An aneurysm is a localised abnormal dilation or bulging of any artery or vein. The most commonly diagnosed type of aneurysm is aneurysm of the large blood vessel leaving the heart (called the aorta), particularly in the vessel's abdominal regions. Aneurysms in the small blood vessels of the brain are also rather common.
One way to imagine an aneurysm is as a car or bike tyre which has been weakened sufficiently in one location for it to 'balloon' out. The point at which the tyre has done this is similar to an aneurysm.
The factors that can contribute to the likelihood of developing an aneurysm include:
White men over 55 years of age are at the greatest risk of developing an aneurysm. Most particularly aneurysms of the aorta, especially in in the abdomen, which is called an abdominal aortic aneurysm (AAA). As such, aneurysms are among the top ten causes of death in this group and around 5% of white males 80 or over will have developed an aneurysm at some point in their lives.
Aneurysms pose a risk to health from their potential to:
Rupture of an aneurysm in the brain causes a stroke.
Rupture of an aneurysm in the abdomen causes vascular shock1.
Causes Of Aneurysms
Atherosclerosis is the most frequent cause of aneurysm and particularly causes them to develop in the descending aorta.
Atherosclerosis is a degenerative disease of the arteries that results in the formation of plaques or atheroma, which consist of dead cells, lipids, and cholesterol crystals. Most commonly it will give rise to a heart attack, as a result of artery occlusion (blockage) when the plaque grows to a sufficient size to reduce blood flow to the heart in the coronary artery.
However, the death of cells in the atherosclerotic region can cause thinning and weakening of the vessel wall, making it prone to further damage - and can therefore also cause the formation of an aneurysm.
Syphilis, formerly the major cause of aortic aneurysm, has become uncommon; however, it may still give rise to the condition.
In those with untreated syphilis the infection can spread, weaken the blood vessel wall and lead to an aneurysm. Most commonly, infection spreads to the portion of the aorta nearest the heart, producing a thoracic aortic aneurysm around 15 to 30 years after the first signs of syphilis.
As with other forms of aneurysm the consequences can be varied depending on location, size and the likelihood of rupture.
This type of aneurysm is caused by severe infection of a major artery. Infection may lead to inflammation and other processes which can damage the vessel wall and once sufficient this damage can lead to aneurysm development.
As the name suggests 'mycotic' refers to fungal infection, though most cases are in fact bacterial in origin and should be categorised as 'bacterial aneurysm'.
Mycotic aneurysms frequently occur as a complication of bacterial infection of the heart lining or heart valves (infective endocarditis), when the infection spreads from the heart to blood vessels. Circulating organisms may also directly infect the vessel wall.
Presentation And Diagnosis Of Aneurysms
Abdominal aortic aneurysms present to the clinician with varying clinical symptoms due to several consequences of aneurysm presence:
Rupture of the aneurysm into the space outside organs (peritoneal cavity) may give rise to massive or fatal haemorrhage.
Impingement on an adjacent structure, for example compression of the ureter or erosion of vertebrae can produce detectible symptoms.
Blockage of a vessel by mural thrombus 2 formation or direct application of pressure on adjacent vessels, particularly of the vertebrae branches.
Emboli; a detached piece of the blood clot or atheroma that can travel through the bloodstream to a distant site to lodge and obstruct or block (occlude) a blood vessel.
Detectible symptoms can be produced in cases where the aneurysm reaches a sufficiently large size for it to be perceived in the abdomen, often being mistaken for cancerous growth.
Abdominal and thoracic aneurysms can be diagnosed medically with an X-ray. Echocardiography is sometimes used to identify aortic root aneurysms. In most cases, however, CT and MRI scans give the most accurate results.
Cerebral aneurysms can be discovered in several ways. Unfortunately they are most often discovered when the aneurysm ruptures into the surrounding area, called the subarachnoid space, causing a subarachnoid haemorrhage.
Rupture of cerebral aneurysm, producing subarachnoid haemorrhage usually causes a sudden, severe headache, often accompanied by:
Subarachnoid haemorrhage can cause the development of vasospasm; an abnormal constriction of arteries in the brain, usually occurring a week or two after surgery. Vasospasm can cause stroke or other neurological damage, which is often more severe than the original haemorrhage.
