2.4.1 Timing of TEVAR and Aortic Remodelling
Several studies showed that the long-term aortic remodelling is influenced by the timing of intervention and the different zones of dissection. This was addressed in a recent review by Jubouri et al. [4] who summarised the evidence in the literature on timing on TEVAR in relation with results, and concluded that performing TEVAR during the subacute phase of un-TBAD yield optimum results, with those being comparable to the acute phase but superior to the chronic phase. A comparative study demonstrated a post-TEVAR reduction in the maximum aortic diameter in the acute and early chronic groups (-4.3 ± 9.3 vs. -5.2 ± 6.9). In contrast, the late chronic group showed a significant increase in the maximum thoracic aortic diameter post-TEVAR. (2.5 ± 4.6 mm, P<0.001) [56]. Similarly, Torrent and colleagues [7] reported a non-significant difference in the degree of dissection extension between interventions in the acute vs. subacute stages of TBAD.
The VIRTUE registry showed a significant reduction in FL area in acute and subacute groups compared to the chronic group [54]. Patients with acute TBAD showed a more consistent degree of remodelling (thoracic FL thrombosis in 80% to 90%) than those with a chronic TBAD (38% to 91%) following TEVAR [55]. Yuan and colleagues [5] further validated these results, concluding that the first three months after the onset of symptoms of TBAD represented the optimal aortic plasticity for intervention for remodelling post-TEVAR.
While timing showed no association with FL thrombosis in the proximal and distal descending thoracic aortic zones (P > 0.3), FL thrombosis inferior to the diaphragm was significantly lower in patients with a chronic TBAD(P=0.035) [54].
True lumen expansion results from the stent-graft placed within the TL of the thoracic aorta covering the primary entry tear. By sealing off the primary entry tear, it provides scaffolding support and ensures blood flows distally within the TL. The ADSORB trial [17] showed a significant increase in TL diameter and reduction in FL diameter within the first year of follow-up following TEVAR. Yet, the INSTEAD-XL trial [16] required a five-year follow-up to demonstrate this significant difference. This difference could be attributed to the timing of patient enrolment in both trials as the ADSORB trial patients were enrolled during the acute phase of dissection (<14 days). Those in the INSTEAD-XL trial were recognized as stable and were enrolled during the subacute phase. Conversely, results from the VIRTUE registry [54] showed that the TL expansion was not related to the timing of TEVAR. Another study by Andacheh et al. [57] showed that the rate at which the TL expands after TEVAR is not parallel to that at which the FL shrinks; the expansion of the TL plateaued after 12 months, while the shrinkage in the FL continued beyond 12 months post-TEVAR. This can further ascertain the assumption that most of the TL expansion is attributed to the scaffolding effect of the stent-graft implantation.
Although the FL regression and the TL expansion seem to demonstrate a significant change following the treatment with TEVAR, this is not the case with the reduction in the maximum aortic diameter. In fact, some studies showed a further slight expansion of the maximum aortic diameter [52, 58]. See table (3)
Although FL thrombosis is considered as a primary predictor of aortic remodelling, there is evidence not all thrombosed FLs have positive remodelling outcomes. A study by Kitamura showed post-TEVAR FL thrombosis was achieved in 74%, whilst only 47% achieved descending thoracic aortic remodelling. Therefore, descending aorta remodelling was reported to remain uncertain, especially when associated with an initially large aortic diameter [59].
Another study by Omura et al. [58], patients with un-TBAD in the acute-subacute timing exhibited smaller aortic diameter. All the patients offered TEVAR in this time period showed significant shrinkage of the proximal aorta. However, distal to the stent-graft, shrinkage was observed in 50% only. This study has demonstrated the importance of radiological surveillance post-TEVAR. Up to 17.8% who underwent TEVAR showed aortic dilation at the distal landing zone [58]. Furthermore, Xie et al. [8] studied abdominal and thoracic FL as separate entities in the follow-up and showed greater than 50% have a patent abdominal aortic FL post-TEVAR in both the acute and subacute phases. Several other studies established this segment-specific approach for the long-term evaluation of aortic remodelling and FL thrombosis [8, 44].
The surveillance of the aortic parameters might be as vital as the primary intervention for the long-term outcomes. Careful assessment of the aortic diameter and the degree of FL thrombosis over time can be very informative about the natural history of the underlying dissection and its sequelae. The risk of developing dSINE must be considered when evaluating an expanding aortic diameter post-TEVAR. The emerging data concerning the aortic expansion distal to the stent-graft needs further investigation and follow-up to better understand the long-term outcomes following TEVAR.