Key Words
Aortic arch variation - Left vertebral artery - Aorta - Variation prevalence
Introduction
The left vertebral artery (LVA) usually arises from the left subclavian artery medial
to the thyrocervical trunk. It then enters the foramen transversarium of C6, passes
through the transverse processes of all superior cervical vertebrae, and finally enters
the foramen magnum while supplying the brain and spinal cord.
The aortic arch normally gives rise to three vessels: the brachiocephalic trunk (BT),
left common carotid artery (LCC), and left subclavian artery (LSA). Variations on
this have been observed numerous times and even classified[1]. This report is concerned with the “Adachi TYPE C” variation in which a fourth vessel,
an anomalous left vertebral artery, also arises from the aortic arch. The frequency
of this variation has generally been reported between 0.68% and 5.8%[2]
[3]
[4]
[5]
[6]
[7]. Two additional studies have reported frequencies of 7.41% and 15.3%[8]
[9]. It is also worth noting that these data indicate that angiography, computed tomography
(CT) and MRI studies may underreport the prevalence of this anomaly compared with
autopsy-based studies.
Case Presentation
While performing a cadaveric dissection of the superior mediastinum, an abnormality
was noticed at the location of the aortic arch. Upon further examination, it was observed
that the aortic arch gave rise to four vessels. The BT, LCC, and LSA were noted to
be in their normal locations. A fourth vessel, arising from the aortic arch proximal
to the LSA was subsequently found to be the LVA ([Figure 1]). Further dissection revealed that a branch of the thyrocervical trunk also formed
an anastomosis with the aberrant LVA. The LVA itself entered the foramen in the transverse
process of C4, while the anastomotic branch may have supplied the cervical vertebrae
below C4.
Figure 1. Panel A and Panel B. Views of the superior mediastinum and neck (magnified in Panel B). AA = aortic arch;
BT = brachiocephalic trunk; LSA = left subclavian artery; LVA = left vertebral artery;
LCC = left common carotid; TT = thyrocervical trunk and anastomoses with LVA (denoted
by asterisks).
We subsequently explored the aortic arches of the remaining 26 cadavers in the Gross
Anatomy course to document other arch variations. In each case, the manubrium was
removed, the superior mediastinum was exposed, and the aortic arch and great vessels
were dissected. After examination, it was concluded that 4/27 (14.8%) cadavers had
an aberrant LVA originating from the aortic arch. No other abnormalities were noted.
The three additional instances of this variation did not have the additional anastomosis
between the LVA and a branch of the thyrocervical trunk. The four cadavers had lifespans
of 87, 81, 89, and 60 years. Three of the four specimens were Caucasian. The cause
of death was cardiopulmonary arrest or otherwise unspecified. None of the cadavers
exhibiting this anomaly came with any accompanying history of central nervous system
symptoms. Interestingly, all four affected cadavers were female. There were 15 female
cadavers in total, corresponding to a 26.7% prevalence rate of this aberration in
females.
Discussion
Several aortic arch variations have been previously described including examples where
the LCC arises from the BT[10] and where both vertebral arteries originate as additional branches of the aortic
arch[11]. The variation examined in our case report (LVA origination in the aortic arch)
has been previously reported[12]
[13] at varying frequencies, with most studies reporting a prevalence between 3% and
8%[14]. We found a frequency of 14.8% (n = 27). A previous study recorded five out of six of their anomalous cadavers as female[15]. Combined with our findings, this suggests that this anomaly is more prevalent in
females.
The development of this variation may involve several factors. Blood vessels develop
with the association of growth factors such as vascular endothelial growth factor-A164/165
and placenta growth factor[16]. The latter especially induces the formation of large, stable blood vessels. Release
of these factors may be increased, decreased, or temporally varied to allow the LVA
to arise from the aortic arch rather than the LSA.
Embryologically, cervical intersegmental arteries appear longitudinally from the aortic
arch as the heart and aorta move caudally. The seventh cervical intersegmental artery
gives rise to the LSA, while the dorsal division gives rise to the LVA[17]. The sixth cervical intersegmental artery usually disappears, but in this case it
may have remained, allowing blood flow directly from the aortic arch[18]. Because of reduced blood flow through the typical origination of the LVA (i.e.,
the dorsal division of the seventh cervical intersegmental artery), origination of
the LVA from the aortic arch was then possible. Additionally, there was an observed
anastomosis between a branch of the thyrocervical trunk and the aberrant LVA in the
primary case in this report. This may have occurred due to a portion of the blood
flow persisting through the dorsal division of seventh cervical intersegmental artery.
It seems as if this variation may be more common than described in the literature.
Our findings support studies reporting higher prevalence rates[8]
[9] and female predominance. Perhaps the discrepant prevalence rates and the serendipitous
nature of these discoveries call for more systematic examination of the aortic arch
of each cadaver dissected in laboratories. The aberrant left vertebral is unlikely
to have any effect on blood flow, but its presence may well complicate aortic arch
surgical and endovascular procedures. Disconnection or compromise of the aberrant
vessel could also have cerebral consequences. If the incidence is indeed as high as
we suspect, the anomalous vertebral artery is at higher risk in surgical procedures
requiring isolation of arch vessels or access to the aortic arch itself. At that point,
disruption of cerebral blood flow could be significant.
