By Ali Khoynezhad
Acute type B dissection is a devastating medical condition and its treatment paradigm has evolved significantly. Open aortic replacement for complications was standard of care for three decades. In recent years, primary stent grafting for complicated type B aortic dissection has been applied with favourable results. Thoracic aortic endovascular repair (TEVAR) can be offered with a relatively low perioperative morbidity and mortality in experienced hands, and it is considered standard of care. The rate of mortality and ischaemic spinal cord injury are typically reduced by 50%, when compared to open surgery.
Classic indications for TEVAR are currently complicated type B aortic dissection such as malperfusion to distal branch-vessels or (impending) rupture as well as high-risk uncomplicated patients with type B aortic dissection. These patients, at significantly higher risk for early rupture, include patients with persistent pain, resistant hypertension, enlarged descending thoracic aorta above 4cm upon presentation, false lumen above 2.2cm and fusiform index over 0.64.
The classification system for aortic dissection dates also back to the open surgical era. DeBakey, Stanford, Najafi, UAB and MGH classifications all stratify based on propagation and extent of the aortic dissection. The location of intimomedial tear, the most important information in endovascular era, is not part of these classifications. This is due to the fact that sophisticated CT scan algorithm with EKG-gating and thin cuts demonstrating the intimomedial tear were not available three decades ago. For example, a patient with an acute retrograde type A aortic dissection, without evidence of rupture or aortic regurgitation, may be offered TEVAR, if sole entry tear is in the descending thoracic aorta. Based on old classification schemes, the patient would have an open-heart operation, putting the patient at higher risk for morbidity and mortality. The recently publicised DISSECT mnemonic by DEFINE investigators is a useful tool for emergency room doctors and referring physicians, but it basically summarises old classification schemes with some clinical findings.
In a quest for a new classification, it is critical to predict successful reverse aortic remodelling, thereby obviating need for future aortic interventions. The author reviewed patients with type B aortic dissection with an average follow-up of four years, and found that location of intimomedial tears and size of the tears has a ramification on clinical malperfusion and reverse aortic remodelling: each patient had an average of 2.8±2.1 tears visible on gated CTA with a mean surface area of 0.63cm2. In addition, 80.5% of tears were above the celiac artery, and 62% of patients with intimomedial tears distal to the celiac artery had evidence of clinical or radiological malperfusion. While the number of tears did not correlate to malperfusion, the size of the tear did: 50% of patients with intimomedial tears >1.9cm2 and 72% with tears >4.8cm2 had malperfusion.
The classification predictive of reverse aortic remodelling after TEVAR was first proposed by our group in a publication in the Journal of Vascular Surgery, based on the follow-up data of patients after TEVAR for type B aortic dissection.
Based on this study, patients with type 1 and 2 (intimomedial tears visible proximal to the celiac artery, 97%) had reverse aortic remodelling at latest follow-up, if the intimomedial tears were excluded successfully during the index TEVAR procedure. This is thought to be due to rerouting the blood flow completely through the true lumen, and enabling reverse aortic remodelling. Type 3 and 4 (right portion of the figure two, intimomedial tears in or below abdominal aorta) had reverse remodelling below 50% at follow-up. Khoynezhad type 0 patients (not shown here) had a tear across the left carotid artery or more proximal and were not amenable to TEVAR with favourable reverse aortic remodelling using conventional endovascular techniques.
The utility of this classification is in the predictive value of reverse remodelling. This may help extend the indications to uncomplicated type B aortic dissections, if we can predict with reasonable certainty that false lumen thrombosis and regression will occur after TEVAR, obviating need for intervention in 30% of patients with chronic type B dissection due to aneurysmal degeneration. This classification may have ramification in future when clinicians start offering TEVAR to patients with uncomplicated type B aortic dissection as the consequence form data from many single-centre studies and the INSTEAD XL. As the literature increases supporting the concept of scaffolding of the descending thoracic aorta as a function of survival benefit and remodelling advantage at long-term follow-up, selection of the patients with potential for reverse aortic remodelling becomes increasingly important.
Ali Khoynezhad is director of Aortic Surgery, professor of Cardiovascular Surgery, Cedars Sinai Hear Institute, Los Angeles, USA