Less commonly, a cerebral aneurysm can rupture and bleed into the brain itself, called intracerebral haemorrhage. This can produce a variety of symptoms depending of the size and location of the haemorrhage.
Unruptured cerebral aneurysms can sometimes be discovered when they cause neurological symptoms, such as headaches, double vision, and trouble walking. This may occur when the section of the blood vessel that has the aneurysm, 'balloons' out sufficiently to apply pressure to regions of the brain, interfering with their functioning.
Sometimes cerebral aneurysms are discovered as an incidental finding when a head CT or MRI scan, or angiogram is performed for some other reason.
The diagnosis of a cerebral aneurysm is usually confirmed with an angiogram, a radiological picture of the blood vessels in the brain made by taking X-rays of the head after giving an injection of a contrast agent (such as water and iodine salts) into the blood vessels. Recent advancements in CT and MRI scanning allow these techniques to also help diagnose aneurysms, but an angiogram usually provides the most detail of an aneurysms size and location.
Categorisation Of Aneurysms
There are several means of categorising aneurysms, one of which first subdivides aneurysms into 'true' and 'false' aneurysms.
True Versus False Aneurysms
A true aneurysm is an aneurysm bounded by complete but often weakened arterial wall components with altered structure. The blood within a true aneurysm remains within the confines of the circulatory system - this category includes atherosclerotic, syphilitic and congenital (present from birth) aneurysms.
In contrast to true aneurysms, false aneurysms (pseudo-aneurysm or pulsating haematoma) are extra-vascular haematomas that communicate with the intra-vascular space. In such an aneurysm the vascular wall is breached, and hence the wall of the aneurysm consists only of outer layers of the arterial wall. A leak at the junction (anastomosis) of a vascular graft with a natural artery can produce this form of aneurysm.
Dissecting aneurysms occur when blood enters the middle layer of the blood vessel wall, called the media. Blood therefore dissects between the layers of the vessel wall, creating a cavity within the vessel's wall.
Dissecting aneurysms are an important complication of the Marfan syndrome and of pregnancy. Most cases occur in middle-aged or elderly men with high blood pressure. The most common aortic aneurysms are abdominal. These occur in elderly overweight men with high blood pressure, they are rarely fatal as such individuals are more likely to die from heart attacks first. The treatment, if the aneurysm is discovered (usually as an incidental finding of an abdominal CT or MRI scan), is often surgical. Treatment typically involves inserting an artificial lumen through the clot.
Dissection may also occur as a result of trauma - such as road traffic accidents, shootings, or stabbings - in which the blood vessel wall becomes damaged.
Dissecting aneurysms present a diverse range of symptoms due to the variation in secondary consequences of the aneurysm, for example iliac artery occlusion. However if the aneurysm ruptures then it usually presents as a 'tearing' pain of extreme severity and sudden onset. The site of the pain depends upon the location of the dissection and moves as the dissection progresses. The pain most commonly starts in the anterior or posterior chest. Intervals of freedom from pain may occur or the pain may involve other regions such as the neck or arms.
On examination, patients appear pale and sweaty with a raised heart rate (tachycardia), and although there may be the appearance of shock, blood pressure is usually within normal limits.
Other signs depend on which branches become blocked. Thus, one or more of the arteries to the limbs may become impalpable. 3 Other complications include aortic regurgitation, where a large volume of blood is regurgitated back into the left ventricle - with each contraction of the ventricle, this increases the work load of the left ventricle, leading to ventricle damage, and ventricle enlargement.
Dissecting aneurysms are most simply classified into two types: type A dissections which account for about two-thirds of cases and type B dissections which account for one-third. Typically, type A dissections are best managed by early surgery, whereas type B dissections can be managed medically.
In type A dissections, there is a strong likelihood of further dissection, rupture and death unless surgical repair is undertaken. Patients are first medically stabilised. Blood pressure is normally lowered to reduce the strain on the aorta - for example, by use of beta-blockers. The choice of surgical procedure depends on the extent of dissection. If involvement of the aortic root is present, it may be necessary to insert a Dacron tube graft with re-suspension or replacement of the aortic valve and re-implantation of the coronary arteries.
In type B dissections, the risks of surgery are generally considered to exceed the risks of medical management, and antihypertensive therapy is usually continued long term to prevent recurrent dissection.
