Predicting type B aortic dissection outcomes when the patient arrives

Rachel Clough
Rachel Clough

Stanford type B aortic dissection carries a better prognosis compared with type A dissection and 90% of patients survive to hospital discharge. The management and long-term outcome of these patients is challenging, writes Rachel Clough, and in some series only 50% are alive at five years. The main late complication of type B aortic dissection is progressive enlargement of the false lumen resulting in aneurysm formation, rupture and death.

The ADSORB and INSTEAD randomised controlled trials were designed to investigate the role of endovascular treatment in the management of acute and chronic uncomplicated patients respectively. These trials demonstrated the long-term benefit of endovascular treatment to prevent aortic-related mortality. They also highlighted the need for a patient-specific approach, to identify early those patients at high risk of developing complications and to offer them endovascular intervention. Pre-emptive endovascular treatment has not traditionally been considered a solution for all patients because of the incidence of adverse events such death, stroke and paraplegia.

In order to identify these high-risk patients, a number of computed tomography (CT) anatomical studies have been undertaken. Measurements of aortic diameter have been shown to be able to provide prognostic information and determine the need for intervention. An analysis of patients in the International Registry of Aortic Dissection showed that in-hospital mortality for patients with an aortic diameter ≥5.5cm was approximately four times greater than those with a diameter of less than 5.5cm. Other studies have shown that a false lumen diameter of ≥22mm in the upper thoracic aorta can predict late aortic expansion with 100% sensitivity and 76% specificity. Features related to the primary entry tear have also been studied and shown to be important; a primary entry tear at the concavity of the distal aortic arch can be a predictor of complications and an entry tear of ≥10mm is associated with a higher incidence of dissection-related events compared with an  entry tear measuring <10mm.

Despite these studies, the use of these parameters to identify patients that require early endovascular intervention is currently not widespread in clinical practice.

Presently there is enthusiasm for serial imaging of patients with type B aortic dissection in the acute phase to identify patients that are undergoing rapid anatomical change. These patients are then offered endovascular treatment in the subacute phase, when the risk of complications such as retrograde dissection is thought to be lower, and the amount of aortic remodelling following endovascular intervention is similar to the amount seen when patients are treated in the acute phase.

The aim of this serial CT imaging is to capture the dynamic changes taking place in the aorta in the acute phase. This protocol has the disadvantage of conferring relatively high doses of radiation to the patient, but has the benefit of providing high-spatial resolution images of the entire aorta and surrounding structures.

Other imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) have been investigated to identify high-risk patients in the acute phase as they are able to measure dynamic and functional information. MRI studies have shown that changes in aortic morphology can be related to intra-aortic haemodynamics, such as flow patterns and pressure. Studies using PET have shown that the amount of biological activity in the aorta can be used to identify the age of the dissection and may be related to the incidence of complications. Studies using these imaging techniques have previously been limited to relatively small series but work is now underway to evaluate the true ability of this imaging to identify high-risk patients.

In conclusion, and in response to the title of this article, we do have methods to identify patients at high risk of complications, but despite this, their use is not currently widespread in clinical practice. The current vogue is serial CT imaging of patients in the acute phase to identify patients that are undergoing rapid anatomical change and to select these patients for endovascular intervention in the subacute phase. Functional imaging may represent a way to identify these high-risk patients in a single radiation-free scan and to deliver a more patient-specific solution.

Rachel Clough is a vascular surgeon at the Hôpital Cardiologique, CHRU Lille, Lille, France