Is Imaging the Holy Grail of Aftercare in Aortic Dissection?

The AHA Position paper addresses an important topic in contemporary cardiovascular medicine and shines a highlight on to a precious set of patients who have survived aortic dissection¹. These patients deserve better care and personalised attention as they had already faced a significant threat to their lives from both the impact of aortic dissection and from the trauma of major surgery, with long circulatory arrest time, hypothermia and selective cerebral perfusion (in experienced centres), or endovascular interventions in distal dissection. Realising that even the best repair strategies are associated with early survival rates of around 80% after successful surgical treatment, leaving every surviving patient with a life-long condition, underlines the urgent need for a standardised and comprehensive surveillance protocol with longitudinal imaging as centrepiece.

Similarly, patients prior to surgery with known aortic pathology (or at risk for aneurysmal formation or dissection), such as patients with underlying hereditary connective tissue disorders, also deserve surveillance both from a clinical as well as from an imaging perspective. Together these groups of patients, both at risk or after surgery, are growing in numbers and represent an increasing health care burden with a population-based prevalence of 28 to 42/100,000/per year.

This scientific statement offers up-to-date guidance by showing the needs for structured imaging surveillance and demonstrating ways to actually implement imaging surveillance in daily practice by referencing the evidence of late complications and degenerative changes in patients with aortic conditions. As a bonus it provides a glimpse into the near future by introducing the concept of predictive imaging and insights into pathophysiology and mechanobiology of dissected aorta as derived from advanced functional MR and CT imaging. Optimistic speculation suggests that prognostic information beyond morphology and anatomy can soon be derived from contemporary scanning protocols by use of smart algorithm; such outlook is certainly of academic interest and not yet backed by validating data.

Nevertheless, the community has understood that even longitudinal changes in morphology alongside the entire aorta, such as growing dimension of an aneurysm and early expansion of dissected aorta are important risk factors and not just by-standing observations that should often trigger swift proactive measures. For instance, in up to 7% of chronic Type B dissections a retrograde Type A dissection may develop with subtle or no classic symptoms, and in up to 49% of operated Type A dissection later aneurysmal degeneration with a risk of rupture may develop in the remainder of the dissected aorta. Hence, direct communications of even subtle signs of progression are equally important and highlights the need for integrated interaction with aorto-vascular surgeons and interventional specialists in an aortic team to address any lurking problem in the follow up of patients. The group of authors, although spearheaded by imaging specialists, combines cardiac surgeons, endovascular specialists and epidemiologists and is an excellent example of a multilateral approach in the aftercare of chronic aortic dissection. Moreover, indications for surgery and thresholds for open or endovascular interventions are in fact moving targets with updated rules and recommendations, especially in the context of hereditary connective tissue disorders. While thresholds for indications tend to get tighter, in many of these patients however, decisions are not based on reconstructed images alone, no matter how advanced they are, but also on age, family history, dynamics of progression, or even patients' preference (PEARS procedure²), highlighting the need for interdisciplinary discussion rather than an image - only guided intervention.

While the statement provides a broad overview of the number of aspects around management of patients' chronic aortic dissection, key elements focus on why image surveillance is important, how this should be put in place and what parameters should be measured.

Why? Imaged surveillance in patients with chronic dissection is highly likely to have prognostic impact since timely detection of asymptomatic progression allows repair or scaffolding and avoid rupture³. Expansion of the False Lumen, in the case of dissection or retrograde progression of a chronic distal dissection are often clinically silent but could be detrimental if not managed properly. Conversely documentation of remodelling of dissected aorta after endovascular or open repair may herald an excellent long-term prognosis⁴. The impact of endovascular True Lumen interventions in chronic aortic dissection such as stent grafting, axillary staged procedures including stent extension, or false lumen interventions (embolization or occlusion techniques) should be assessed and followed even after successful procedures by serial imaging^{5, 6,}. Assessment of longitudinal changes is not only important for aortic dissection but certainly for patients with connective tissue disorders at risk for aneurysm formation or other features of progression and may even apply to 1^st degree family members of patients diagnosed with such as condition. A gap in knowledge is the ideal interval for serial imaging surveillance with lack of supporting evidence. Moreover, the use of emerging functional information from 4D MRI simulations and Finite Element modelling of aortic stress maps is uncharted territory. It is intriguing that follow-up surveillance can be individualised based on several input parameters including clinical variables and biomarkers. Ideally, a personalised surveillance plan would not miss any evidence of progression, but also avoid too many unnecessary scans and protect resources. Unfortunately, we are still far from understanding such a complex algorithm that will be composed of more than just imaging data.

