Site and classification of dissection

Sixty to sixty-five per cent of dissections originate in the ascending aorta, 5 to 10 per cent in the aortic arch, and 30 to 35 per cent in the first part of the descending aorta. Subsequent dissection usually occurs distally but may also extend proximally. The original De Bakey classification divides dissection into three types based on the extent of aortic involvement (not the site of the intimal tear, which may be difficult to identify) ( Fig 1).

I. This involves both the ascending and descending aorta and usually extends into the abdomen. Ten to fifteen per cent of such patients have an intimal tear distal to the left subclavian artery; in the remainder the origin is in the proximal aorta.

II. This is the least common type and involves the ascending aorta only, stopping just proximal to the innominate artery. This type of dissection is seen in Marfan's syndrome and may occasionally be found incidentally in operations for ascending aortic aneurysms.

III. This originates distal to the left subclavian artery and extends downwards. Type Ilia dissections are confined to the descending thoracic aorta only, whereas the more common type IIIb dissection extends into the abdominal aorta and may involve the iliac arteries. The majority of these aneurysms have an intimal tear just distal to the origin of the left subclavian artery. Type III dissections tend to occur in older patients.

The alternative Shumway (Stamford) classification divides dissection into two types only. This classification is less descriptive but is well suited to clinical decision-making (Fig 1).

Type A. Involvement of the ascending aorta (De Bakey types I and II). Type B. No involvement of the ascending aorta (De Bakey type III).

In autopsy series, type A dissections predominate in a ratio of approximately 2 to 1. In most clinical series, types A and B appear almost equally common, probably because type A dissections are more rapidly fatal and may pass unrecognized.

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