Aneurysm

An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus for clot formation and embolization. As an aneurysm increases in size, the risk of rupture increases, which could lead to uncontrolled bleeding. Although they may occur in any blood vessel, particularly lethal examples include aneurysms of the circle of Willis in the brain, aortic aneurysms affecting the thoracic aorta, and abdominal aortic aneurysms. Aneurysms can arise in the heart itself following a heart attack, including both ventricular and atrial septal aneurysms. There are congenital atrial septal aneurysms, a rare heart defect.

Etymology

The word is from Greek: ἀνεύρυσμα, aneurysma, "dilation", from ἀνευρύνειν, aneurynein, "to dilate".

Classification

Aneurysms are classified by type, morphology, or location.

True and false aneurysms

A true aneurysm is one that involves all three layers of the wall of an artery. True aneurysms include atherosclerotic, syphilitic, and congenital aneurysms, as well as ventricular aneurysms that follow transmural myocardial infarctions.
A false aneurysm, or pseudoaneurysm, is a collection of blood leaking completely out of an artery or vein but confined next to the vessel by the surrounding tissue. This blood-filled cavity will eventually either thrombose enough to seal the leak or rupture out of the surrounding tissue.
Pseudoaneurysms can be caused by trauma that punctures the artery, such as knife and bullet wounds, as a result of percutaneous surgical procedures such as coronary angiography or arterial grafting, or use of an artery for injection.

Morphology

Aneurysms can also be classified by their macroscopic shapes and sizes and are described as either saccular or fusiform. The shape of an aneurysm is not specific for a specific disease. The size of the base or neck is useful in determining the chance of for example endovascular coiling.
Saccular aneurysms, or "berry" aneurysms, are spherical in shape and involve only a portion of the vessel wall; they usually range from in diameter, and are often filled, either partially or fully, by a thrombus.Saccular aneurysms have a "neck" that connects the aneurysm to its main artery, a larger, rounded area, called the dome.
Fusiform aneurysms are variable in both their diameter and length; their diameters can extend up to. They often involve large portions of the ascending and transverse aortic arch, the abdominal aorta, or, less frequently, the iliac arteries.

Location

Aneurysms can also be classified by their location:
File:Ultrasonography of a thrombosed great saphenous vein aneurysm.jpg|thumb|Ultrasonography of an aneurysm of the great saphenous vein due to venous valve insufficiency.

Arterial and venous, with arterial being more common.
The heart, including coronary artery aneurysms, ventricular aneurysms, aneurysm of sinus of Valsalva, and aneurysms following cardiac surgery.
The aorta, namely aortic aneurysms including thoracic aortic aneurysms and abdominal aortic aneurysms.
The brain, including cerebral aneurysms, berry aneurysms, and Charcot–Bouchard aneurysms.
The legs, including popliteal artery aneurysms.
The kidney, including renal artery aneurysms and intraparenchymal aneurysms.
Capillary aneurysms are flesh-colored solitary lesions, resembling an intradermal nevus, which may suddenly grow larger and darker and become blue-black or black as a result of thrombosis.
The large vessels such as external and internal jugular veins

Cerebral aneurysms, also known as intracranial or brain aneurysms, occur most commonly in the anterior cerebral artery, which is part of the circle of Willis. This can cause severe strokes leading to death. The next most common sites of cerebral aneurysm occurrence are in the internal carotid artery.

Size

Abdominal aortic aneurysms are commonly divided according to their size and symptomatology. An aneurysm is usually defined as an outer aortic diameter over 3 cm, or more than 50% of normal diameter that of a healthy individual of the same sex and age. If the outer diameter exceeds 5.5 cm, the aneurysm is considered to be large.
The common iliac artery is classified as:

Normal	Diameter ≤12 mm
Ectatic	Diameter 12 to 18 mm
Aneurysm	Diameter ≥18 mm

Signs and symptoms

Aneurysm presentation may range from life-threatening complications of hypovolemic shock to being found incidentally on X-ray. Symptoms will differ by the site of the aneurysm and can include:

Cerebral aneurysm

Symptoms can occur when the aneurysm pushes on a structure in the brain. Symptoms will depend on whether an aneurysm has ruptured or not. There may be no symptoms present at all until the aneurysm ruptures. For an aneurysm that has not ruptured the following symptoms can occur:

Fatigue
Loss of perception
Loss of balance
Speech problems
Double vision

For a ruptured aneurysm, symptoms of a subarachnoid hemorrhage may present:

Severe headaches
Loss of vision
Double vision
Neck pain or stiffness
Pain above or behind the eyes
Abdominal aneurysm

Abdominal aortic aneurysm involves a regional dilation of the aorta and is diagnosed using ultrasonography, computed tomography, or magnetic resonance imaging. A segment of the aorta that is found to be greater than 50% larger than that of a healthy individual of the same sex and age is considered aneurysmal. Abdominal aneurysms are usually asymptomatic but in rare cases can cause lower back pain or lower limb ischemia.

