Aortic Aneurysms

An aneurysm is a local dilatation of an artery or chamber of the heart caused by a weakening of its wall. Aneurysms can occur almost anywhere but the most frequent place is the aorta.

Aneurysms are also commonly seen in the popliteal artery behind the knee and less frequently in the carotid and subclavian arteries in the neck. They can vary in shape and can be saccular, diffuse or fusiform. This depends to a certain extent as to their original cause.

Aneurysms can also occur as the result of dissection of the lining of the aorta by the ingress of blood stripping it off. This can lead to blocking of branches to the vital organs.

Causes

Genetic disorders

These include conditions such as Ehlers-Danlos syndrome, Marfan's disease, Tuberous sclerosis, Takayasu's disease and familial multiple aneurysm syndrome. These diseases are fascinating but very rare and do not constitute a large group.

Inflammatory conditions

Aneurysms have been known about for centuries and the commonest cause before the advent of 20th Century antibiotics was syphilis which was introduced into Europe by Spanish sailors returning from the newly discovered New World. Syphilis spread rapidly and unabated for four centuries and untreated became a common cause of aneurysm of the aorta.

Fortunately, untreated syphilis is now a rarity and with its passing, aneurysms due to this cause have died out in modern societies only to be replaced by degenerative aneurysms of old age.

Infection of the arterial wall can occur either from ingress of bacteria following local trauma or from another site such as an infected heart valve or septicaemia. Bacteria can take hold in particularly vulnerable areas such as an ulcerated, atheromatous plaque or the site of an injection. Salmonella enteritides occurs not uncommonly in infected aneurysms and usually affects older people following an intestinal infection picked up from eggs or poultry. Drug addicts are another vulnerable group due to the use of dirty needles. the organism in that case is usually Staphylococcus aureus which lives on the skin surface. Infective or mycotic aneurysms as they are known have a particularly bleak outlook as they can expand rapidly and rupture early without aggressive surgical treatment.

Degenerative

These aneurysms now constitute 95% of aortic aneurysms seen in this country. The absolute true prevalence is not known since less than 15% of deaths result in a post-mortem examination. Nevertheless, in one study 4.2% of men and 1.2% women died of rupture of the aorta. Aortic aneurysms become more prevalent the older the population gets and as everyone knows the average age of the population is increasing. It is clear that the simplistic view that atherosclerosis was the primary cause of aneurysm formation is not correct. People with atherosclerosis undoubtedly get arterial disease and atherosclerosis worsens as we get older so it is hardly surprising to find both conditions co-existing. However, this does not prove cause and effect and analysis of the facts show that people with atherosclerotic vessels with occlusive disease tend to be younger, less healthy and have a reduced life expectancy compared with people with aneurysmal disease. Although the two conditions can overlap, people with aneurysms, tend to have relatively healthy vessels beyond the aneurysm compared to the calcified, reduced calibre arteries of the typical heavy smoking individuals with widespread atherosclerosis.

The exact mechanism as to why aneurysms occur has not been fully worked out but it has something to do with a loss of collagen and elastin associated with ageing. The half life of elastin, the protein that gives the artery its spring, is thought to be 70 years. There is little or no turnover of this substance in adult life but breakdown does nevertheless gradually occur. The associated strong collagen also degenerates with time. This may explain why aneurysms occur but the full story is considerably more complex with trace metals, enzymes, hormones, genetic factors and some drugs all believed to have some part to play. The stretchy wrinkly skin of older relatives is recognizable to us all and occurs for similar reasons. It is thought that if we were all to live long enough everyone would eventually develop an aortic aneurysm.

Who gets an aortic aneurysm?

Without doubt abdominal aortic aneurysms occur mainly in men. The greatest prevalence occurs between 70 and 80 years of age. The men outnumber women in this condition by over 10:1 in the 60-70 age group and by 6-7:1 in the most prevalent age group. It is true to say that even in the 90 + age range men still outnumber women by 3:1 despite the fact that women dominate this population sample of longevity.

Size Matters

Like so much in life, size is an important factor in aortic aneurysm. The bigger an aneurysm is, the more likely it is to rupture and lead to an early demise. Remarkably it has never been firmly established what exactly constitutes an aneurysm but it is generally agreed that an abdominal aortic aneurysm is present if the antero-posterior diameter is 4.0 cm or more or if the diameter is 0.5 cm greater below the renal arteries when compared to that above them.

Abdominal aortic aneurysms are divided into three main categories according to size:

• 6.0 cm or greater
• 4.0 - 5.9 cm
• Less than 4.0 cm

This distinction is important since it is known that when the aneurysm gets over 6.0 cm in diameter further expansion is rapid and the chances of death from rupture is over 60% within three years.

The chances of dying during or within 30 days of elective surgery assuming that the person concerned is fit enough to undergo an anaesthetic are approximately 5% in expert hands so the odds in favour of surgical repair are obvious. However, in the case of aneurysms less than 6.0 cm the rupture rate is 6% per annum over 3 years so odds of dying at 5% from an operation do not look so attractive. This explains why most surgeons are reluctant to operate on patients in this group and is demonstrated by the hypothetical graph above. Moreover, the expansion rate of an aneurysm less than 6.0 cm is generally less than that of a bigger size. Rapid expansion, pain or other complication would tip the balance in favour of repair however and this takes experience and judgement.

What else can go wrong?

Rupture of an abdominal aortic aneurysm is by far the most important and devastating complication that can occur but other things do happen:

• Embolus into the leg vessels
• Rupture into the intestine
• Rupture into the inferior vena cava
• Inflammatory aneurysm
• Thrombosis

Embolus is a not an uncommon complication of aortic aneurysm and leads to an acute blockage of one or both arteries supplying the legs. The lining of the aneurysm is composed of a soft mixture of atheroma and clot and can sometimes be unstable and fall off. The aneurysm may have to be dealt with along with the offending embolic material. Fortunately, abnormal communications between the intestine and the inferior vena cava are relatively rare. Thrombosis of an aortic aneurysm can occur after an embolus but is very unusual. It does occur with devastating consequences in popliteal aneurysms however.

Treatment

Assuming that the person unfortunate enough to possess a large abdominal aortic aneurysm is fit enough to undergo surgery, the treatment of first choice is surgery. It is remarkable that prior to the great French surgeon, Charles Dubost's original description of resection of an aortic aneurysm and replacement with a human homograft in 1951, the only hope of treatment was to thrombose the aneurysm by blocking it with coiled wire or by tying off the neck of the sac; wholly unsatisfactory treatment with poor results. Nowadays, a cloth-like material called Dacron is used to replace the widened section of the aorta by stitching a tube of comparable size to the neck inside the sac. This is an excellent procedure that has stood the test of time since it was first described by the American surgeons, Blakemore and Voorhees.

However, the operation is formidable especially in those considered high risk. In selected cases it has become possible to consider placing a graft made of Dacron reinforced with a Nitinol alloy wire stent inside the aorta using X-ray techniques via the arteries in the groin thereby avoiding a massive abdominal operation. This technique is the subject of an ongoing trial in the UK and the results will be carefully scrutinized over time. At present patients considered too high risk or too old for a standard open operation are being considered. Not all aneurysms are suitable for this new technique so it does not represent a complete alternative.