Aortic Aneurysm: Thoracic and Abdominal aortic Aneurysm

An aneurysm is a permanent localized pathologic dilation of a segment of a blood vessel over the normal diameter. Therefore an aortic aneurysm is a permanent localized pathologic dilation of the walls of the aorta.

As age increases, arteries become stiffer, wider (aneurysm) and longer (tortuosity).

Causes of aneurysms

Aneurysms arise from conditions that cause degradation or abnormal production of the structural components of the aortic wall collagen and elastin.

Most aneurysms are caused by degenerative disease affecting the vessel (atherosclerosis)

Other causes are:

• Structural weakness and Haemodynamic forces.
• Damage to, and loss of intima.
• Reduction in the elastin and collagen content of the media.
• Collagen; tensile strength, adventitia.
• Elastin; recoil capacity, media.

Risk factors or aneurysms

Risk factors associated with aneurysms include:

• Smoking,
• Hypertension,
• Hypercholesterolaemia

Laplace’s law

Laplace’s law states that Tension varies directly with radius when pressure is constant.

For every increase in the radius, there is a large increase in tension, leading to further enlargement of the aneurysm.

Rare causes of aneurysms

1. Congenital causes such as Marfan’s syndrome, Berry aneurysms. Mutations of the gene that encodes fibrillin-1 are present in patients with Marfan’s syndrome.
2. Post-stenotic causes i.e. Coarctation of the aorta, Cervical rib, Popliteal artery entrapment syndrome
3. Traumatic causes eg Gunshot, stab wounds, arterial punctures. These most commonly affect the descending thoracic aorta just beyond the site of insertion of the ligamentum arteriosum.
4. Inflammatory causes like (Vasculitide) Takayasu's disease, Behcet’s disease
5. Mycotic origins.
   A mycotic aneurysm is a rare condition that develops as a result of staphylococcal, streptococcal, Salmonella, or other bacterial or fungal infections of the aorta, usually at an atherosclerotic plaque. These aneurysms are usually saccular.
   Blood cultures are often positive and reveal the nature of the infective agent such as Bacterial endocarditis, syphilis, tuberculosis, and other bacterial infections
6. Pregnancy-associated aneurysms ie Splenic, cerebral, aortic, renal, iliac & coronary

Classification of aortic aneurysm

Aneurysms can be classifieds to as:

True aneurysm.

A true aneurysm occurs as a result of dilatation involving all the three layers of the wall of the blood vessel.

False (pseudo) aneurysm.

False aneurysm refers to the ones that arise due to traumatic breach in the wall. The intimal and medial layers are disrupted and the dilated segment of the aorta is lined by adventitia only and, at times, by a perivascular clot.

One characteristic is that the sac made up of the compressed surrounding tissue.

Classification according to their gross appearance.

Fusiform aneurysm

This kind is spindle-shaped involving the whole circumference of the blood vessel. It, therefore, results in a diffusely dilated artery.

Saccular aneurysm

A saccular aneurysm involves a small segment of the wall which then balloons or out pouches due to localized weakness.

Classification according to location

Aortic aneurysms also are classified according to location into an abdominal and thoracic aneurysm. Aneurysms of the descending thoracic aorta are usually contiguous with infra diaphragmatic aneurysms and are referred to as thoracoabdominal aortic aneurysms

Incidence of atherosclerotic aneurysms

• More than 90% affecting the abdominal aorta
• An infra-renal segment in about 95%
• Male: Female ratio 4:1
• More common in western countries
• 5% over 50s, 15% over 80s
• Associated with iliac aneurysms in 30%
• Associated with popliteal aneurysms in 10%

Thoracic aortic aneurysm

Clinical features of Thoracic Aortic Aneurysm

Most thoracic aortic aneurysms are asymptomatic and incidentally discovered during a clinical examination or radiologic investigation; However, compression or erosion of adjacent tissue by aneurysms may cause symptoms such as;

• Acute Chest pain,
• Shortness of breath,
• Cough,
• Hoarseness.

