Risk stratification in acute type A aortic dissection: angle or elongation—2 sides of the same coin?

In this issue of the journal, Della Corte et al. [1] present an observational study focusing on geometrical features of the aorta for potential risk stratification regarding acute type A aortic dissection (ATAAD). With their present work, the authors expand on their previously published data [2] by incorporating markedly more patients through a multicentre approach, while also differentiating between patients with bicuspid or tricuspid aortic valves.

Three main findings are highlighted: (i) a strong association between a narrow ascending-to-arch angle and ATAAD, (ii) the correlation between root length and the aforementioned ascending-to-arch angle also in non-ATAAD patients and (iii) a more marked association of the ‘root phenotype’ with ATAAD in bicuspid patients (BAV) compared to patients carrying a tricuspid aortic valve. Some of these key findings regarding geometrical peculiarities were already described in their initial pilot study [2]. Therefore, it stands to reason asking what their current iteration adds to the body of evidence and what the potential implications and future perspectives are?

‘WHAT IS KNOWN?’—IN ADDITION TO THE AORTIC DIAMETER, OTHER SPECIFIC ANATOMICAL FEATURES SEEM TO PLAY A PIVOTAL ROLE IN THE DEVELOPMENT OF AORTIC ANEURYSM AND ATAAD

For a long time the absolute diameter of the ascending aorta remained the decisive variable for pre-emptive surgery in patients with ascending aortic aneurysm. However, various studies have shown that diameter alone may not be the definite risk factor since it is oftentimes not significantly different when comparing patients with normal aortas to patients suffering from ATAAD [3, 4]. Since sufficient pre- and post-dissection imaging data of the same patient are generally scarce, many authors utilized estimated pre-dissection diameters [5]. As a consequence of many such observational studies, particular attention has been directed to specific anatomical features of the diseased aorta as risk factors for ATAAD [3, 4]. This ultimately resulted in the incorporation of factors like ‘root phenotype’ aortopathy and aortic size indices—alongside the absolute diameter—into the latest ‘Guidelines for the Diagnosis and Management of Aortic Disease’ [6]. The length of the tubular ascending aorta in particular (aortic elongation) has been identified as a potential factor associated with an increased risk for ATAAD [3, 4, 6]. Regarding the relative prevalent BAV, it is estimated that ∼40% of patients develop aneurysmal enlargement and that the ‘root phenotype’, hence the predominant dilatation at the level of the sinuses of Valsalva, represents an especially malignant and rapidly progressive aortopathy [7].

‘WHAT IS NEW?’—AORTIC ELONGATION AND ASCENDING-TO-ARCH ANGLE ARE NOT DIRECTLY ASSOCIATED

Apart from the fact that the present study by Della Corte et al. draws conclusions from a larger cohort, a particular improvement is that through meticulous patient follow-up, valuable data of patients with pre- and post-dissection imaging were acquired. Although only 8% of such data sets were available, the authors also utilized estimated, algorithm-based pre-dissection diameters and differentiated between bicuspid and tricuspid patients to further improve their model. Nevertheless, a reoccurring criticism remains the idea that the introduced ascending-to-arch angle as the key variable represents merely a surrogate for aortic elongation. However, the authors demonstrate that the degree of elongation in patients with aneurysms compared to patients suffering from ATAAD is the same. They were able to show that only the ascending-to-arch angle is significantly different when comparing patients with aneurysms and patients suffering from ATAAD, while aortic elongation is present in both groups. This somewhat unexpected finding does, however, not necessarily contradict the notion that elongation still represents a risk factor for dissection. Rather it seems to add to the concept that not all elongated aortas are predetermined to dissect. Interestingly, in BAV, the root tract was significantly more elongated in ATAAD patients compared to patients with aneurysms, while the length of the ascending tubular tract was not significantly different. This resulted in a two-fold higher prevalence of ‘root phenotype’ in the BAV cohort compared to their tricuspid counterpart. Based on the presented results, it remains uncertain whether the ascending-to-arch angle or the length of the root tract should be considered as the principal risk factor of ATAAD.

Undoubtedly the most valuable data in all retrospective analyses regarding risk stratification for ATAAD are pre- and post-dissection images of the same patient. The present study by Della Corte et al. reveals that in all patients, for whom pre-dissection images were available, the mean ascending-to-arch angle was narrowed. Although potentially indicatory for having a relevant influence, the low number of patients, especially regarding BAV patients, remains a major limitation for a decisive interpretation of the results.

‘WHAT IS NEXT?’—FUTURE PERSPECTIVES

Prospective studies regarding specific ‘high-risk’ anatomical features in ATAAD are not a realistic option. Therefore, alternative study approaches are important. Apart from the aforementioned pre- and post-dissection images, a reasonable complementary approach—specifically for the results demonstrated by the authors—may be combining their findings with other imaging based methods. Since it is speculated that the increased angulation exacerbates mechanical stresses on the aortic wall, investigating a narrowed ascending-to-arch angle using wall stress models may underscore the impact of such a variable. It has already been demonstrated that circumferential and longitudinal stresses are greater in BAV compared with tricuspid patients suffering from ATAAD and are not correlated with aortic diameter in this specific cohort [8]. Since peak wall stress can indicate the areas susceptible to the formation of an intimal tear and subsequent aortic dissection [9], correlating the ‘angulation’ (narrowed ascending-to-arch angle and/or elongated root) with peak wall stress and subsequently analysing other factors such as vessel wall composition may shed light on some of the unresolved key questions. Especially for the BAV cohort, such analyses could potentially greatly improve our current risk stratification models.

Despite the limitations of the presented study—most notably the few pre- and post-dissection images and the overall heterogeneity of the study population—the authors did an exceptional job by openly addressing these issues and using alternative ways as surrogate. By expanding their initial cohort using multicentre data and approximating diameters by utilizing algorithm-based pre-dissection calculations, the results do corroborate various ideas also from other researchers regarding the potential influence of specific anatomical features on ATAAD. Whether the ascending-to-arch angle represents the key risk marker for ATAAD remains unclear. However, in the absence of 1 feature reliably predicting ATAAD, we all can agree on, the ascending-to-arch angle seems to nicely combine more recent notions of risk prediction, namely the ‘root phenotype’ in conjunction with ‘aortic elongation’, potentially resulting in a more specific rather than sensitive stratification marker.

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