheters that are connected to the left heart bypass.²⁷ Paraplegia may complicate thoraco-abdominal aneurysm repair in up to 20% of cases and is increased in more proximal aneurysms, with lengthy clamp time, renal impairment, advanced age and emergency presentations. Paraplegia results from damage to the spinal cord due to a combination of division of spinal cord arteries, prolonged spinal cord ischaemia, reperfusion injury and postoperative hypotension. Maintenance of spinal cord blood supply may be achieved by reimplantation of patent intercostal arteries and by distal aortic perfusion. CSF pressure increases during aortic clamping, and CSF drainage has been advocated to reduce paraplegia.^28,29 A functional approach to the problem of neurological deficit aims at intraoperative monitoring of the spinal cord function. Somatosensory evoked potentials are widely used to detect spinal cord ischaemia during aortic cross-clamping and to identify vessels critical to spinal cord blood supply, which may then be perfused and reimplanted. An alternative approach is to use motor evoked potentials, which have been reported to improve paraplegia rates.^30,31

Results of surgical repair of thoraco-abdominal aneurysms

The mortality and morbidity rates following conventional repair of TAAAs are still significant, with specialised centres reporting mortality rates of 5–16%, with paraplegia rates of 4–11%.32–34 However, these excellent results are not representative of outcomes outside of these centres, with national/community mortality rates exceeding 20% at 30 days and 30% at 1 year.^35–37 Identifiable patient risk factors predicting adverse outcomes include emergent presentation, previous aortic replacement, diabetes, smoking history, renal and cardiorespiratory impairment, age beyond 80 years, acute dissection and the extent of TAAA.