Aortic Aneurysms

See related article ruptured aortic aneurysm, aortic dissection, and the general article on aneurysms.

An aneurysm is an irreversible dilatation that is generally fusiform in shape. In the aorta, most are in the proximal part of the ascending aorta or just above the bifurcation of the abdominal aorta. Arterial aneurysms can affect many other arteries.

There is no fixed definition for abdominal aortic aneurysms (AAA) but the most pragmatic and widely used definition is an aorta that is dilated at least 1.5 times that of the adjacent intact aorta (usually the intrarenal or suprarenal aorta). Any abdominal aortic diameter ≥3.0cm may also be considered aneurysmal. The normal is 2.1cm in males and 1.8cm in females ≥55yrs).

Atherosclerosis is no longer considered to be the principal cause, and the term "degenerative" is better to emphasise the multifactorial aetiology.

Metabolic

Metabolic derangement can develop from infection including bacterial endocarditis, sepsis, and syphilis. The more proximal the lesion, the greater the probability of syphilis and syphilitic aneurysm is rare below the diaphragm. Cystic medial degeneration, arteritis (particularly Takayasu's disease), and an autoimmune basis are also factors.

Structural

The arterial pressure wave increases as the diameter reduces and this is compounded by reflected pressure waves from the extremities. The abdominal aorta contains less elastin and more collagen than the thoracic part. The aging process, with changes in collagen and elastin, as well as atherosclerosis, compromises repair. This can weaken the wall leading to dilatation, and a vicious circle of increased wall tension and greater wall stress according to the Laplace' Law (wall tension is proportional to pressure x radius).

Genetic

With thoracic aneurysms, 20% of patients who require surgery have a relative with the condition and it is usually inherited as an autosomal dominant of low penetrance but increasing with age. OMIM recognises a type 1¹ and a type 2² inherited form. Marfan's syndrome is associated with thoracic aortic aneurysm, usually proximal and there may be involvement of the aortic valves. Ehlers-Danlos syndrome is another associated genetic connective tissue defect.

Other Factors

• Hypertension increases the risk of aortic aneurysm both above and below the diaphragm, in accordance with Laplace's Law.
• A previous aortic dissection with persistent false channel may produce aneurysmal dilatation.
• Trauma can cause aortic aneurysm but it is uncommon below the diaphragm. Thoracic lesions can result from shearing forces in a road traffic accident.
• Smoking is a major risk factor as is COPD. It would seem that smoking is important in initiating the condition.³

• About 6,000 men a year die from ruptured aortic aneurysm in England and Wales accounting for about 2% of deaths in men. Women are much less frequently affected.
• The incidence is around 4% in men over the age of 64, rising to 7-14% of those with both hypertension and peripheral vascular disease, and 11-28% of those with a first degree relative with AAA.
• AAA affects mostly men between 40 and 70.
• 95% of aortic aneurysms are below the renal arteries
• Whereas mortality and morbidity associated with coronary artery and cerebro-vascular disease have been declining, incidence and mortality associated with AAA have increased.

• Most aortic aneurysms occur in the abdominal aorta.
• Most AAs are asymptomatic at discovery, and an incidental finding on chest x-ray, or on clinical or ultrasound examination of the abdomen.
• It may be found at screening.
• In up to half of patients with an asymptomatic AAA, there may be intermittent claudication, and half will be found to have a pulsatile abdominal mass.
• There may be no symptoms until the aneurysm dissects or ruptures, and the classical triad for ruptured AAA of abdominal or flank pain, shock, and pulsatile abdominal mass, is found in only 24-42% of cases.

Aortic dissection is described elsewhere and so is given only brief mention here. Dissection and rupture should not be confused.

A large thoracic aortic aneurysm puts pressure on the surrounding organs in the chest but there may be no symptoms or signs.

Symptoms

• Upper back pain
• Coughing and wheezing
• Hoarseness due to impingement on the recurrent laryngeal nerve

Signs

• If it causes turbulent blood flow there will be a bruit
• There may be oedema of the neck or arms
• Horner's syndrome

Abdominal aortic aneurysms produce less trouble from compression. Again, there may be no symptoms or signs.

Symptoms

• The patient may be aware of a pulsating mass
• Pain if leaking or dissecting, often referred to the back
• Ask about erectile dysfunction
• There may be intermittent claudication

Signs

• It may be possible to feel a pulsating mass just above the umbilicus
• If it causes turbulent blood flow, there will be a bruit
• It may cause renal artery stenosis and hypertension
• There may be weakness or absence of peripheral pulses
• A thoracic aneurysm at the level of the brachial artery will cause unequal blood pressure in the two arms.

Do not forget to check blood pressure in all cases of aortic aneurysm.

The diagnosis of AAA is often missed and clinical signs may be limited.

• 80% of those with ruptured aneurysms have never been diagnosed as having a AAA.
• 24-42% of those with ruptured AAAs are misdiagnosed on first presentation, and have double the mortality rate of those diagnosed correctly.
• Misdiagnoses include:
  • Renal colic
  • Diverticulosis
  • Strangulated hernia
  • GI bleeding
  • Myocardial infarction
  • Lumbar spine disease
• Common features of patients in this category include:
  • Only 26% had pulsatile abdominal mass.
  • Only 9% had the clinical triad.
  • Only 12% had back pain.

