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This article is part of the operation script for grafting an abdominal aortic aneurysm. The article can be used on its own, or to go to the next part of the operation, click on NEXT PART OF THE OPERATION at the bottom of this page.
AORTIC ANEURYSM 060 - PATHOLOGY REVIEW
Macroscopic appearance
- The word aneurysm comes from the Greek word, meaning widening up
- Abdominal aneurysms can be looked on as a variant of occlusive atheromatous arterial disease.
- Atheromatous disease is always present as well.
- An aortic aneurysm is a swelling of the aorta more than 1.5 times the normal diameter (ie more than 30mm.).
- Lesser degrees of swelling are called ectasia or dilatations.
- Aneurysms can exceed 150mm. in diameter.
- The aneurysms are saccular (local blow-outs) or fusiform (spindle shaped).
Microscopic appearance
- Microscopically there is a degeneration of all three layers (intima, media and adventitia) of the aortic wall.
- The wall is weakened by a loss of elastin fibres.
- The wall thins and stretches.
- Stitches will tear through this weak wall.
- There is also infiltration of the aneurysm wall with atheromatous and calcified plaques.
- These plaques may show up on straight X-rays of the abdomen.
Platelet thrombus inside abdominal aortic aneurysms
- With loss of the intima, the aneurysm becomes lined with platelet thrombus.
- This accumulation of platelets is a soft, yellow jelly-like mass, accurately described as chicken fat thrombus.
- The thrombus almost completely fills the aneurysm.
- It may amount to several handfuls of thrombus in a large aneurysm.
- There is a central lumen to the thrombus, along which the blood flows.
- An angiogram may show an apparently normal lumen without displaying the aneurysmal sac.
- Thrombus may embolise into the vessels of the lower limbs.
- This may happen before, during or after the surgery.
Progress of the disease
- As an aortic aneurysm enlarges, it draws the relatively healthy subrenal aorta forwards from the front of the spine.
- This gap is an important plane of dissection and a preferred site for clamping the aorta.
- The aorta, as with any aneurysm, will lengthen as well as dilate.
- This leads to distortion of the anatomy.
- For instance, elongation of the aorta and the iliac arteries may displace the origin of the common iliac arteries sideways and posteriorly out of sight.
- There is nearly always some degree of atheromatous disease in the aorta.
- Both proximal to the aneurysm and in the vessels more distally.
- ie At the sites for suturing the graft.
- These changes often lead to difficult graft anastomoses.
- Atheroma tends to form initially on the posterior walls of the vessels.
- The aortic surgeon seeks healthy sites on the anterior walls of distal vessels for placing the distal limbs of grafts.
- Calcified plaques around the origins of the lumbar arteries may need to be removed before the arteries can be stitched off.
- Clinically, aneurysms expand at a rate of about 2mm. per year.
- The rate of expansion is more rapid in some patients.
- The rate of expansion acclelerates once the diameter exceeds 5cm.
- The larger the aneurysm, the greater the chance of rupture.
- Aneurysms of the subrenal aorta are most amenable to surgery.
- 70% of subrenal aneurysms will have a 2-5cm. segment of reasonably normal aorta just below the renal arteries.
- This segment will usually hold sutures well enough to secure the graft.
- This segment becomes shorter as the aneurysm increases in diameter and length.
- Aneurysms of the renal and supra renal aorta are often inoperable.
- This is due to involvement of the renal and superior mesenteric arteries and the coeliac axis,
Other arteries affected by occlusive arteriosclerotic disease
- Atheromatous disease may cause occlusion of the arteries of the lower limb, coronary and cerebral arteries.
- Assessment of these vessels is a standard part in the clinical work up.
- The inferior mesenteric artery is often occluded by the thrombus long before an aneurysm presents clinically.
- This will encourage anastomotic pathways to the bowel.
- It explains the low risk of ischaemia of the bowel when this artery is tied off during aortic
- surgery.
- The gonadal arteries are similarly affected with atheromatous disease
- Collaterals are adequate in protecting the testes from ischaemia.
- The lumbar arteries, branching from the back wall of the aorta, often become occluded. The lumbar arteries normally supply blood to the spinal arteries.
- Collateral blood supply to the spinal arteries is normally adequate to protect the spinal cord from ischaemia when the lumbar arteries are stitched off during the operation
Arteriosclerotic aneurysms elsewhere in the body
There may be a generalised aneurysmal disease.
- eg Aneurysms of cerebral, thoracic aorta, suprarenal abdominal aorta, iliac, femoral or popliteal arteries.
- These should be sought clinically and on special tests.
- In the presence of aneurysms (and occlusive disease) in the iliac and femoral vessels, aortic operations need to be carefully planned:
- To maintain the blood flow to pelvis and lower limbs.
- To minimise the chance of rupture of these secondary aneurysms.
Dissecting aneurysms
- Dissecting aneurysms of the aorta are quite distinct from arteriosclerotic aneurysms.
- Dissecting aneurysms are caused by necrosis of just the media of the aorta.
- Starting in the thoracic aorta, a stream of blood dissects between the intima and the adventitia of the aortic wall, blocking the lumen.
- This stripping process may extend down as far as the abdominal aorta.
- Dissecting aneurysms are often seen in young adults with Marfan’s syndrome (arachnodactyly, high arched palate, lens dislocation and dissecting aneurysm).
Mycotic aneurysms
- Mycotic aneurysms are rare aneurysms associated with infection such as infected pancreatic cysts.
- Staphylococcus or Salmonella may be found in otherwise typical atheromatous aneurysms.
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