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A Very Rare Origin of the Left Vertebral Artery and its Clinical Implications

The 2 vertebral arteries join to form a single midline basilar artery in the hind brain. Even though there is variability in the origins of the left vertebral artery (LVA) along the aortic arch, however, it had been reported that about 5% of the left vertebral artery (LVA) will have a variable origin, most usually between the left common carotid artery (LCCA) and the left subclavian artery (LSA). Jayanthi V et al. (2010) reported an anomalous origin of the LVA from the arch of the aorta [1].


INTRODUCTION
The usual origins of the vertebral arteries are from thepostero-superior surfaces of their ipsilateral subclavian artery, the artery coursing superiorly medial-ward, usually to enter the transverse foramen of the 6th cervical vertebrae.
The artery is typically divided into 4 segments V1 (preforaminal): origin to transverse foramen of C6, V2 (foraminal): from the transverse foramen of C VI to the transverse foramen of C2, V3 (atlantic, extradural or extraspinal): from C2 to the dura and V4 (intradural or intracranial): from the dura to their confluence to form the basilar artery.
The 2 vertebral arteries join to form a single midline basilar artery in the hind brain. Even though there is variability in the origins of the left vertebral artery (LVA) along the aortic arch, however, it had been reported that about 5% of the left vertebral artery (LVA) will have a variable origin, most usually between the left common carotid artery (LCCA) and the left subclavian artery (LSA). Jayanthi V et al. (2010) reported an anomalous origin of the LVA from the arch of the aorta [1]. Gluncic V et al, (1999) described case of an unusual origin of both vertebral arteries in a singular cadaver. On the left, the artery arose directly from the common trunk of vertebral and subclavian artery at the aortic arch and enters the transverse cervical foramina at C VI. The right vertebral artery (RVA) on the other hand originated from the right common carotid artery (RCCA) and enters the transverse foramina at C III [2].
LVA arising as the most distal of the branches off the aortic arch is very rare compared with the one originating between the LCCA and the LSA? Case D et al., (2015) however presented a case report of an anomalous origin of the RVA from the aortic arch distal to the LSA along with a review of cases reported to date in the literature [3].  Einstein EH et al, (2016) reported the case of an anomalous origin of the LVA from the aortic arch, arising between the LCCA and the LSA [4]. Shiva Kumar GL, Pamidi N, Somayaji SN, Nayak S, Vollala VR (2010) reported an anomalous branching pattern of the aortic arch, reporting the LCCA arising from the BCT, and a trifurcated brachiocephalic (BCT) with the RVA arising between the LSA and RCCA [5]. Of all cases, vertebral arteries that arose from the aortic arch were much more likely to not only have a more medial course (especially their preforaminal segment) over the cervical vertebra bodies but also to enter a transverse foramen that was more cranially located than the normal C6.

CASE REPORT
During routine dissection of the thoracic region in our facility, we observed an aortic arch branching variation with four arising vessels arising from the arch, the BCT as the first branch followed by the LCCA. There was no observed truncus bicaroticus morphology as the BCT and LCCA arose independently off the arch. The LCCA was followed by the LSA, but on careful inspection of the LSA, we did not observe the usually arising left vertebral artery from the s supero-posterior surface of the LSA. However following a more careful search, we observed the LVA arising poster inferiorly from the aortic arch distal to the LSA as the last terminal branch, resulting in 4-vessel aortic arch morphology (Fig). The observed LVA coursed superiorly in the mediastinum, travelling medial-ward to run posterior to the LSA. We observed tortuosity in the morphology of the LVA just proximal to the LVA entering the cervical foramina at the level of C5 to continue its cranial-ward cervical course in the more superior cervical foramina (Fig1). We did not observe any other abnormality distal to the LVA.
The LVA as the last branch off the aortic arch was carefully examined to ensure it is not an aberrant right subclavian artery (RSA) another supra aortic arising variant vessel sometimes reported as the last branch off the aortic arch. There were no other abnormalities observed along the aortic arch. The BCT arose normally as the first branch, bifurcating to yield the RSA and the RCCA. The RVA originated normally from the posterosuprior surface of the RSA. The LCCA also originated independently off the aortic arch, without the presence of a bovine morphology (Fig1).

