30: Abdominal Aortic Aneurysm

Key Points

• Disease summary:

  • Abdominal aortic aneurysms (AAAs), defined as dilatations of the aorta, are a complex disease with both genetic and environmental risk factors.

  • Prevalence: 1% to 2% of Caucasian populations harbor AAAs in industrialized countries. Prevalence increases with age to about 10% among men over 65 years of age.

  • AAA is the 17th leading cause of death in the United States, with approximately 15,000 deaths per year.

  • Risk factors: The most important known risk factors are smoking, positive family history, advanced age, and male sex. Elevated cholesterol levels and hypertension are mild risk factors. Diabetes, African-American ethnicity, and female sex are protective factors.

  • AAAs have a complex, poorly understood pathophysiology, in which inflammation, smooth muscle cell apoptosis, reactive oxygen species, extracellular matrix degradation, and activation of matrix metalloproteinases play a role.

• Differential diagnosis:

  • Diverticulitis, renal colic, and gastrointestinal hemorrhage belong to the differential diagnosis of ruptured AAA.

• Monogenic forms:

  • Aortic aneurysms can be found in patients with the Ehlers-Danlos syndrome type IV, the Marfan syndrome, and fibromuscular dysplasia. These Mendelian diseases are far less common than AAA and patients with these diseases constitute only a very small fraction of AAA patients, representing a rare form of AAA.

• Family history:

  • Family history of AAA is an important risk factor for AAA with an odds ratio (OR) = 1.96 (95% CI: 1.68-2.28) determined in population-based screening studies. Two formal segregation analyses have been published for AAA. Both studies demonstrated statistically significant evidence for a genetic model. In one study an autosomal recessive model for a major gene had the best fit, whereas in the other study an autosomal dominant model for a major gene was the best fit for the familial aggregation of AAA in their sample. A third report, in which a large collection of AAA families with 233 multiplex families was described, found that 72% of the AAA families were consistent with an autosomal recessive pattern, while 25% showed an autosomal dominant pattern of inheritance. Not finding a single mode of inheritance is consistent with AAA being a multifactorial disease where different loci can have distinct modes of inheritance.

• Twin studies:

  • Only four case reports on twins with AAA have been published.

• Environmental factors:

  • Smoking is the most important identified risk factor with an OR = 3.34 (95% CI: 3.04-3.67) for men and 3.80 (95% CI: 1.57-9.20) for women with a history of smoking.

• DNA linkage studies:

  • Shibamura and colleagues studied 119 AAA families using an affected relative pair (ARP) approach with sex and number of affected relatives, and their interactions as covariates, allowing for genetic heterogeneity. Using a two-phase, two-stage study design, a whole genome scan was completed using approximately 400 microsatellite markers. The first phase analyzed 36 multiplex families with 78 ARPs using sex and the number of first-degree relatives as covariates. Twelve regions on eight chromosomes (3, 4, 5, ...