Aortic diseases: aortic dissection, aortic aneurysm

01Definition and overview

The aorta is an elastic tube with three layers: the intima, a thick media of elastic fibres and smooth muscle, and the adventitia. Its normal diameter is about 3 to 3.5 cm at the root and about 2 cm in the abdomen, larger in men, tall people and with age.

An aneurysm is a permanent, localised dilatation of an artery to at least one and a half times its normal diameter. In the abdomen this is a diameter of 30 mm or more. In a true aneurysm all three layers form the wall. A false aneurysm (pseudoaneurysm) is a leak contained by the surrounding tissue, as after a femoral artery puncture. Aneurysms are named by their place:

The acute aortic syndromes are emergencies in which the aortic wall gives way:

• Aortic dissection: a tear in the intima lets blood into the media, which splits along its length to form a false lumen separated from the true lumen by a flap.
• Intramural haematoma: bleeding into the media without a visible tear, which can progress to dissection or rupture.
• Penetrating atherosclerotic ulcer: an atherosclerotic plaque that ulcerates through the intima into the media.

They are classified by whether the ascending aorta is involved, which decides the treatment:

A dissection of the arch that spares the ascending aorta is called non-A non-B. European guidance adds the site of the entry tear and the presence of malperfusion of organs (the TEM classification). A dissection is acute in the first 2 weeks, subacute up to 3 months and chronic after that; the risk of death and extension is greatest in the first days.

02Etiology and pathophysiology

Why the wall gives way

How an aneurysm grows and a dissection spreads

1. 1

   Weakening of the media

   In an abdominal aneurysm, chronic inflammation and enzymes (matrix metalloproteinases) break down elastin and collagen and smooth muscle cells die. In the ascending aorta, medial degeneration loses elastic fibres and muscle cells, as in Marfan syndrome, where faulty fibrillin weakens the fibres and increases TGF-β signalling.
2. 2

   Growth

   The wall tension rises with the diameter and the pressure, so a wider aneurysm is stressed more and grows faster. Abdominal aneurysms grow by about 2 to 3 mm a year on average, faster when large and in smokers, and are often lined by thrombus that can embolise.
3. 3

   The tear

   In dissection, blood under arterial pressure enters the weakened media through an intimal tear, usually in the ascending aorta just above the valve or just beyond the left subclavian artery, and the false lumen spreads, mostly forwards.
4. 4

   The complications

   The false lumen can rupture outwards; in the ascending aorta into the pericardium, causing tamponade. The flap can block the branches, causing myocardial infarction, stroke, paraplegia, and ischaemia of the gut, kidneys or limbs (malperfusion), and it can tear the support of the aortic valve, causing acute aortic regurgitation.

03Clinical features

Acute aortic dissection

• Pain: sudden and severe, at its worst at onset, described as sharp, tearing or ripping. It is felt mostly in the front of the chest in type A and more often in the back or abdomen in type B, though the two overlap widely, and it may move as the dissection spreads. A few patients, particularly older ones, have little pain.
• Signs of branch involvement: a pulse deficit or a difference in systolic pressure of more than 20 mmHg between the arms, a stroke or fainting, paraplegia, a cold leg, abdominal pain with a rising lactate from gut ischaemia, and kidney injury.
• Signs of the ascending aorta: the early diastolic murmur of aortic regurgitation, tamponade with hypotension and a raised jugular venous pressure, and myocardial infarction, usually inferior from the right coronary artery.
• The blood pressure is often high in type B. Hypotension in type A suggests tamponade, rupture or acute aortic regurgitation.

A dissection can present as a myocardial infarction, a stroke or an acute abdomen, and thrombolysis or anticoagulation for these can be fatal. Probability is judged by the aortic dissection detection risk score, one point for any feature in each of three groups:

Aneurysms

• Abdominal aneurysms are usually silent and found by screening, by chance on imaging, or as a pulsatile, expansile mass above the umbilicus, which is easily missed in obesity. Some patients have a popliteal aneurysm as well.
• Rapid growth causes pain and tenderness in the abdomen or back. A tender or painful aneurysm is treated as about to rupture.
• Emboli from the thrombus in the sac cause blue toes ("trash foot"). An inflammatory aneurysm can trap the ureters; a rare fistula into the duodenum causes major gastrointestinal bleeding.
• Rupture: sudden abdominal, back or loin pain with collapse and a pulsatile mass; the full triad is present in only about half. In an older man, "renal colic" may be a ruptured aneurysm.
• Thoracic aneurysms are usually found on a chest X-ray or CT. Large ones press on nearby structures: hoarseness from the recurrent laryngeal nerve, stridor, cough, difficulty swallowing; they can erode into the airway or oesophagus.

