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Treating aneurysmal disease
Aneurysms can affect vessels of any size. Clearly, not every aneurysm requires immediate treatment but, when indicated, successful endovascular management requires exclusion of the aneurysm from the circulation. The main options can broadly be thought of as:
Embolization techniques which sacrifice the target vessels:
This is only used when it is safe to occlude part of the circulation, i.e. there is a collateral pathway or the end target can be sacrificed; this will be covered in detail in the management of haemorrhage section.
Techniques that preserve flow through the target vessel:
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Stent-grafting to restore a normal-calibre lumen. Most often used in larger-calibre vessels (>6 mm)
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Packing the sac with coils sometimes combined with stenting across the aneurysm. This technique is most commonly used to manage aneurysms in the cerebral circulation and is a useful adjunct in the visceral arteries where flow must be preserved and a stent graft cannot be used.
Each of these strategies has its place and, as always, the real skill lies in choosing who, when and how to treat. There is not enough space to cover the nuances of every clinical scenario and each device, so the emphasis here is on the generic aspects of terminology, assessment for stent-grafting and the basic principles of deployment of devices.
Aneurysm terminology
As a general rule, a region is deemed aneurysmal when it is at least 1.5× greater than the adjacent, normal arterial segment. When commenting on less dilated vessels, consider using ‘ectatic’ rather than ‘aneurysmal’; this generates less alarm and work. Aneurysms are classified and discussed in several ways.
Morphology:
Useful really only as a descriptive term.
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Fusiform – a roughly symmetrical swelling of an artery
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Saccular – an asymmetric outpouching from the artery wall.
Aetiology:
An important consideration when deciding treatment.
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Degenerative/atheromatous: commonest type, usually age-related
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Infective/mycotic: often irregularly shaped with adjacent inflammatory change
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Inflammatory: classically abdominal aorta with evidence of thickening of vessel wall and lack of separation between aneurysm and adjacent structures
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Traumatic: more often saccular and associated with other injuries, presentation may be delayed
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Abnormal connective tissue: often multiple sites, e.g. Ehlers–Danlos.
Structure:
‘True’ or ‘False’ aneurysm?
True aneurysms
involve all the layers of the artery wall, these have a more predictable prognosis, e.g. risk of rupture of an abdominal aortic aneurysm (AAA) increases with increasing diameter.
False aneurysms
( aka pseudoaneurysms) form due to a defect in the artery wall and the blood is constrained only by adjacent soft tissue, e.g. following trauma such as arterial puncture. The absence of a wall makes a false aneurysm prone to catastrophic rupture.
Alarm
Improved technology makes more aneurysms amenable to treatment by endovascular means; it does not follow that all aneurysms need to be treated or that the endovascular approach is the best option for the patient.
When to treat aneurysms?
Aneurysms are more frequently detected now due to the prevalence of multislice computed tomography (CT). Many will be incidental findings. The threshold for intervention varies depending on the size, site, the anticipated impact of rupture and our level of knowledge about the benefits and risks of intervention. Diameter thresholds for treating aortic and thoracic aneurysms are well established, fairly well agreed for visceral aneurysms and some larger vessel peripheral aneurysms but for smaller aneurysms, the natural history is often uncertain. Guidelines and treatment thresholds ( Table 48.1 ) are a useful starting point but management decisions will also reflect other factors including aetiology, symptomatology and speed of aneurysm growth. Patients with a symptomatic, very large or rapidly growing aneurysms are at increased risk of rupture and may have a greater benefit from treatment. Case-by-case discussion is essential, especially for less common scenarios.
Table 48.1
Accepted thresholds for intervention a
| Thoracic aorta | 55 mm if atherosclerotic; 45 mm if Marfans b |
| Abdominal aorta | 55 mm or growth rate >10 mm year |
| Iliac | 40 mm |
| Popliteal | 12 mm |
| Visceral aneurysms | 20 mm in diameter or larger in women beyond childbearing age and in men |
Guidance for imaging prior to endovascular repair
Endovascular aneurysm repair requires far more detailed assessment than is necessary for open surgery. Computed tomography with multiplanar reconstructions is the method of choice and is able to accurately measure the diameters, lengths and angulation of the aneurysm, anchorage sites and the access vessels ( Figs 48.1 , 48.2 ).
Tip
Use suitably wide windows when reviewing the images to allow you to distinguish between calcification in the vessel wall and contrast enhancement in the lumen.
Imaging assessment
What to look for?
The variety of scenarios and devices makes it impossible to be dogmatic and each manufacturer has their own restrictions in the ‘indications for use’ for the device; if in doubt check with the ‘sizing chart’. There are however some general points to consider:
Access:
Usually the common femoral and iliac arteries.
Stent grafts are relatively large and inflexible devices. Check the size (≈French size/3), currently available devices for endovascular abdominal aortic aneurysm repair (EVAR) are introduced via ≈18–28F (6–9 mm) sheaths. Ask yourself whether it will be possible to introduce the device (percutaneously or by surgical cut down) into the chosen access vessel and advance it from there to the target. Normal vessels will usually straighten with a stiff wire but heavily calcified, narrow and tortuous vessels are unlikely to be winners. In the presence of significant disease, review alternative points of access.
Tip
If there is uncertainty whether a calcified artery will straighten, perform a test catheterization using the stiff wire needed for EVAR.
Alarm
Severe disease in the common femoral artery (CFA), iliac arteries or anywhere else en route is likely to be problematic for either introducing or removing the delivery system.
