Hypertension is the major risk factor for aortic dissection. Aortas in hypertensive patients show narrowing of the vasa vasorum associated with degenerative changes in ECM and variable loss of medial SMCs, suggesting that diminished flow through the vasa vasorum is contributory. Abrupt , transient increase in blood pressure, as may occur with cocaine abuse, is also known to cause aortic dissection. Most other dissections are related to heritable or acquired connective tissue disorders that give rise to abnormal aortic ECM, including Marfan syndrome, Ehlers-Danlos syndrome type IV, and defects in copper metabolism.
The trigger for the intimal tear and subsequent intramural hemorrhage is not known in most cases. Nevertheless, once the tear has occurred, blood under systemic pressure dissects through the media along laminar planes. Accordingly, aggressive pressure-reducing therapy may be effective in limiting an evolving dissection. In rare cases, disruption of the vasa vasorum can give rise to an intramural hematoma without an intimal tear.
MORPHOLOGYInmostdissections,theintimaltearmarkingthepointoforiginisfoundintheascendingaortawithin10cmoftheaorticvalve(see
Fig.10.20A ).Suchtearsusuallyaretransverseorobliqueinorientationand1to5cmlong,withsharp,jaggededges.Thedissectionplanecanextendretrogradetowardtheheartordistally,occasionallyasfarastheiliacandfemoralarteries,andusuallyliesbetweenthemiddleandouterthirdsofthemedia(see
Fig.10.20B
Externalrupturecausesmassivehemorrhage,orresultsincardiactamponadeifitoccursintothepericardialsac.Insome(fortunate)instances,thedissectinghematomareentersthelumenoftheaortathroughaseconddistalintimaltear,creatingasecondvascularchannelwithinthemedia(so-called“doublebarreledaorta”).Overtime,suchafalsechannelbecomesendothelializedformingachronicdissection.
Inmostinstances,nospecificunderlyingcausaldefectisidentifiedintheaorticwall.Themostfrequenthistologicallydetectablelesionisthecystic medial degeneration discussedearlier;thisischaracterizedbySMCdropoutandnecrosis,elastictissuefragmentation,andaccumulationsofamorphousproteoglycan-richECM(see
Fig.10.18 ).Inflammationischaracteristicallyabsent.Recognizablemedialdamageappearstobeneitheraprerequisitefordissectionnoraguaranteethatdissectionisimminent.Occasionally,dissectionsoccurinthesettingofseeminglytrivialmedialdegeneration,whilemarkeddegenerativechangesmaybeseenatautopsyinindividualswithoutdissection.
The clinical manifestations of dissection depend primarily on the portion of the aorta affected; the most serious complications occur with dissections involving the proximal aorta and arch. Thus, aortic dissections generally are classified into two types (
Fig. 10.21 • Proximal lesions (type A dissections), involving the ascending aorta, with or without involvement of the descending aorta (DeBakey type I or II, respectively) http://ebooksmedicine.net
Fig. 10.21 Classificationofdissections.TypeAdissections(proximal)involvetheascendingaorta,eitheraspartofamoreextensivedissection(DeBakeytypeI),orinisolation(DeBakeytypeII).TypeBdissections(distal,orDeBakeytypeIII)ariseafterthetakeoffofthegreatvessels.TypeAdissectionstypicallyhavethemostseriouscomplicationsandthegreatestassociatedmortality.
• Distal lesions (type B dissections), usually beginning beyond the subclavian artery (DeBakey type III)