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Source: Adapted with permission from reference 17.

Source: Adapted with permission from reference 17.

Moreover, hypertension was much less common as a risk factor among young patients, and only 25% of young patients were hypertensive on presentation.

A number of reports have identified cocaine abuse as a risk factor for aortic dissection, typically among young, black, hypertensive men. However, cocaine likely accounts for less than 1% of cases of aortic dissection, and the mechanisms by which it causes dissection remain speculative18.

For decades, there has been recognition that there exists an unexplained relationship between pregnancy and aortic dissection, typically occurring in the third trimester or in the early postpartum period. In IRAD, only 0.2% of all aortic dissection cases were associated with pregnancy, which might suggest at first that it is a relatively minor contributor16. However, given that 93% of patients in IRAD were ^40 years old and the majority of patients were men, only a small minority could even have had pregnancy as a risk factor. Alternatively, if one considered only a cohort of those IRAD patients who were female and under the age of 40, one would find that 12% were associated with pregnancy, implying that there is some causal association. However, it is quite likely that pregnancy is a precipitant of aortic dissection among women

Figure 1.3. Frequency of dissection symptom onset during four periods of the day. Adapted from reference 20.

otherwise at risk (due to some underling cystic medial degeneration), rather than the primary cause of aortic pathology.

Trauma can also cause aortic dissection. Blunt trauma and deceleration injuries tend to cause localized tears, hematomas, or frank aortic transection but only rarely cause classic aortic dissection. More commonly, iatrogenic trauma is associated with true aortic dissection and accounts for 5% of cases in IRAD19. Both the manipulation of catheters and wires within the aorta and the insertion of intraaortic balloon pumps may puncture the aortic intima induce aortic dissection. In addition, cardiac surgery also entails a very small risk (0.12-0.16%) of acute aortic dissection, which is usually discovered and repaired intraoperatively. Aortic dissection appears to occur more often as a late complication of cardiac surgery, typically occurring months to years after the procedure; in fact, 22% of those with acute aortic dissection have a history of prior cardiac surgery, 9% had preexisting aortic disease (prior thoracoabdominal aortic aneurysm or dissection repair), and 5% had prior aortic valve replacement. The association of dissection with aortic valve replacement may reflect the fact that many of those having undergone aortic valve surgery did so because of an underlying dysfunctional bicuspid aortic valve and thus would likely have had underlying cystic medial degeneration as well. It could then be the congenital cystic medial degeneration rather than the cardiac surgery itself that predisposed this group to late aortic dissection. Nevertheless, 6% of those with aortic dissection had only a CABG as their surgical procedure.

Data from IRAD indicate that there are chronobiological patterns of acute aortic dissection20. There is diurnal variation in the onset of aortic dissection (see Figure 1.3), with dissection occurring most often in the early daytime i/i c

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