Imaging

• Echocardiography can be used to assess aneurysms in the chest⁴ and to assess the aortic valve.
• Ultrasound is a very good way to assess the size and progress in the abdomen. With abdominal aortic aneurysms the following policy is based on the Gloucestershire Aneurysm Screening Project:⁵
  • <26mm diameter can be ignored
  • 26 to 39mm screen annually
  • ≥40mm and above refer to surgeon. He may screen every 6 months if <55mm but 56mm and above or increasing in size by 10mm a year or more will probably require operation.
• Magnetic resonance angiography (MRA) is likely to replace traditional angiography in future.
• Before operation the surgeon will arrange arteriography to assess the lumen and to look for involvement of other arteries.

Other Investigations

• Check renal function in case renal arteries are involved.
• Investigations as for coronary heart disease should be performed and if not already diagnosed check for diabetes.

Risk Reduction

• The risk of rupture⁶ of abdominal aneurysms is high if the patient continues to smoke
• Treat any cause such as hypertension or syphilis (watch out for the Jarisch-Herxheimer reaction).
• Control hypertension if present, but beware of possible renal artery stenosis if using ACE inhibitors.

Surgical

• Surgical repair should be considered for all aneurysms⁷ of the ascending aorta.
• Abdominal aortic aneurysms require operation if >56mm in diameter or expanding by 10mm a year of more.
• At any level the aneurysm is usually replaced by a Teflon graft.
• In cases of high operative risk from other factors, intravascular stenting is an option.

• Rupture, includes rupture into adjacent structures.
  • Rupture into the the vena cava causes signs of AV fistula with CCF
  • Rupture into the duodenum produces a herald GI bleed followed by exsanguination
  • Rupture can occur into the oesophagus
• Chronic contained rupture may occur in 5-10%. It can develop an expanding haematoma, although the majority will finally rupture within 3 months. Usually presenting with abdominal pain, they may produce unusual manifestations such as jaundice from compression common bile duct, ureteral obstruction, or inguinal hernia.
• Dissection is often confused with AAA rupture, and can occur with or without a coexisting aneurysm. However dissection in the abdominal aorta is rare, accounting for only 2-4% of aortic dissections.
• Emboli may be released, producing livedo reticularis of feet, or blue toe syndrome.
• Thrombosis of a small AAA produce acute claudication.
• Pressure on other structures. Compression of the stomach causes satiety, nausea and vomiting whilst venous compression leads to deep vein thrombosis.

• If surgery is required it should be performed before dissection or rupture occurs as the results for elective surgery are vastly superior to those for emergency surgery.
• Elective surgery for abdominal aneurysms carries a mortality below 5% compared with over 60% for emergency repair.⁸
• 5 year survival rates after aneurysm repair vary from 49-84%, much better than those unrepaired, but below age-matched controls probably due to co-existing coronary artery disease.
• In Marfan's syndrome the aortic valve often does not need replacing and outcomes are good.⁹

• Arteriosclerotic aneurysms require the same prevention as for CHD but with special emphasis on hypertension and smoking.
• Diagnose and treat syphilis early.
• With thoracic aneurysms give genetic counselling and screen relatives. Not all genetic cases are Marfan's syndrome.
• Marfan's syndrome:
  • Prophylactic beta-blockers have been shown to slow aortic root dilatation.
  • A balanced magnesium intake has been advocated.
  • Gene therapy, using a hydrogel angioplasty balloon to deliver antisense ribozyme to the target area is being examined for the future.

The value of routine screening for abdominal aneurysms is discussed in an article in the BMJ.⁵ It is supported by a systematic review.¹⁰ It involves ultrasound examination and is offered just to men. Whilst it is generally accepted that screening women for AAA is neither clinically indicated nor economically viable,¹⁰ there may possibly be a subgroup of women who are worth screening.¹¹ It costs about £28,000 per added year of life and that is comparable to other screening programmes.¹² It reduces deaths by 42%. Screening is performed once at age 65 and if the diameter of the aorta is less than 26mm they are discharged. This is 95% of all those screened. A single negative result around the age of 65 is suitable for excluding significant AAA disease for life.¹³

A study from Denmark found that screening men between 64 and 73 years old produced favourable results over 10 years with reduced mortality from AAA of 73%, and the frequency of emergency operations was reduced by 68% compared with controls.¹⁴

A study based on general practice and offering it to local practices had a 71.7% attendance rate and 4.1% of the screened population had an AAA.¹⁵ This is compatible with most series but attendence could be improved by more education. The cost per scan was £17.39 and this equates to around £400 per case detected. There is no problem of managing false positives as with other screening procedures¹¹ and distinct benefit from early detection. It will be interesting to see what priority this receives in the developing NHS and how it may be influenced by practice based commissioning.

Anreas Vesalius (or Wesalius) is regarded as the "Father of anatomy". He recognised that Galen had never dissected a human. He reorganized the study of anatomy, and published his remarkable textbook of anatomy, "Fabrica" in 1543. He is credited with the first description of an abdominal aortic aneurysm.

Pierre Laplace, French physicist and mathematician, briefly Napolean's Minister of the Interior, is famous for his "Celestial Mechanics", which translated the geometrical study of mechanics used by Newton, to one based on calculus, known as physical mechanics.