ANEURYSMS AND DISSECTION OF BLOOD VESSELS PATHOLOGY

 

An aneurysm is a balloon-like bulge in an artery. Aneurysms can form in arteries of all sizes. An aneurysm occurs when the pressure of blood passing through part of a weakened artery forces the vessel to bulge outward, forming what you might think of as a blister.

• Aneurysms are congenital or acquired dilations of blood vessels or the heart.
• Aneurysms are abnormal vascular dilations.

CAUSE

Any condition that causes the walls of the arteries to weaken can lead to an aneurysm. The following increase the risk of an aneurysm or an aortic dissection:

• Atherosclerosis (a build-up of fatty plaque in the arteries).
• High blood pressure.
• Smoking.
• Deep wounds, injuries, or infections of the blood vessels.
• A congenital abnormality (a condition that you are born with).
• An inherited disease such as Marfan syndrome.
• A defective gene.

SYMPTOMS

Symptoms depend on the type and location of the aneurysm and result from the pressure caused by an aneurysm’s pressing against nearby organs, nerves, and other blood vessels. For example,

• Aortic aneurysms may cause shortness of breath, a croaky or raspy voice, backache, or pain in your left shoulder or between your shoulder blades.
• Aortic dissection may cause sudden and severe pain, and patients often feel like something is ripping or tearing inside of them. The pain is mainly felt in the chest, but it can spread to the back or between the shoulder blades. Aortic dissection may also cause sudden stomach pain, lower back pain, or flu-like symptoms. If blood leaks from the dissection and builds up in the chest, the blood may enter the pericardial space (the sac that surrounds the heart) and prevent the heart from filling properly. This can lead to a life-threatening condition called cardiac tamponade.
• Abdominal aortic aneurysms may cause pain or tenderness below your stomach, make you less hungry, or give you an upset stomach.
• Cerebral (brain) aneurysms may have no symptoms, although you may have headaches, pain in your neck and face, or trouble seeing and talking

RISK FACTORS

Risk factors for aortic dissection include:

• Uncontrolled high blood pressure (hypertension)
• Hardening of the arteries (atherosclerosis)
• Weakened and bulging artery (pre-existing aortic aneurysm)
• An aortic valve defect (bicuspid aortic valve)
• A narrowing of the aorta at birth (aortic coarctation)

Other potential risk factors include:

• Sex. Men have about double the incidence of aortic dissection.
• Age. The incidence of aortic dissection peaks in the 60s and 80s.
• Cocaine use. This drug may be a risk factor for aortic dissection because it temporarily raises blood pressure.
• Pregnancy. Infrequently, aortic dissections occur in otherwise healthy women during pregnancy.
• High-intensity weightlifting. This and other strenuous resistance training may increase the risk of aortic dissection by increasing blood pressure during the activity.

COMPLICATIONS

An aortic dissection can lead to:

• Death due to severe internal bleeding
• Organ damage, such as kidney failure or life-threatening intestinal damage
• Stroke
• Aortic valve damage (aortic regurgitation) or rupture into the lining around the heart (cardiac tamponade)

TRUE ANEURYSMS

A true aneurysm is bounded by all three vessel wall layers (i.e., intima, media, and adventitia), although the layers can be individually attenuated.

FALSE ANEURYSMS

A false aneurysm (pseudoaneurysm) is an extravascular hematoma that communicates with the intravascular space; part of the vessel wall has been lost.

DISECTION

A dissection occurs when blood enters the arterial wall itself, as a hematoma dissecting between the layers. In arterial dissections, pressurized blood gains entry to the arterial wall through a surface defect and then pushes apart the underlying layers.

CLASSIFICATION OF ANEURYSMS

Aneurysms can be classified by shape.

Saccular aneurysms are discrete outpouchings ranging from 5 to 20 cm in diameter, often with a contained thrombus

Fusiform aneurysms are circumferential dilations up to 20 cm in diameter; these most commonly involve the aortic arch, the abdominal aorta, or the iliac arteries

PATHOGENESIS

Arteries are dynamically remodeling, and the various constituents are constantly turning over.

Aneurysms can occur due to:

Poor intrinsic quality of the vessel matrix:

 In Marfan syndrome, inadequate fibrillin synthesis leads to aberrant transforming growth factor-b (TGF-b) activation and progressive loss of elastic tissue matrix; in Loeys-Dietz syndrome, TGF-b receptor mutations likewise cause elastic tissue loss. In both cases, aneurysms result from progressive remodeling of an inelastic media. Defective collagen III synthesis in Ehlers-Danlos syndrome, or defective collagen cross-linking in vitamin C deficiency (scurvy) also lead to aneurysm formation.

Imbalance of matrix synthesis matrix and degradation:

 Increased MMP activity by inflammatory cells (e.g., in atherosclerotic plaque or in vasculitis) can cause a net loss of medial ECM.

Loss of medial SMC or change in SMC matrix synthesis:

Ischemia of the innermost aspect of the aortic media occurs when thick atherosclerotic plaque impedes adequate oxygen and nutritional diffusion from the lumen. Similarly, mid-medial ischemia of the aorta can occur when vessels of the vasa vasorum are stenosed due to inflammation or hypertension. Such ischemia is reflected in SMC loss and/or “degenerative changes,” with loss of normal ECM synthesis and increased production of amorphous ground substance (glycosaminoglycan). These changes are collectively denoted as cystic medial degeneration and can be seen in a variety of settings including Marfan syndrome or scurvy.

CAUSE

The most common causes of aortic aneurysms include;

• atherosclerosis (particularly in the abdominal aorta)
• hypertension (particularly in the ascending thoracic aorta)
• syphilis
• trauma
• vasculitis
• congenital defects (e.g., berry aneurysms)
• septic embolization (e.g., from bacterial endocarditis)
• systemic bacteremia