An aneurysm is the permanent and irreversible dilatation of a blood vessel to 1.5 times its normal diameter. It is commonly due to a weakness in the arterial vessel wall. The normal aortic diameter is on average 2cm. The diameter naturally increases with age. However, particular risk factors, environmental and genetic influences can hasten the progress requiring surgical intervention. The abdominal aorta, below the renal arteries, is the most common site followed by aneurysms in the thoracic, iliac, femoral and popliteal regions.
Abdominal aortic aneurysms (AAA) are seen in 5-8% of the population. Males over the age of 65 are most commonly affected. Deaths from AAA account for approximately 2% of all deaths in men aged over 65. The incidence is steadily rising due to the increasing ageing population. The number of people aged over 65 is expected to double by 2020. This indicates the need for primary and secondary prevention to lower the cases of abdominal aortic aneurysms.
A combination of risk factors can predispose certain individuals to aneurysms. There is a strong genetic component increasing the likelihood of aneurysm formation. Many of the risk factors are similar to that of atherosclerosis, hence the need for primary and secondary prevention. The risk factors are as follows:
It has been suggested 1 in 25 males over 65, without having additional risk factors, will be found to have an AAA. However, 1 in 6 males with accompanying risks factors, will have an AAA. This has lead to NICE developing a screening programme soon to be implemented in England.
Aneurysms are considered either true or false. A true aneurysm is a dilatation of the artery involving all three layers of the vessel wall. Alternatively, a false aneurysm, also known as a pseudo-aneurysm, involves a traumatic event to the vessel wall leading to a leaking hole whereby a haematoma forms outside of the lumen, and is contained by surrounding tissues. Typically they expand slowly and are felt as a pulsatile mass (NOT expansile). False aneurysms are often associated with arterial interventions such as angiograms or angioplasties and intravenous drug users.
Aneurysms can also be described in terms of appearance; saccular or fusiform. Saccular aneurysms are asymmetrical and appear to 'blow out' on one side of the vessel wall. Fusiform aneurysms are symmetrical and involve both sides of the vessel wall.
Mycotic aneurysms arise from an infection of the vessel wall leading to an inflammatory response and in some cases aneurysm formation as a result of damage to the vessel wall. It may be a systemic or local infection. Infective endocarditis can be the initial source of infection disseminating from heart valves to the major vessels. Whilst mycotic infers a fungal origin, the majority are associated with bacterial infections.
Berry aneurysms are typically cerebral in original, largely affecting the circle of willis. They are characterised by a stem followed by an aneurysm; much like the appearance of a hanging berry. As they are congenital, the majority are unaware of them.
SITE OF ANEURYSM
Infra-renal abdominal aortic aneurysms are the most common. Thoracic aneurysms account for less than 10% and are predominantly asymptomatic. However patients can complain of substernal and neck pain, with associated dysphagia, stridor, hoarseness of the voice, and dyspnoea. Peripheral aneurysms include those at the popliteal region which account for 80% and tend to be bilateral. When they occur, they are seen with aortic aneurysms. The second most common seen in the periphery is a femoral aneurysm which is felt as a mass in the groin.
NB. It is important not to confuse an aneurysm with an aortic dissection. In aortic dissection, the layers of the vessel wall separate to form a new channel which runs parallel to the lumen.
Infrarenal abdominal aortic aneurysms are the most common. The majority are asymptomatic and are found incidentally on examination. Symptomatic aneurysms cause either continuous or intermittent back pain. Pain can radiate to the groin or loin depending on the site. It is important to look for distal limb ischaemia owing to embolisation. By checking the pulses of the lower limb, you can note if the aneurysm is of the abdominal or the popliteal artery. On examination you can feel for a mass in the abdominal region which will characteristically be expansile, meaning it will expand and contract. Beware of pulsatile masses as they may indicate structures overlying the aorta rather than an aneurysm itself. It may be worthwhile at this point to look for signs of hyperlipidaemia such as xanthelasma and check the blood pressure (these are both risk factors for aneurysm formation).
The process of aneurysm formation is complex and not fully understood. It is believed there are four main processes in the formation of an AAA:
An aneurysm has two life-threatening complications: rupture and dissection. It is for this reason that regular monitoring is required. Other complications to consider are thrombosis and emboli, as well as pressure on adjacent structures.
RUPTURED ABDOMINAL AORTIC ANEURYSM
The risk of an AAA rupturing is largely dependent on the size and rate of increase of the diameter. Symptoms of sudden back pain, tachycardia, tachypnoea, sweating, syncope and collapse are common. This is due to hypovolaemic shock. Extreme pain can also be an indicator for impending rupture from a rapidly expanding aneurysm. On examination you can look for grey-turners sign, a sign of retroperitoneal haemorrhage. The mortality rate is 80%. Of the patients who make it to theatre, only 50% survive the surgery.
Initial investigations include blood tests for full blood count, clotting factors, renal and liver function, and cross match if surgery is being considered. Ultrasound scanning is the investigation used for monitoring as it is cheap, easy to use, non-invasive and relatively accurate. CT is used if a more detailed anatomical image is required to reveal any additional arteries affected, and looking for the 'crescent sign' - sign of impending rupture.
Aneurysms expanding at a rate of >1cm a year are at a high risk of rupture. The intensity of monitoring correlates with the diameter and risk of rupture.
At 5cm the risks associated with rupture are much greater than the risks of surgery.
NICE have issued a report to introduce a national screening programme to offer a single ultrasound scan for males aged 65. Ultrasound can give an immediate positive or negative result in addition to the size and location of an aneurysm. If an aneurysm is found it will be followed up with a CT or CT angiogram, whereby a more accurate picture of the size and complexity of the aneurysm can be gauged.
Surgery is the mainstay of treatment for aneurysms measuring greater than 5.5cm. The risks associated with rupture outweigh the risks of surgery. Currently there are two interventions that can be used:
Elective open laparotomies are the most commonly performed. It involves clamping of the aorta and iliac arteries and removing the aneurysmal area. It is replaced by a graft which is well tolerated and durable. The risk of mortality stands at 5-7%, the normal risk of anaesthesia. For emergency repair, the mortality rate stands at 50%. Regular check-ups are not required, however control of co-existing risk factors such as hypertension is important. Complications include cardiorespiratory difficulties, renal failure, wound infection, haemorrhage and a risk of paraplegia from spinal cord ischaemia.
Endovascular repair (EVAR)
Access is obtained via the femoral arteries and a stent graft placed within the aneurysm by x-ray guidance. At least 1-2cm of normal aorta is needed to secure the stent graft securely. This forms a new channel completely contained in the aneurysm. The risk of mortality stands at 2%. A move towards EVAR would be ideal due to its minimally invasive technique. However, complication rates are higher and there is a need for life-long follow-ups to monitor endoleaks and displacement of the stent graft. This is done with regular CT scans following the surgery. The use of EVAR is highly dependent on the properties and location of the aneurysm.
Medical management involves secondary prevention of cardiovascular risk factors including smoking cessation, blood pressure control and prescribing statins. It improves mortality rates for those with an aneurysm.
The patient must notify the DVLA if their aneurysm is 6cm or greater. License is revoked if the aneurysm is greater than 6.5cm or 5.5cm in the case of driving a LGV/HGV.
Key points: Abdominal aortic aneurysms can often present with claudication as blood flow is often slow around edges of the aneurysm sac leading to thrombus formation and ultimately emboli occluding peripheral arteries.
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