How? Current standards of surveillance are still ill-defined despite a sensible recommendation to use 2D and 3D reformatting of contrast enhanced CT angiographic acquisition with ECG gating. With the use of retrograde gating and phasic acquisition of CT images, of course the argument of 3 times higher radiation needs to be considered. In particular for younger individuals gadolinium enhanced MR angiography provides high quality 3D datasets for both morphological and functional imaging to assess blood flow dynamics or thrombus formation in the false lumen, which may have predictive information.

Ultrasound techniques have a place in monitoring the aortic root and ascending aorta (even after surgical repair) and elegantly assess function and morphology of the aortic valve as well. Transoesophageal ultrasound is less useful in the scenario of chronic dissection but of paramount importance in the context of endovascular or open repair manoeuvres and possibly undervalued in this paper. The abundance of information provided by peri- interventional transoesophageal ultrasound about morphology and immediate changes with endovascular procedures both of flow of the true and false lumen and on transformation to thrombosis is highly important and requires deep knowledge of aortic pathology and procedures. This new emerging field is likely to produce an interventional echocardiographer as an integral part of an Aortic Team. As there is an issue of reproducibility in all of the imaging modalities it appears sensible to use the identical modality for longitudinal evaluation in any given patient. Realising however that TTE is only useful to visualise the aortic valve and proximal aorta.

What? Subject of longitudinal surveillance is certainly each individual patient, but the substrate of imaging is mostly dimension, threshold diameters, and possibly flow information, all considered moving targets as there is a trend to lower thresholds at least in hereditary aortopathy patients to indicate the need for intervention. We still miss out on direct assessment of the aortic wall thickness and composition, on valid description of elastic properties of the aorta and functional assessment of the wall. Moreover, information derived from phasic CT imaging or 4D MRI including stimulation strategies are still experimental and far from being clinically validated⁷. However, for the time being utilisation of our available toolbox can be optimised as nicely shown in the position paper. For instance, reporting and visualisation of longitudinal changes in a graphical format is a fantastic tool to convey information amongst all shareholders including surgeons, interventionalists, and family physicians or specialised nurse led clinics involved in follow-up. The presented format demonstrates nicely how the progression of disease or even regression of the false lumen dimension after treatment, in a convincing way. Display of temporal changes over time in just one figure has also a highly educational element even in the communication with patients and families.

A risk stratification based on serial imaging is certainly the Holy Grail of surveillance, but is it enough? And did we make much progress over the last 25 years using image information? We still use primarily dimensions, and morphology rather than functional information and established thresholds to roughly estimate the risk of rupture in patients with chronic aortic dissection. Non-morphologic parameters are not consistently used for risk prediction and are far from being validated, although it is intriguing to incorporate markers of inflammation, potential drug effects, and certain comorbidities, in a smart and adaptive risk prediction model. Instead of a holistic risk prediction algorithm with image information as important integral component, current efforts are still focused on documentation of longitudinal changes in morphology, or evidence of remodelling after stent-grafting or adjunctive measures over time. Computer simulations of future evolution is a compelling new field of research but invokes assumptions to model the near or far outcome of natural course, or the result of an intervention. Validation will be critical for this concept to stand the test of time.

On aggregate, congratulations for the author group for timely highlighting the concept of surveillance imaging in patients with chronic aortic dissection, for introducing the latest development in suitable imaging techniques and for a glimpse into the near future of predictive imaging. Nevertheless, surveillance imaging should be embedded into a framework of multidisciplinary corporation in an Aortic Centre or care group, and risk prediction requires a holistic approach on an individual patient with morphology as one component.