DISCUSSIONS
The most common anomaly of the usual aortic arch is the direct LVA origin off the aortic arch between the LCCA and the LSA, converting the arch into four vessel morphology. Such a variant has been widely reported in many surgical, radiologic, post-mortem and cadaveric studies, the reports are either incidental or in large sample studies. Or as incidental findings, with a Bhatia K, et al., (2005) reported a 7.41% incidence [6] while Budhiraja V et al., (2013) reported a much higher 15.3% incidence of the LVA arising from the arch between the LCCA and the LSA [7].
TheLVA arising from the aortic arch is one of the aberrant supraaortic branching variations. Other supra-aortic variants include the LCCA sharing a common trunk with the BCT (the bovine arch morphology), the LCCA arising from the BCT, an aberrant RSA originating from the aortic arch as the last branch distal to the LSA and the LVA sharing a common stem of origin with the LSA directly off the aortic arch. Nelson ML, Sparks CD (2001) reported an unusual instance of the four arteries arising from the arch of the aorta, sequenced as RSA LSA, RCCA and LCCA [8]. However instances of the LVA arising as the last branch of the aortic arch distal to the LSA are very rare and with yet undetermined incidence, not even in large sample studies. Saeed UA however reported 3 cases of aberrant RVA arising as the last branch of the aortic arch, a very rare variant [9]. Of all cases, vertebral arteries that arose from the aortic arch were much more likely to not only have a more medial course (especially their preforaminal segment) over the cervical vertebra bodies but also to enter a transverse foramen that was more cranially located than the normal C6 entrance of the vertebral artery. Spine surgeons who approach the anterior cervical spine should be aware that an aortic arch origin of the LVA is likely to be closer to the midline and less protected above the C6 vertebral level. Tardieu et al (2017) [10]. SIKKA A reported the instance of LVA which took origin from the arch of aorta and entered the foramen transversarium of the fourth cervical vertebra. While RVA took origin from the RSA close to its origin and entered the foramen transversarium of the third cervical vertebra [11]. Our reported variant like in other vertebral arteries of aortic origin showed a medial-ward coursing and less protected above C6 since this was found entering at the C5 transverse foramina. Most aberrant vertebral artery which including those of the RVA which may arise from a trifurcating BCT or those arising directly from the LCCA or from the thyrocervical trunk are often asymptomatic. The normally arising R or L vertebral artery, arising from its ipsilateral subclavian artery postero superiorly serves an important path physiological role in instances of occlusion occurring at the proximal origin of its ipsilateral subclavian artery, a path physiological state named the Subclavian Steal Syndrome (SSS). In assuming an occlusion at the proximal origin of the LSA in the case we are reporting and a hypothetic retrograde flow in the aberrant RVA, such retrograde flow will not dump into the LSA, but into the arch of the aorta proceeding to form the descending thoracic aorta, resulting in a termed -Descending Aorta Steal Syndrome (DASS), of a therapeutic role should an occlusion occur in the arch distal to the LSA (Illustration 2). In the typical occlusion proximally occurring in the LSA and a normally arising LVA, would result in the obstructed subclavian artery obtaining its blood supply from a retrograde flow its ipsilateral LVA, a retro grade flow derived from the origin of the basilar artery formed by the union of the 2 vertebral arteries in the caudal Pons region. This is termed the SSS as depicted in Illustration 1.

Illustrations1
Illustration1depicts what happens in a normally arising morphology of the aortic arch with occlusion proximally in the left subclavian artery (LSA) and a retrograde flow in the left vertebral artery (LVA)-the standard Subclavian Steal Syndrome (SSS).
Illustration1. Image depicts the typical supra-aortic anatomy and retrograde flow in LVA when a proximal occlusion occurs in the LSA ─ a Subclavian Steal Syndrome (SSS).

Illustration2
Illustrate 2 depicts that even in a simulated retrograde flow in the LVA that arises from the aortic arch , flow goes into the arch and not into the subclavian artery. An LVA that arises distally from the arch, like the one being reported will be beneficial when there is an occlusion in the arch distal to the left subclavian artery-producing a ‖Descending Aortic Steal Syndrome (DASS).
RSA; right subclavian artery, RCCA; right common carotid artery; BCT; brachiocephalic trunk, LCCA, left common carotid artery, LSA; left subclavian artery.LVA, left vertebral artery. In instances of the LVA not found in its usual origin, diligent search has to be carried out to determine possible abnormal origins of this artery. In the age of increasing endovascular surgical procedures and vertebral stents, knowledge of the various aberrant origins of the VA is important in determining the different origins of the artery, its sizeand coursing so as to limit incidence of iatrogenic ligation of such aberrant LVA. Traumaticinjury to an unprotected longer than normal segment of the pre-foraminal VA due to its usually longer length may lead to an hemorrhagic stroke.

ACKNOWLEDGEMENT
Immense thanks to Prof. Stephen Peiper, sent my way to correct academic injustices, and as Chai of the Dept, encourages scholarship and providing a healthy working environment through a caring leadership.
Thanks to the Human Gift who in death still continues to contribute knowledge.