Comment
In this study, we examined our clinical CT scan database to define arch
anomalies, its variants, and the possible association with TAA. We found
the reported prevalence of arch anomalies on initial survey to be 2.8%,
with bovine aortic arch being the most common anomaly (58.7% of all
anomalies and 1.6% in this population), and aberrant left subclavian
artery combined with right sided arch to have the highest association
with TAA (33%, P<0.001) 12-14. The reported
prevalence of 2.8% of arch anomalies was much lower than what we found
in the validation sample (21% for bovine arch and 6.6% for aberrant
left vertebral artery). The results of the validation sample correspond
to previously published reports on the prevalence of bovine arch
anomaly. 2,16 This deviation from the literature in
the initial survey is due to under reporting in our clinical database,
as the CT scans included in this study were done for indications other
than aorta diseases (such as lung cancer screening, pulmonary diseases,
infection/ sepsis, trauma, pulmonary embolism, esophageal diseases) and
it is possible that the arch anatomy received less attention from the
reader since most of these cases were report by radiologists without
fellowship training in cardiovascular imaging. With this relatively high
prevalence of bovine arch, the clinical significance of it should be
considered. Morrehead et al reported 31.1% prevalence of bovine arch,
including 14.9% type I bovine arch, and 16.2% type II bovine arch in
patients with thoracic aortic pathology.15 Dumfarth et
al reported bovine aortic arch to be the most common anomalous branch
pattern in patients with thoracic aortic disease, with a prevalence of
24.6%. This was followed by isolated left vertebral artery in 6.3% of
patients with thoracic aortic disease, and an aberrant right subclavian
artery in 1.8%.18 Wanamaker et al., reported similar
finding with bovine arch being the most common association with aortic
dissection followed by isolated left vertebral
artery.10
We also noticed that patients with arch anomalies had higher association
with aortic valve disease and hypertension. It is difficult to know if
one factor predisposes the occurrence of the other but we can
hypothesize that it is also possible that fewer /anomalous branching of
the aorta in this region leads to abnormal flow dynamics throughout the
aortic wall in this region putting it under abnormal hemodynamic stress
which when combined with other factors like hypertension or wall stress
caused by abnormal aortic valve geometry could result in an aggressive
phenotype leading to aortic dilatation and even dissection due to the
added/synergistic effect of multiple wall stressors.19
Given the nearly 3 fold higher association with TAA in the arch anomaly
subgroup compared to subjects without arch anomalies in this cohort and
also from the forementioned reports, greater attention needs to be paid
to carefully look and document for bovine arch anatomy in patients
undergoing routine CT chest imaging, with follow-up over time for
development of TAA. To some extent, the argument for this is similar to
incidental diagnosis of bicuspid aortic valve congenital variant, which
occurs in ~1-2% of the general population. These
patients are carefully monitored over time for development of aortic
valvulopathy and ascending aortopathy, although majority of subjects
with bicuspid aortic valve will not need any surgical intervention.
The prevalence of other aortic arch variants noted in our study is in
accordance to that reported in the literature, which is
~1-2% 17. Our study also shows
differential influence of the type of arch anomaly on the size of the
aorta. Right sided aortic arch combined with aberrant left subclavian
had the highest association with TAA (33%), while aberrant left
vertebral artery was not associated with increased risk of TAA (3%).
In this cohort, we also defined independent risk factors for TAA and
found significant association of arch anomaly with incidental aortic
aneurysm (OR 2.85). Other independent factors were: aortic valve disease
(OR=2.95), age, male sex, and increased BSA, while diabetes was found to
be protective (OR=0.75). Given the strong association of arch anomalies
with TAA in this study and also in the literature, and the fact that TAA
in this sub-population tends to dissect more often4,5, we suggest further investigation for inclusion of
aortic arch anomalies in the guidelines for screening for TAA. Current
guidelines for screening and follow-up of patients with known thoracic
aortic disease does not take into consideration the presence of aortic
arch anomalies 20, 21.