Aneurysms and DissectionsAneurysm: A localized abnormal dilation of a blood vesselor wall of heart. When aneurysm is bounded by arterial wallcomponents or attenuated wall of heart calledtrue aneurysm. Atherosclerotic, syphilitic, and congenital vascularaneurysms and left ventricular aneurysm that canfollow a myocardial infarction are true aneuryms.
False aneurysm (also called pseudoaneurysm) isa breach in vascular wall leading to extravascularhematoma that freely communicates withintravascular space (pulsating hematoma ). The most common false aneurysms are: Post-myocardial infarction rupture. Leak at junction (anastomosis) of vascular graftwith natural artery.
Dissection: Arises when blood enters the wall of artery asa hematoma and dissecting between its layers . Dissections may, but do not always, arise inaneurysmal arteries. Both true and false aneurysms, also dissections,can rupture.Causes of Aneurysm: The two most important causes ofaortic aneurysms are:o Atherosclerosis.o Cystic medial degeneration of arterial media.
Other causes include:o Trauma: traumatic aneurysms or arteriovenousaneurysms.o Congenital defects such as berry aneurysms:small spherical dilatations most frequentlyoccur in brain.o Infections (mycotic aneurysms).o Syphilis.o Systemic diseases: in some vasculitides.
Aneurysms can be classified by shape and size :-o Saccular aneurysms: spherical and vary in sizefrom 5 to 20 cm , often partially or completelyfilled by thrombus.o Fusiform aneurysms: involving a long segment,many involve the entire ascending and transverseportions of aortic arch.o These shapes and sizes are not specific for anydisease or clinical manifestations.
ABDOMINAL AORTIC ANEURYSMS ( AAA ):Atherosclerosis is the most frequent etiology.Morphology:o Usually positioned below renal arteries and abovebifurcation of aorta.o Saccular or fusiform.o Prime sites for formation of athero emboli.
Arteriograph showing abdominal aortic aneurysm
Gross appearance of abdominal aortic aneurysm
Pathogenesis: AAAs rarely develop before age 50 andare more common in men. There is a genetic susceptibility to AAA beyondthe genetic predisposition to atherosclerosis. As in Marfan syndrome: genetic defects inconnective tissue component which isresponsible for strength of aorta .
An altered balance of collagen degradationand synthesis mediated by local inflammatoryinfiltrates and destructive proteolytic enzymesthey produce and regulate. In this regard, matrix metalloproteinases (MMPs)in macrophages implicated in development ofaortic aneurysms through increased proteolysisof extracellular matrix proteins.
These enzymes have capacity to degrade allcomponents of EC matrix in arterial wall (collagens,elastin, proteoglycans, laminin, fibronectin). Decreased level of tissue inhibitor ofmetalloproteinases (TIMP) also reported inaortic aneurysms.
Clinical Course: Rupture into peritoneal cavity or retroperitonealtissues with massive fatal hemorrhage. Obstruction of vessel (iliac, renal, or mesenteric)leading to ischemic tissue injury. Embolism from atheroma or mural thrombus. Impingement on adjacent structures such ascompression of ureter, or erosion of vertebrae. Presentation as an abdominal mass (often pulsating)that simulates a tumor.
SYPHILITIC (LUETIC) ANEURYSMS : Obliterative endarteritis, characteristic oftertiary stage of syphilis (lues), showspredilection for small vessels. Syphilitic involvement of vasa vasorum ofthoracic aorta can lead to aneurysmal dilation. Thoracic aortic aneurysms can give rise to signsand symptoms:1. Respiratory difficulties: due to encroachment onlungs and airways.
2. Difficulty in swallowing: due to compressionof esophagus.3. Persistent cough: due to irritation of or pressureon recurrent laryngeal nerves.4. Pain: caused by erosion of bone (ribs and vertebralbodies).5. Cardiac disease: aortic valve dilation with valvularinsufficiency, or narrowing of coronary ostiacausing myocardial ischemia.6. Rupture.