Renal (kidney) aneurysm

Flank pain and tenderness
Hypertension
Haematuria
Signs of hypovolemic shock
Risk factors

Risk factors for an aneurysm include diabetes, obesity, hypertension, tobacco use, alcoholism, high cholesterol, copper deficiency, increasing age, and tertiary syphilis infection. Connective tissue disorders such as Loeys-Dietz syndrome, Marfan syndrome, and certain forms of Ehlers-Danlos syndrome are also associated with aneurysms. Aneurysms, dissections, and ruptures in individuals under 40 years of age are a major diagnostic criteria of the vascular form of Ehlers-Danlos syndrome.
Specific infective causes associated with aneurysm include:

Advanced syphilis infection resulting in syphilitic aortitis and an aortic aneurysm
Tuberculosis, causing Rasmussen's aneurysms
Brain infections, causing infectious intracranial aneurysms

A minority of aneurysms are associated with genetic factors. Examples include:

Berry aneurysms of the anterior communicating artery of the circle of Willis, associated with autosomal dominant polycystic kidney disease
Familial thoracic aortic aneurysms
Cirsoid aneurysms, secondary to congenital arteriovenous malformations
Pathophysiology

Aneurysms form for a variety of interacting reasons. Multiple factors, including factors affecting a blood vessel wall and the blood through the vessel, contribute.
The pressure of blood within the expanding aneurysm may also injure the blood vessels supplying the artery itself, further weakening the vessel wall. Without treatment, these aneurysms will ultimately progress and rupture.
Infection. A mycotic aneurysm is an aneurysm that results from an infectious process that involves the arterial wall. A person with a mycotic aneurysm has a bacterial infection in the wall of an artery, resulting in the formation of an aneurysm. One of the causes of mycotic aneurysms is infective endocarditis. The most common locations include arteries in the abdomen, thigh, neck, and arm. A mycotic aneurysm can result in sepsis, or life-threatening bleeding if the aneurysm ruptures. Less than 3% of abdominal aortic aneurysms are mycotic aneurysms.
Syphilis. The third stage of syphilis also manifests as aneurysm of the aorta, which is due to loss of the vasa vasorum in the tunica adventitia.
Copper deficiency. A minority of aneurysms are caused by copper deficiency, which results in a decreased activity of the lysyl oxidase enzyme, affecting elastin, a key component in vessel walls. Copper deficiency results in vessel wall thinning, and thus has been noted as a cause of death in copper-deficient humans, chickens, and turkeys.

Mechanics

Aneurysmal blood vessels are prone to rupture under normal blood pressure and flow due to the special mechanical properties that make them weaker. To better understand this phenomenon, we can first look at healthy arterial vessels which exhibit . Unlike crystalline materials whose linear elastic region follows Hooke's law under uniaxial loading, many biomaterials exhibit a J-shaped stress-strain curve which is non-linear and concave up. The blood vessel can be under large strain, or the amount of stretch the blood vessel can undergo, for a range of low applied stress before fracture, as shown by the lower part of the curve. The area under the curve up to a given strain is much lower than that for the equivalent Hookean curve, which is correlated to toughness. Toughness is defined as the amount of energy per unit volume material can absorb before rupturing. Because the amount of energy released is proportional to the amount of crack propagation, the blood vessel wall can withstand pressure and is "tough". Thus, healthy blood vessels with the mechanical properties of the J-shaped stress-strain curve have greater stability against aneurysms than materials with linear elasticity.
Blood vessels with aneurysms, on the other hand, are under the influence of an S-shaped stress-strain curve. As a visual aid, aneurysms can be understood as a long, cylindrical balloon. Because it's a tight balloon under pressure, it can pop at any time stress beyond a certain force threshold is applied. In the same vein, an unhealthy blood vessel has elastic instabilities that lead to rupture. Initially, for a given radius and pressure, stiffness of the material increases linearly. At a certain point, the stiffness of the arterial wall starts to decrease with increasing load. At higher strain values, the area under the curve increases, thus increasing the impact on the material that would promote crack propagation. The differences in the mechanical properties of the aneurysmal blood vessels and the healthy blood vessels stem from the compositional differences of the vessels. Compared to normal aortas, aneurysmal aortas have a much higher volume fraction of collagen and ground substance and a much lower volume fraction of elastin and smooth muscles, which contribute to higher initial stiffness. It was also found that the ultimate tensile strength, or the strength to withstand rupture, of aneurysmal vessel wall is 50% lower than that of normal aortas. The wall strength of ruptured aneurysmal aortic wall was also found to be 54.2 N/cm², which is much lower than that of a repaired aorta wall, 82.3 N/cm². Due to the change in composition of the arterial wall, aneurysms overall have much lower strength to resist rupture. Predicting the risk of rupture is difficult due to the regional anisotropy the hardened blood vessels exhibit, meaning that the stress and strength values vary depending on the region and the direction of the vessel they are measured along.