In the majority of cases, AAA results from ECM degradation mediated by proteolytic enzymes released from inflammatory infiltrates in atherosclerotic lesions. Atherosclerotic plaques also compromise the diffusion of nutrients and wastes between the vascular lumen and the arterial wall, alternations that have deleterious effects on SMCs in the media. Due to this combination of effects, the media undergoes degeneration and necrosis, which results in arterial wall thinning. A familial predisposition to AAA, independent of genetic predilections to atherosclerosis or hypertension, appears to be a factor in some individuals. Of note, smokers who develop AAA tend to have more severe chronic obstructive pulmonary disease, particularly emphysema, suggesting that such patients are constitutionally predisposed to develop disorders associated with ECM degradation.
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Fig.
10.19 ).Inthevastmajorityofcases,extensiveatherosclerosisispresent,withthinningandfocaldestructionoftheunderlyingmedia.Theaneurysmsacusuallycontainsbland,laminated,poorlyorganizedmuralthrombus,whichcanfillmuchofthedilatedsegment.Notinfrequently,AAAsareaccompaniedbysmalleriliacarteryaneurysms.Nextwedescribesomeofthelesscommonformsofaorticaneurysms:
Inflammatory AAAs areadistinctsubtypecharacterizedbydenseperiaorticfibrosiscontainingabundantlymphoplasmacyticinflammationwithmanymacrophagesandgiantcells.
AsubsetofinflammatoryAAAsmaybeavascularmanifestationofarecentlyrecognizedentitycalledimmunoglobulin G4 (IgG4)-related disease. ThisdisorderismarkedbytissuefibrosisassociatedwithfrequentinfiltratingIgG4expressingplasmacells.AsdiscussedinChapter5,IgG4relateddiseasescanalsoaffectavarietyoftissues,includingthepancreas,biliarysystem,thyroidgland,andsalivarygland.Affectedindividualshaveaortitisandperiaortitisthatweakenthewallsufficientlyinsomecasestogiverisetoaneurysms.Recognitionofthisentityisimportantsinceitrespondswelltosteroidtherapy.
Mycotic AAAs occurwhencirculatingmicroorganisms(asinbacteremiafrominfectiveendocarditis)seedtheaneurysmwallortheassociatedthrombus;theresultingsuppurationacceleratesthemedialdestructionandmayleadtorapiddilationandrupture.
Fig. 10.19 Abdominalaorticaneurysm.(A)Externalsiteofruptureofalargeaorticaneurysmisindicatedbythearrow. (B)Openedaorta,withthelocationoftherupturetractindicatedbyaprobe. Thewalloftheaneurysmisattenuated,andthelumenisfilledbyalarge,layeredthrombus.
The clinical consequences of AAA include the following:
Obstruction of a vessel branching off the aorta (e.g., the renal, iliac, vertebral, or mesenteric arteries), resulting in ischemia of the kidneys, legs, spinal cord, or gastrointestinal tract, respectively
Embolism of atheromatous material (e.g., cholesterol crystals) or mural thrombus
Impingement on adjacent structures (e.g., compression of a ureter or erosion of vertebrae by the expanding aneurysm)
Rupture into the peritoneal cavity or retroperitoneal tissues, leading to massive, often fatal hemorrhage