Aneurysms are congenital or acquired dilatations of blood vessels or of the heart. We have several types which are easiest explained by this figure:
True aneurysms involve all three layers of the vessel, while false aneurysms only involve extravascular connective tissue. Saccular aneurysms are outpouchings ranging from 5 – 20 cm in diameter, while fusiform aneurysms are circumferential dilations up to 20 cm.
Risk factors for abdominal aneurysms include:
- Old age
- Tertiary syphilis
The pathogenesis of aneurysms involves one or more of these factors:
- Abnormal wall structure due to genetic diseases
- Marfan syndrome
- Ehlers-Danlos syndrome
- Disturbed balance between collagen synthesis and degradation
- Recall from biochemistry that matrix metalloproteinases (MMPs) are enzymes that can break down connective tissue components like collagen, elastin, proteoglycans and so on.
- If MMPs are overexpressed will collagen in the arterial wall be degraded, which weakens the wall. MMPs are overexpressed by macrophages in atherosclerotic plaques and in vasculitis
- Smooth muscle cell loss or decreased ECM synthesis
- This can occur in ischaemic damage of tunica media in atherosclerosis or hypertension
- Metabolic causes
- Scurvy causes deficient collagen synthesis
- Blocking of the vasa vasorum
- End-arteritis in tertiary syphilis
There are two important types of aneurysms, abdominal aortic aneurysm and thoracic aortic aneurysm.
Abdominal aortic aneurysms (AAAs) occur in the abdominal aorta. They usually start below the renal arteries and end above the iliac bifurcation. AAAs are especially characteristic in atherosclerosis, but can also be seen in systemic inflammation, IgG4-related disease and infections. They can be saccular or fusiform, often with atherosclerotic plaques and large mural thrombi, like in our preparation.
Thoracic aortic aneurysms (TAAs) occur in the thoracic aorta. They are characteristic in hypertension, Marfan syndrome and especially tertiary syphilis. Common symptoms include:
- Dyspnoea, due to compression of the airways
- Dysphagia, due to compression of the oesophagus
- Permanent cough due to compression of the recurrent laryngeal nerve
- Cardiac problems due to aortic valve insufficiency
Complications of aneurysms include:
- Rupture of the aneurysm, potentially causing stroke or hypovolaemic shock
- Mural thrombosis due to the turbulence inside the aneurysm
- Compression of nearby structures
Aortic dissection occurs when the layers of the tunica media inside the wall of the aorta get separated, and blood starts flowing between the layers. The aorta then ends up with two “lumen”, one real lumen and one pseudolumen between the layers of the aorta.
Blood gets into the aortic wall via a rupture in the tunica intima. Blood will then flow inside the pseudolumen and create a new tear in the intima, where it returns into the true lumen.
The biggest risk factors for aortic dissections are:
- Marfan syndrome
- Ehlers-Danlos syndrome
Two classification systems exist for aortic dissections, the Stanford classifications and DeBakey classifications. We’ll look at the former and ignore the latter. Stanford type A aortic dissection originates from the ascending aorta and may extend all the way to the iliac bifurcation. Stanford type B aortic dissection originates from the beginning of the descending aorta and may also extend to the bifurcation.
The blood flow inside the pseudolumen is logically much poorer than the blood flow inside the real lumen. Many arteries originate from the aorta, and in this case are especially the coronary, celiac, mesenteric and renal arteries important. Any of these arteries can, after the dissection, originate from the pseudolumen instead of the real lumen. Because of the poor blood flow in the pseudolumen will any artery that now originates from it receive very poor blood flow, so supplied organs undergo coagulative necrosis.
The most dangerous Stanford type is type A, because in this case can the coronaries originate from the pseudolumen. This can cause extensive acute myocardial infarct with cardiogenic shock. Both Stanford types can be fatal however, because they can both cause haemorrhagic bowel infarct, which can lead to perforation and sepsis.
73. Congenital heart diseases
75. Pathogenesis, classification and clinicopathology of vasculitides. Vascular tumours