Morphology: Inflammatory involvement begins in aortic adventitia,particularly involving vasa vasorum. Inducing obliterative endarteritis rimmed byan infiltrate of lymphocytes and plasma cells(syphilitic aortitis). The narrowing of lumina of vasa causes ischemicinjury of aortic media, with patchy loss of medialelastic fibers and muscle cells, followed byinflammation and scarring.
With destruction of media, the aorta loses itselastic recoil and become dilated. Luetic aortitis may also cause aortic valve ringdilation resulting in valvular insufficiency, Through stretching of valve cusps, widening ofcommissures, and thickening and rolling of freemargins. Owing to aortic insufficiency, left ventricular wallcan undergo volume overload hypertrophy,sometimes to 1000 gm (three times normal weight),referred to (cows heart).
Figure 1: A large swelling on left sideof anterior chest wall.Figure 2: Chest roentgenogram showingwidening of mediastinum witha radiopaque mass extending into superiormediastinum occupying left upper andmid-zone with right tracheal shift.
Figure 3: Three-dimensionalreconstructive image ofcardiac angiogram showinganeurysm of arch of aortaand ascending aortaindicated by arrows.Figure 4: Elastic Van-Gieson stain showingwall of aneurysm composed of fibrohyalinetissue with fragmented elastic fibers .
AORTIC DISSECTION (DISSECTING HEMATOMA) : Dissection of blood between and along laminarplanes of media, with formation of blood-filledchannel within aortic wall which often rupturesoutward causing massive hemorrhage. Aortic dissection may or may not be associatedwith marked dilatation of aorta. For this reason the older term "dissecting aneurysm"is discouraged.
Aortic dissection occurs principally intwo groups of patients:o More than 90% occur in men between agesof 40 and 60 with antecedent hypertension.o The second major group of patients,usually younger, has a systemic or localizedabnormality of connective tissue that affectsaorta (Marfan syndrome).
Dissection can also be iatrogenic,as a complication of arterial cannulation(during diagnostic catheterization orcardiopulmonary bypass). Rarely, for unknown reasons, dissection of aortaor its branches including coronary arteriesoccurs during or following pregnancy
Morphology: Detectable aortic wall pathology is not alwayspresent in dissection. The most frequent histologically detectable lesionis medial degeneration (cystic medial degeneration). Characterized by elastic tissue fragmentation andseparation of elastic and fibro muscular elementsof tunica media by small cleft like spaces. These are not truly "cysts“.
Thus, the term "cystic medial necrosis" is inaccuratebecause neither necrosis nor cysts are present. Inflammation is absent.This aortic dissection occurred just abovethe aortic root in a patient with Marfanssyndrome.
Microscopically, the tear (arrow) in this aorta extends through the media,blood also dissects along the media (asterisk).
Pathogenesis: Hypertension is the major risk factor , but itscontribution to aortic medial damage is uncertain. Some dissections are related to inherited connectivetissue disorders (Marfan syndrome) an autosomaldominant disease of connective tissue fibrillincharacterized by skeletal, cardiovascular, andocular manifestations. The cause of spontaneous dissections not associatedwith hypertension or genetic disorders is unknown.
Clinical Course: Depend strongly on level of aorta affected. Classified into two types :o More common (more serious ) proximal lesions:involving either ascending portion only, or bothascending and descending aorta (type A).o Distal lesions: not involving ascending part andusually occur distal to subclavian artery (type B).
The classic clinical symptom is sudden onset ofexcruciating pain, beginning in anterior chest,radiating to back, and moving downward asdissection progresses. This intense pain can be confused with that ofacute myocardial infarction. The most common cause of death is rupture ofdissection outward into any of three body cavities(pericardial, pleural, or peritoneal).
Retrograde dissection into aortic root can causedisruption of aortic valvular apparatus. Extension of dissection into great arteries of neck ;or into coronary, renal, mesenteric, or iliac arteries,causing critical vascular obstruction. Compression of spinal arteries may causetransverse myelitis. Aortic dissection was usually fatal.