An unusual origin of the right vertebral artery from the thyrocervical trunk – a case report

Raveendranath Veeramani* Nachiket Shankar**

*Assistant Professor,Department of Anatomy, Sri Manakula Vinayagar Medical College,Pondicherry, India

**Assistant Professor, Department of Anatomy, St. John’s Medical College, Bangalore,India

 

ABSTRACT

 

     The vertebral artery (VA) usually arises from the first part of the subclavian artery. During routine dissection of an elderly female cadaver the VA was seen to arise from the dorsal aspect of the thyrocervical trunk at the bifurcation of inferior thyroid and ascending cervical artery on the right side. It was subsequently identified to enter the foramen transversarium of the fifth cervical vertebrae. The left VA arose from the first part of subclavian artery, coursed with moderate tortuosity and showed the presence of an unusual stenosed segment. The length of the first part of the VA was noted to be 19.2 mm on the right side and 31.5 mm on the left side. The external diameter of on the right and left side was 1.8mm and 5.38 mm respectively. Previous studies have shown that it is rare for the right VA to arise from the thyrocervical trunk. The origin of VA is clinically important for vascular surgery in the head and neck as well as in the interpretation of four vessel angiography for endovascular interventional procedures and diagnostic radiology.

 Key words: Vertebral artery, Variations, Thyrocervical trunk

  Introduction

          The vertebral artery (VA) usually arises from the posterosuperior aspect of the first part of the subclavian artery. It passes through the foramina in the transverse processes of all of the cervical vertebrae except the seventh, curves medially behind the lateral mass of the atlas and enters the cranium via the foramen magnum. At the lower pontine border it joins its fellow to form the basilar artery.1 Variations in the origin of the VA are commonly described. We describe a case report of an unusual origin of the right VA from the thyrocervical trunk and discuss its clinical implications.

 

Case report

          During routine dissection of the right side of the neck of an elderly female cadaver at St. John’s Medical College, Bangalore, India, the authors found an artery arising from the thyrocervical trunk and passing upwards. On tracing the artery proximally and distally, it was identified as the VA. The VA arose from the dorsal aspect of thyrocervical trunk at the bifurcation of inferior thyroid and ascending cervical artery (Figure 1a,Figure 1b).  It coursed superomedially to enter the foramen transversarium of the fifth cervical vertebrae. The right VA did not show any tortuosity. The length of the first part of the VA from its origin to the point of entry into the foramen transversarium of the fifth cervical vertebra was 19.2 mm and its caliber was 1.8 mm. Proximally the thyrocervical trunk on the right side gave off the suprascapular and transverse cervical arteries. 

        The left VA was a branch from the cranial aspect of the subclavian artery. The artery entered the foramen transversarium of the sixth cervical vertebrae. There was moderate tortuosity of the artery. The length of the artery from its origin to the point of entry into the sixth cervical vertebrae was 31.5 mm. The left VA showed a constricted portion  which was 6.21mm from its origin and at the point of constriction the external diameter was 1.63 mm and at other sites the external diameter was 5.38 mm. No other unusual features were noted on the left side.

 Discussion

          An anomalous origin of the VA is not very common. These variations of the VA more commonly involve the left side (5%) than the right (1-3%).2,3,4 The right VA may arise from the brachiocephalic artery (1%),  directly from aortic arch (3%) or from the right common carotid artery (0.28%).5 An origin from the right internal carotid artery as well as double origin have also been described.6 Lippert and Pabst described a case of double origin of the right VA in which the right VA arose  from the right subclavian artery and right thyrocervical trunk.7 Daseler and Anson reported that the incidence of the right VA arising from the thyrocervical trunk was 2.6%.8 Abnormal origin of left VA from the thyrocervical trunk has been reported by Strub et al.9 The thyrocervical trunk normally gives off the inferior thyroid, transverse cervical and suprascapular arteries, one of the branches of the inferior thyroid artery being the ascending cervical artery. In the present case, the VA arose from the dorsal aspect of thyrocervical trunk at the bifurcation of inferior thyroid and ascending cervical artery.    

          The first part of the VA is formed from the dorsal division of the 7th cervical intersegmental artery bilaterally. The second part of the VA is formed from the post-costal anastomosis between the more cranial cervical intersegmental arteries. The thyrocervical trunk however is formed from the pre-costal anastamosis between the cervical intersegmental arteries.  Anomalous origins of the VA can be explained by occurrence of aberrant anastomosis at any time during embryonic development.  The time and location of this anastomosis will determine the ultimate adult anomalous origin.10 In the present case, the VA on the right side is likely to have arisen as a branch from the pre-costal anastomosis of the cervical intersegmental arteries, rather than as a dorsal branch of the 7th cervical inter-segmental artery.

          The VA may enter the foramen transversarium of the 2nd to 6th cervical vertebra.3,5. In the present study the right VA entered the foramen transversarium of the fifth cervical vertebrae. The VA enters the foramen transversarium of C6 vertebrae in 88% of the cases,C7 vertebrae in 5% and C5 vertebrae in 7% of the cases.5 However the entry into foramen transversarium at C5,C7 and C4 was 4.5,1.2 and0.7% in other studies.3,11  The vertebral arteries are usually unequal in size, the left being larger than the right.5  In the present case as well, the caliber of the VA on the left was much greater than the right. In addition the length of the first part of the VA was less on the right as compared to the left. A possible explanation for the bilateral differences in the length and caliber of the VA is that the 1st part of the VA on the right probably arose from the pre-costal anastomoses rather than the dorsal branch of the 7th cervical intersegmental artery as on the left side. The left VA probably had to compensate for the smaller than usual right VA by having a large caliber and greater blood flow. This could have predisposed the left VA to early atherosclerotic changes, thus possibly contributing to the stenosis and tortuosity that was observed.12         

          The VA forms one of the major sources of cerebral blood flow. Variations of the VA are clinically important in the interpretation of diagnostic or interventional angiograms and vascular surgeries of the head and neck. If the VA cannot be found in the usual position, the possibility of such a variant must be considered. It has been documented that in most cases of anomalous origin of the VA, the subjects were asymptomatic.13 However some authors hypothesize that an anomalous origin and distribution of the VA can cause alterations in cerebral hemodynamics, that may predispose to aneurysmal formation with a greater risk of cerebrovascular accidents.10,14 Thus, whenever there is an anomalous origin of the VA, a thorough search for coexisting aneurysms should be conducted, so that endovascular therapy of an intracranial aneurysm can be performed before patients present with subarachnoid hemorrhages.10 Additionally the pre-vertebral segment of the VA is a frequent site for atherosclerosis.15

          In conclusion, the unusual origin and course of the VA as described in this study has to be borne in mind as it is of extreme importance for the endovascular interventionist to identify the vessel and the diagnostic radiologist to avoid misinterpretation of angiograms.

 

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