Aortic dissection

Introduction

Aortic dissection is an uncommon yet potentially lethal condition. Owing to the broad range of presenting signs and symptoms, a high index of suspicion is required to diagnose it. Investigations must be carefully chosen and rapidly performed to confirm the diagnosis. Treatment should be commenced as soon as the diagnosis is suspected, because when AD is untreated, the mortality rate is approximately 1% per hour for the first 48 hours.

Aortic dissection is one of a group of similar conditions that constitute the acute aortic syndrome, which comprises several related life-threatening aortic pathologies including aortic dissection, intramural haematoma, penetrating aortic ulcer and traumatic aortic transection with incomplete rupture. These conditions all progress to the same pathophysiological end point: separation of the aortic intima from the outer aortic layers, with predictable and often devastating consequences. There is considerable overlap in the signs, symptoms and principles of management of the conditions that constitute acute aortic syndrome.

Epidemiology, pathophysiology and classification

The incidence of aortic dissection is 3 patients per 100,000 population per year. One-third to one-half of all cases are diagnosed at autopsy. Although the overall incidence is low, aortic dissection is the most common catastrophe of the aorta, being two to three times more common than rupture of the abdominal aorta.

Most cases occur in males, particularly those between the ages of 50 and 70. Proximal dissections involving the aortic arch have a peak incidence 10 years earlier than distal dissections. Risk factors—among which hypertension is the single most important—are shown in Box 5.10.1 . The diagnosis of aortic dissection must be considered in any patient with a history of hypertension who presents with sudden severe chest, back or abdominal pain. Approximately 5% of all cases involve patients with a history of Marfan syndrome.

Arterial hypertension and degeneration of the aortic media are the two key elements of aortic dissection. Dissection occurs when blood is forced along a low-resistance tissue plane within the wall of the aorta created by a diseased and weakened media. Two pathophysiological processes have been proposed to initiate the dissection. The traditional explanation requires a breach in the intima (an intimal tear) to initiate the dissection process. The tear occurs at sites where hydrodynamic and torsional forces on the aorta are greatest, most commonly a few centimetres above the aortic valve (60%to 65%) or just beyond the insertion of the ligamentum arteriosum (30% to 35%). A column of high-pressure aortic blood gains access to the media and dissects through the weakened tissue plane, creating a false lumen. The dissection can extend in an antegrade or retrograde direction. The alternative mechanism suggests that diseased or unsupported vasa vasorum within the media rupture, creating an intramural haematoma. The haematoma dissects through the media as it expands, subjecting the unsupported media to increased shearing forces during diastolic recoil of the aorta. Eventually—but not necessarily—this may lead to a tear in the intima. In this mechanism, the intimal tear is a consequence of the dissection, not an initiating factor. An intimal tear is not identified in 12% of autopsies, suggesting that it is not a mandatory precursor of aortic dissection.

Regardless of the primary process producing dissection, the sequelae are identical. As the dissection extends, any structures caught in its path may be affected. Branch vessels of the aorta may be distorted or occluded, resulting in signs and symptoms of ischaemia to the organs they supply. Proximal dissection may produce acute aortic valve incompetence, and continued proximal extension may enter the pericardial sac, tamponading the heart. The false lumen created by the dissection may also partially or completely obstruct the true lumen. It may end in a blind sac or rupture back into the true lumen at any point. The false lumen may also rupture outwards through the adventitia. If this occurs and the haematoma is not contained, rapid exsanguination will occur. Common sites of external rupture are into the left pleural cavity or mediastinum.

Once dissection begins, propagation is dependent on the blood pressure and the gradient of the arterial blood pressure wave, which is a function of the velocity of left ventricular contraction. This explains why urgent pharmacological treatment is aimed at lowering arterial blood pressure and reducing the ventricular contractile force.

Classification

The Stanford and De Bakey systems are two commonly used, anatomically based classification systems. Both describe the site of the dissection, providing information that assists management.

The Stanford system divides aortic dissection into two types ( Fig. 5.10.1 ). Type A (65% to 70% of cases) involves the ascending aorta, with or without the descending aorta. The presence or absence of an intimal tear, the site of the tear and the extent of distal extension are not considered in this classification. Type B dissections (30% to 35%) involve the descending aorta only, which, by definition, begins distal to the origin of the left subclavian artery. The Stanford system is simple, easy to remember and has become the primary classification system referred to in the literature.

The De Bakey classification system divides aortic dissection into three types (see Fig. 5.10.1 ). Type I involves both the ascending and the descending aorta. Type II involves the ascending aorta only. Type III involves the descending aorta only and is subdivided into type IIIa, which is confined to the thoracic aorta, and type IIIb, which extends into the abdominal aorta.

Aortic dissection is classified as acute if symptoms are present for fewer than 14 days or chronic if are present for longer than 14 days.

Clinical features