Anchorage:
At either end of the aneurysmal segment (aka neck) . Secure fixation is essential to forming a seal. Failure to achieve this will lead to type I endoleak or graft migration! The ideal neck is straight, parallel-sided, disease-free and of suitable diameter and length to allow a seal. Atheroma, calibre discrepancy and tapered necks will make deployment more challenging. Too short and you will risk occluding branch vessels or landing in the sac.
Alarm
Significantly oversized grafts may not seal due to folds in the graft material.
Target site:
Is the target artery straight or tortuous? Ask yourself whether the graft can be delivered to the target and whether it is sufficiently flexible to conform to the arterial anatomy. If not, ask yourself what will happen if the stent graft straightens out the vessel?
Alarm
There is a real risk of kinking at the end of the stent graft leading to occlusion.
Aneurysm sac
Wall:
Look for evidence of inflammatory change including thickening and loss of fat planes.
Diameter:
This will serve as a baseline for comparison with evolution during follow-up.
Thrombus:
It is also customary to note the degree of thrombus within the sac. In general, the greater the amount of thrombus, the fewer branch vessels arising from the sac.
Branch vessels:
Are there any branch arteries arising from the target area? As the stent graft will cover them they will either occlude, leading to ischaemia, or alternatively they may compromise repair by retrograde perfusion of the aneurysm (type II endoleak). In either case, ask yourself: can these be safely sacrificed if they are occluded or by pre-emptive embolization?
Aortic disease
Endovascular abdominal aortic aneurysm repair (EVAR) and thoracic EVAR are the commonest stent-graft procedures. Ruptured abdominal aortic aneurysm (AAA) carries a 75% mortality, roughly half the patients die within 30 min and a further 50% will die despite undergoing repair. Hence, the management strategy aims to prevent rupture by screening to identify patients with asymptomatic AAA and then following up until they reach a diameter of 5.5 cm, show accelerated growth or become symptomatic.
The majority of infrarenal abdominal aortic aneurysms are secondary to atherosclerotic disease. Rarer, but still seen with reasonable frequency, are inflammatory aneurysms; surgical dissection can be difficult in these patients and endovascular repair is often preferred if possible. Even rarer are mycotic aneurysms; they are often diagnosed by a combination of unusual site/morphology and a good history. Decision-making is complex and will depend on the underlying cause and prognosis. Treatment is usually considered when aneurysm diameter is >55 mm or becomes symptomatic or fast-growing (10 mm/year). As well as CT, the patient should undergo physiological assessment of cardiorespiratory function to assess suitability for EVAR. Patients with an asymptomatic infrarenal AAA >5.5 cm in diameter who are deemed ‘fit enough for surgery’ should be offered treatment by EVAR or open surgery. Patients who are judged unfit for surgery are unlikely to live long enough to benefit from aneurysm repair and should be managed conservatively.
Assessment for EVAR
When assessing the aorta, before making measurements, use the guidance for imaging and make a rapid review of factors that will preclude or may complicate EVAR and consider possible solutions. These include:
Access issues
Small CFA/iliac artery <7 mm
Solutions:
Consider the use of a temporary graft conduit to allow device delivery.
Tortuous, calcified vessels especially with more than one ≥90 degree angulation
Solutions:
Angioplasty for focal stenosis, consider alternative access for more extensive disease, e.g. cutdown onto the iliac arteries ± placement of a temporary conduit.
Narrow distal aorta
This can prevent access for a contralateral iliac limb or cause narrowing of both limbs of a bifurcated graft. Prevention is better than cure!
Solutions:
Consider an aorto-uni-iliac (AUI) graft and femoro-femoral cross-over graft, but do not forget to occlude the contralateral common iliac with an Amplatzer plug to prevent type II endoleak.
Short, angulated, tapered and diseased anchorage sites
Length:
Should be ≥10 mm for the proximal neck but limits vary from device to device.
Configuration
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Angulation: Ideally should be ≤60 degrees, though some devices will cope with greater angles
Tip
If the neck is short and angulated, then use reformats to assess the optimal obliquity to use for angiography/fluoroscopy during graft deployment.
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Shape: Ideally the neck should be parallel-sided. If conical, does the diameter increase or decrease towards the aneurysm?
Alarm
Short cone-shaped necks which enlarge towards the aneurysm increase the risk of migration.
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Neck quality: Thrombus, eccentric calcification or atheroma can impact on sealing. Usually recorded as 25%, 50%, 75% of the circumference.
Solutions:
In theory, most issues with the proximal neck can be remedied by the use of grafts with suprarenal fixation ( Fig. 48.3 ), fenestrated grafts which have a greater contact with the wall or branched grafts, which anchor proximally in the ‘normal’ aorta. In practice, consider whether this will be better for the patient than an open repair.
Branch vessels
Review the imaging, looking for expected and variant anatomy:
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Important branches: decide whether the vessel is essential, e.g. hypertrophied inferior mesenteric artery (IMA) in the presence of superior mesenteric artery (SMA) disease or if it can be sacrificed, e.g. a small accessory renal artery.
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Unwanted vessels: decide whether pre-procedure embolization will improve outcome and treat as required.
Measurements for EVAR
With conventional bifurcated and aorto-uni-iliac (or AUI as the ‘cool’ operators term them) devices, roughly one-third of patients with AAAs will be straightforward to treat. A further 30–50% will be treatable but would be expected to be more challenging with limited implantation sites, awkward angulations or difficulty with access. Such cases will be higher risk with less certain long-term outcomes. These should only be undertaken after careful consideration by the clinical teams and discussion with the patient. Juxtarenal, suprarenal and thoracoabdominal aneurysms are treatable but their management should be undertaken in tertiary centres and is beyond the scope of this book.
For AAA repair, it is conventional to know the following measurements:
Aorta proximal neck
Remember to note any thrombus/atheroma.
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