John P. Veinot MD Pathology, Ottawa Hospital, Civic Campus


Aneurysm Definition – a localized abnormal dilatation of a blood vessel.

Types: True vs False

True aneurysms show a wall composed of all three layers (intima, media and adventitia) of a blood vessel or a heart chamber (endocardium, myocardium and epicardium).

False aneurysms represent repaired or healed ruptures and are composed of only thrombus, blood clot and fibrous tissue.

Types: saccular and fusiform

A saccular aneurysm is a sac or bag-like localized dilatation of a vessel.

A fusiform aneurysm is shaped like a sausage and represents a localized swelling or dilatation of the blood vessel. fusiform aneurysm

Most aneurysms occur in arteries, chiefly the aorta and muscular arteries.

Aneurysms may also be congenital or acquired. The most common congenital aneurysms are berry aneurysms, usually of the small muscular arteries of the brain.

Acquired aneurysms may be degenerative, infectious, immune/inflammatory, traumatic, mechanically derived,  
drug-related, metabolic, neoplastic, genetic, or idiopathic.


The most common aneurysm is the atherosclerotic aneurysm. Most of these aneurysms occur in the infrarenal abdominal aorta. These aneurysms produce localized swellings of the aorta and are often lined by abundant amounts of thrombus. atherosclerotic aortic aneurysm

Aneurysm complications: 

Mass effect
Thrombosis – luminal obstruction – emboli


Aneurysms may rupture producing bleeding and death. They may also form fistulas with adjacent organs.


Other important examples of aneurysms include:

Ascending aortic aneurysm – related to degenerative changes of the media, post syphilis, and systemic arterial hypertension
Coronary artery aneurysms – post Kawasaki disease.



An aortic dissection is an aortic tear followed by disruption of the aortic wall by a hematoma. The tear usually has origin in the ascending aorta, shortly after the aortic valve. The entry of the blood into the aortic wall causes separation of the aorta into outer and inner layers. Entry tear

 Microscopy aortic dissection ; Tear aorta ; Entry tear aortic dissection

Most aortic dissections relate to media degeneration of multiple causes including: 

Collagen vascular disease including Marfan’s syndrome and Ehlers-Danlos.
Bicuspid aortic valve
Aortic coarctation
Hormone – estrogen in pregnancy

 Dissection may:

form an intramural hematoma,
cause dissection with splitting of the aorta or
rupture through the adventitia, which may cause death.

 Dissection complications: Aorta cross sections with lumen compression

Branch compromise
Aortic insufficiency


Aortic dissection may be divided into two types, A and B. Types of aortic dissection

Type A involves the ascending aorta; Type B involves the descending aorta.

Type A are usually treated by surgery whereas Type B may be treated medically.