04Diagnosis and management

Tests for an acute aortic syndrome

• ECG, to look for ischaemia; it is normal or nonspecific in most dissections, and an inferior infarction may be caused by the dissection.
• Chest X-ray: a widened mediastinum or an abnormal aortic outline in many, but a normal film does not exclude dissection.
• Bedside transthoracic echocardiography: a flap in the aortic root, aortic regurgitation and pericardial fluid.
• CT angiography of the whole aorta, gated to the ECG, is the test of choice, from the neck to the pelvis: it shows the flap, the extent, the entry tear, the branches and any rupture. Transoesophageal echocardiography is used in an unstable patient in theatre or intensive care, and MRI in stable patients and for follow-up.
• D-dimer: when the risk score is 0 or 1, a D-dimer below 500 ng/mL makes a dissection very unlikely, and with a high score the patient goes straight to CT.

Management of acute aortic dissection

After the acute phase, lifelong treatment keeps the blood pressure below about 130/80 mmHg, usually with a beta-blocker, and CT or MRI follows the aorta, after a medically treated type B dissection at 1, 3, 6 and 12 months and then yearly, because the dissected segment often enlarges. Heavy straining is avoided. A dissection in a young patient, or with a family history, leads to genetic testing and screening of first-degree relatives.

Abdominal aortic aneurysm

• Screening: one ultrasound scan of the abdominal aorta at about 65 detects most aneurysms in men and reduced deaths from rupture by about 40 per cent in a large trial. European cardiology guidance recommends it for men from 65 who have smoked and for first-degree relatives of patients from 50, and suggests it may be considered for all men from 75 and for women from 75 who smoke or have hypertension. National programmes differ: the UK and Sweden screen all men at 65.
• Surveillance by ultrasound, more often as the aneurysm grows:

• Medical treatment: stopping smoking, which slows growth and lowers the risk of rupture; blood pressure control, a statin and an antiplatelet drug for the cardiovascular risk, since most of these patients die of heart disease or stroke; exercise. No drug has been shown to slow aneurysm growth. Fluoroquinolones are avoided when another antibiotic will do, but not withheld when needed.
• Repair: at a diameter of 55 mm in men and 50 mm in women, or with symptoms. Rapid growth, 10 mm or more in a year, is first re-measured, because it is often an error of measurement, and then prompts consideration of repair. Below this size, trials found that early surgery did not improve survival compared with surveillance.

Thoracic aortic aneurysm

• Imaging by echocardiography for the root and ascending aorta, and CT or MRI for the whole thoracic aorta, at intervals set by the size and growth.
• Blood pressure control, stopping smoking and avoiding heavy isometric exercise. In Marfan syndrome a beta-blocker, an angiotensin receptor blocker or both slow the growth of the root.
• Surgery for an ascending aneurysm at 55 mm in most patients, earlier (above about 52 mm) in a patient with a low surgical risk, and earlier still in inherited disease: in Marfan syndrome at 50 mm, or 45 mm with risk factors such as a family history of dissection or rapid growth, and in Loeys–Dietz syndrome above 45 mm, or at about 40 mm with high-risk features in some genetic forms. A bicuspid valve with a dilated root is operated on at 50 mm or more.
• A descending thoracic aneurysm is repaired at 55 mm, by a stent graft when the anatomy suits, and a thoracoabdominal one usually at 60 mm; in Marfan and other inherited aortic diseases, open surgery is preferred. Repair of the thoracoabdominal aorta carries a risk of paraplegia from spinal cord ischaemia.
• Women with Marfan syndrome need advice before pregnancy, because pregnancy raises the risk of dissection; a root wider than 45 mm is replaced before pregnancy.

05Epidemiology

Abdominal aortic aneurysms are found in about 1 per cent or less of 65-year-old men in current national screening programmes, fewer than in the older trials as fewer people smoke, and several times less often in women. Smoking is the strongest risk factor; male sex, age, a family history, hypertension and atherosclerosis add to it, and diabetes is, unusually, associated with a lower risk. Thoracic aneurysms are less common, about 5 to 10 per 100,000 people a year; most are linked to age and hypertension, about a fifth to an inherited aortic disease, and a bicuspid valve is another important cause.

Acute aortic dissection affects about 6 people per 100,000 a year in a population study, more in men and with age, at an average age in the 60s to 70s, about two-thirds type A. Hypertension is present in most patients, often poorly controlled for years. In young patients an inherited aortic disease, a bicuspid valve or stimulant use are the usual causes.

06Prognosis

Acute dissection is often fatal before hospital: in a population study, about half of the patients with type A died before they could be assessed. Without surgery, mortality in type A has been put at 1 to 2 per cent an hour, about half within 48 hours in older series, and more than half die in hospital in a recent registry. With surgery, about 1 in 5 die in hospital in a large registry. Uncomplicated type B has a lower early mortality, around 1 in 10 or less, but complicated type B carries a much higher risk. Survivors of the first month do fairly well, but the dissected aorta can enlarge and rupture later, which is why imaging and blood pressure control are lifelong.

Small abdominal aneurysms below 55 mm in men rupture in about 1 per cent a year or less; the risk rises steeply with size, and women rupture at smaller diameters, at a higher rate than men with aneurysms of the same size. A ruptured aneurysm kills most patients: many die before reaching hospital, and about a third or more of those who reach emergency repair die within 30 days. Elective repair has a low mortality, and life expectancy after it is limited mainly by coronary disease, stroke and lung disease.