Aneurysmal dilation of the ascending aorta may cause congestive heart failure as a consequence of aortic regurgitation, and compression of the superior vena cava may produce a congestion of the head, neck, and upper extremities

Diagnosis and Investigation of thoracic aortic aneurysm

Chest X-Ray. It shows widening of the mediastinal shadow and displacement or compression of the trachea or left mainstem bronchus.

Echocardiography, particularly transesophageal echocardiography, can be used to assess the proximal ascending aorta and descending thoracic aorta.

Contrast-enhanced spiral CT, magnetic resonance imaging (MRI), and conventional invasive aortography are sensitive and specific tests for assessment of aneurysms of the thoracic aorta and involvement of branch vessels

Electrocardiogram,

Erythrocyte sedimentation rate

Urea and electrolytes

Arteriography.

Treatment of thoracic aortic aneurysm

• β-Adrenergic blockers currently are recommended for these patients, particularly those with Marfan’s syndrome, who have evidence of aortic root dilatation to reduce the rate of further expansion.
• Control of hypertension.
• Angiotensin receptor antagonists may reduce the rate of aortic dilation in patients with Marfan’s syndrome by blocking TGF-β signalling.
• Operative repair with placement of a prosthetic graft is indicated in patients with symptomatic ascending thoracic aortic aneurysms and for most asymptomatic aneurysms, including those associated with bicuspid aortic valves, when the aortic root or ascending aortic diameter is ≥5.5 cm, or when the growth rate is >0.5 cm per year.
• Replacement of the ascending aorta more than 4.5 cm is reasonable in patients with bicuspid aortic valves undergoing aortic valve replacement because of severe aortic stenosis or aortic regurgitation.
• Surgery should be considered in patients with Marfan’s syndrome with ascending thoracic aortic aneurysms of 4–5 cm.
• Operative repair is indicated for patients with degenerative descending thoracic aortic aneurysms when the diameter is more than 6 cm.
• Endovascular repair should be considered when the diameter is more than5.5 cm.
• Repair is also recommended when the diameter of a descending thoracic aortic aneurysm has increased more than 1 cm per year.

Abdominal aortic aneurysms

Before we move unto looking at abdominal aortic aneurysms it is prudent to remind ourselves the basic anatomy and branches of the abdominal aorta.

Anatomy of the abdominal aorta

Abdominal aorta begins at the level of T12, Ends at L4

• Anterior relations
  • Splenic vein, pancreas, duodenum
• Right
  • Cisterna chyli, Inferior vena cava, azygos vein
• Left
  • Sympathetic trunk
• Surface anatomy
  • Just above the transpyloric plane in the midline to a point left to the midline on the supracristal plane.

Branches of the abdominal aorta

• Paired visceral branches
• Suprarenal, renal, gonadal
• Unpaired visceral branches
• Coeliac, SMA, IMA
• Paired abdominal wall branches
• Subcostal, inferior phrenic, lumbar

As opposed to thoracic aortic aneurysm which occurs more in men, abdominal aortic aneurysms occur more frequently in males than in females, and the incidence increases with age.

At least 90% of all abdominal aortic aneurysms >4.0 cm are related to atherosclerotic disease, and most of these aneurysms are below the level of the renal arteries

Risk of rupture correlates with aneurysm size.

An abdominal aortic aneurysm commonly asymptomatic and usually detected on routine examination as a palpable, pulsatile, expansile, and nontender mass, or it is an incidental finding observed on an abdominal imaging study performed for other reasons.

Clinical features of abdominal aortic aneurysm

As abdominal aortic aneurysms expand, however, they may become painful. Some patients complain of strong pulsations in the abdomen; others experience pain in the chest, lower back, or scrotum.

Acute pain and hypotension occur with rupture of the aneurysm, which requires an emergency operation

Patient