Figure 4. Possible TBAD treatment selection algorithm. Similarly reported by Tadros et. al (60). † Based on data summarised inTable 2 ; ‡ Based on data summarised in Table 3 .
Future directions
Next-generation biomarkers
Specific biomarkers for tissue inflammation, matrix degeneration, and fibrinolysis are increasingly encroaching into the clinical realm to better inform current predictors of unTBAD deterioration. There seems to be growing evidence for a role for these novel serologic markers in predicting the development of thoracic aortic aneurysm – many of these are elusive based on small sample studies although CRP and TGF-β appear to be of value (62). More recently, immunological metrics - such as neutrophil-lymphocyte ratio (NLR) (63), monocyte to high-density lipoprotein ratio (64, 65), and long ncRNAs (e.g. LUCAT1) (66) - have all been suggested and further study in these areas is warranted.
Imaging advances
There are known limitations to measuring the diameters of true and false lumens as a means of characterising aortic remodelling and predicting aortic enlargement. The index CT scans could be volumetrically analysed and the ratio of true lumen volume (TLV) to false lumen volume (FLV) used to predict eventual aortic intervention (67). TLV/FLV < 0.8 (OR = 12.2; 95% CI 5-26; p < 0.001) is reportedly highly predictive of aortic intervention whereas > 1.6 is highly predictive of freedom from aortic intervention (67). Incorporating personalized hemodynamic parameters in patient risk stratification could also offer prognostic value. 4D-MRI could detect flow patterns in both lumens and the entry tear, and recent clinical guidelines suggest that these non-invasive methods can be used to deliver additional quantitative information about the flow in the entry tear, and on whether or not arterial vessels are involved (56, 68, 69). 4D-MRI imaging has demonstrated that flow velocity, configuration (e.g., helical), and stroke volume are related to the rate of aortic expansion (70, 71). Since it is challenging to accurately standardize these parameters between patients based on differential aortic morphologies, patient-specific approaches must be used to utilize 4D-MRI for both predicting aortic expansion and as a tool for selecting patients who may require additional procedures based on their clinical conditions (e.g. organ malperfusion) (72, 73). Other innovations such as multidetector CT, high-resolution ultrasound including color-coded Doppler, and contrast-enhanced B-mode imaging have also been reported that may be used to enhance aortic care (74).
Conclusions
TEVAR has enriched the surgical armamentarium to treat acute and subacute TBAD and has reached maturation as a baseline strategy in many patients. A specific subset of acute unTBAD patients at greater risk for developing aortic complications should be strongly considered for timely prophylactic TEVAR intervention. There are a variety of demographic, clinical, laboratory and radiographic/morphological risk factors to consider when selecting these patients. To guide selection, the principles of the treatment scheme presented in this review could be used. The scheme could be refined in future as the reporting of risk quantification ratios across the literature becomes more consistent, and advances in biochemistry and imaging techniques progressively enter the clinical arena. Selection of high-risk patients should also be further informed with randomized controlled trials and multicentre registry analyses.