Commentary: Relining Failing Abdominal Aortic Stent-Grafts With the Nellix Endoprosthesis

Endovascular aneurysm repair (EVAR) has gradually replaced open surgery as the preferential treatment of patients with anatomically suitable abdominal aortic aneurysms, despite the small but definite incidence of late complications. ¹ These complications contribute to the reduced survival of patients treated with EVAR compared with those undergoing conventional surgery. ²

In developed countries, the rise of EVAR has been accompanied by a parallel increase in life expectancy, with a gain of ~3 years in the past decade by those in the seventh decade of life; assuming this trend continues, an average 65-year-old may soon expect to live well in excess of 20 years. ³ Patients with aortic aneurysms may not fare as well as “average” people, but many will still outlive their endograft and require further treatment, sometimes many years after their original repair, when they may no longer be suitable for major surgical intervention. The ultimate failure of EVAR is aneurysm rupture, which is most commonly preceded and caused by type I or type III endoleaks. ⁴ It thus seems inevitable that (endo)vascular specialists will be called to intervene on these complications with increasing frequency over the coming years.

In the February 2017 issue of the JEVT, Youssef and colleagues ⁵ report their initial experience with the Nellix Endovascular Aneurysm Sealing (EVAS) system (Endologix Inc, Irvine, CA, USA) with and without visceral chimneys for post-EVAR secondary intervention for type Ia and type III endoleaks. They treated a small but challenging cohort of patients, many presenting urgently; some would have been difficult, if not impossible, to manage by other endovascular means. The authors’ results are impressive, particularly the 100% technical success and the absence of visceral stent occlusion and endoleaks, albeit during a limited follow-up period. The reader should not assume that these outcomes are automatically reproducible, as they were achieved in large specialist centers with significant experience in both simple and complex EVAS.^6–8 The reader should also be aware that, according to its instructions for use, the Nellix device is designed neither for secondary aortic procedures nor for the concomitant use of visceral chimneys. ⁹

Despite these caveats, the Nellix endoprosthesis offers some obvious advantages in this setting. For type III endo-leaks, where proximal seal extension is not necessary, simple relining of the endograft with 2 Nellix stent-grafts can be performed with ease, even within short-bodied endografts, in which insertion of a second infrarenal bifurcated endograft may be impossible. In the emergency situation, EVAS with visceral chimneys (chEVAS) offers an off-the-shelf solution to those patients with type Ia endoleak who require proximal extension of the seal. ChEVAS also has fewer anatomical constraints than standard extension cuffs with visceral fenestrations (which can be very difficult to deploy), and its use has been recently supported by the announcement of the early results of the ASCEND registry. ¹⁰ Gutter-related endoleaks, which are sometimes encountered after chimney EVAR, do not seem to be a problem with chEVAS. ¹⁰

While there are some self-evident merits in the proposed technique, it is also important to highlight some potential limitations for the benefit of those who may wish to introduce it to their practice. EVAS, when used as primary treatment modality, excludes an aneurysm by occupying its lumen entirely with filled endobags; it does not rely on active fixation, as the Nellix endoprosthesis does not exert any radial force on its landing zones. In theory at least, this “sac anchoring” impedes migration of the stent/endobag complex, as all the potential space for displacement is obliterated. When deployed within another endograft, Nellix stents do not increase distraction forces, ¹¹ but although augmenting columnar strength, they do not provide any added fixation either. It is thus important, prior to embarking on EVAS relining, to make sure that the fixation of the original endograft is preserved.

Readers familiar with EVAS will be aware of the volume/pressure relationship within endobags during their inflation (Figure 1). They would have noticed that pressure in the endobags remains close to the mean arterial pressure during inflation but rises sharply when the luminal volume is filled. At this point, even small further injections result in a major increase in pressure. When EVAS is used as the primary treatment modality, pressure spikes may be blunted by the compressibility of aortic thrombus.^12,13 When the endobags are deployed within the confined space of a rigid endograft, a small volume excess may result in extremely high endobag pressure, with possible prolapse (as encountered by the authors, fortunately without clinical consequences) or even rupture. Slow, steady endobag inflation, coupled with awareness of the potential consequences of hyperinflation, will help reduce this risk. Furthermore, “unfurling” of the endobags with a small volume of saline prior to stent deployment may also help to eliminate folds in the endobags and potential pockets (within said folds) in which high pressure may be generated. The use of contrast during prefill, as proposed by the authors, may also help detect endobag herniation when it occurs, but only if this is in a plane perpendicular to the fluoroscopy beam.

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Figure 1.

Relationship between pressure and volume during inflation of the endobags. Pressure remains around mean arterial values (MAP) for much of the inflation and quickly rises when the final volume is approached. The rise is steeper when endobags are inflated in smaller volumes.

Like the authors, we also have had a positive experience with EVAS and chEVAS in secondary aortic interventions for life-threatening situations where other therapeutic options were not feasible. In these scenarios, where long-term outcome is much less of a concern than in other circumstances, the technique can prove to be life-saving. We strongly recommend, however, that specialists considering its use should become thoroughly familiar with the Nellix device as well as advanced aortic endovascular techniques prior to introducing it to their practice.