This is a small spherical dilatation rarely exceeding 1-1.5 cm in diameter; these are most common in the brain. Rupture of a berry aneurysm is the most common and pathologically significant cause of sub-arachnoid haemorrhage, carrying a relatively high mortality.
Although the berry aneurysm is the most common occurring in the brain's circulatory system, congenital saccular aneurysms are also rather common, with atherosclerotic fusiform aneurysms being rather rare. Approximately 90% of aneurysms occurring in the brain are located in anterior circulation, typically being located near branch points.
The aorta is the main artery leaving the heart and carrying oxygen-rich blood to all parts of the body. Most aortic aneurysms are fusiform, or 'general', and affect the entire circumference of the aorta. Aortic aneurysms may also be 'localised' or 'saccular'; where one region of the vessel wall 'balloons' out to form a 'sac', or 'pouch' like distension of the vessel wall.
Aortic aneurysms usually occur in the abdominal region of the aorta, but may also occur in the chest region.
Saccular and fusiform aortic aneurysms
These aneurysms are localised distensions of the aortic wall.
Saccular aneurysms are essentially spherical, only involving a portion of the vessel wall. Normally they are between five and 20 cm in diameter, typically being totally or partially filled with blood clot.
A fusiform aneurysm is a gradual progressive dilation of the complete circumference of the vessel, again varying in size up to a maximum of about 20 cm. Most affect the aorta in the chest region, but fusiform aneurysms may also affect large sections of the abdominal aorta or the iliac arteries.
Fusiform aneurysms are often found in association with conditions such as Marfan syndrome and coarctation of the aorta.
Treatment Of Aneurysm
High blood pressure is a major contributory factor to development and rupture of aortic (and other) aneurysms; reduction of blood pressure is often a major first step in reducing complications of aortic aneurysms. In those aneurysms caused by atherosclerosis (atherosclerotic aneurysms), reducing associated risk factors:
are central to reducing aneurysm progression and the likelihood of rupture.
Surgery is an option for some aortic aneurysms, depending mainly on its size and location, and its likelihood of rupture. The risk of surgery is weighed against the risk of non-surgical intervention. The decision to operate or not is taken on a person-by-person basis on the evidence of their particular case and circumstances.
The main surgical procedure involves inserting a piece of 'Dacron' tubing into the aorta at the point of the aneurysm. The tube needs to be the same length as the aneurysm and gives it the support necessary to prevent further enlargement. If the aneurysm affects the area of the aorta that branches to the legs, then a 'branched' graft may be required.
Alternatively, a catheter can be inserted into the artery at the groin. This carries a tube called an 'endovascular stent' to the aortic aneurysm. The stent stays in place to support the aneurysm all the way round.
Treatment depends on how the aneurysm is diagnosed; if it is discovered before rupture, then the aim of treatment is to stabilise the aneurysm, and prevent rupture or further growth. In aneurysms that are discovered by virtue of their rupturing, treatment requires immediate stabilisation to prevent further bleeding, and to allow maximal therapy in order to prevent or limit vasospasm.
Aneurysm surgery usually involves placing a small metallic clip across the base of the aneurysm (often called the aneurysm 'neck'). In doing so, the weak abnormal aneurysm wall is protected from blood flow thus the risk of rupture or thrombus formation is eliminated. At the same time, the normal artery wall is reconstructed to maintain blood flow to the brain.
Endovascular treatments for cerebral aneurysms are now fairly well established, providing partial or complete treatment in many cases.
Treatment typically involves threading a catheter from an artery in the leg up to the aneurysm and under X-ray guidance placing a metal coil into the aneurysm. After applying an electric current, the blood in the aneurysm clots, thereby filling the aneurysm.
Other techniques to fill an aneurysm include injected biological glue materials or sialastic balloons. Endovascular techniques are still developing but have shown utility in treating certain aneurysms which are not suited to surgery as determined by the neurosurgeon.
1 The sudden loss of blood caused by an aneurysm reduces blood supply to all organs of the body, hampering the functioning of virtually every organ and tissue in the body.
2 A thrombus or blood clot formed on and attached to a diseased area of the lining of the heart (endocardium).
3 A pulse cannot be found in the artery