Multidetector computed tomographic angiography is becoming the modality of choice for evaluation of the supra-aortic circulation in acute stroke imaging. Variations of the supra-aortic circulation, in particular of the extracranial arteries, are common. This article aims to provide a pictorial description of the variant anatomy of the aortic arch and extracranial arteries. Knowledge of the presence and clinical relevance of normal variants such as anomalies, duplications and embryological persistence plays a clinically relevant role in the diagnosis and management of neurological and surgical conditions, particularly as we enter an era of increasing extracranial intervention.
Accurate knowledge and greater understanding of the variations in extracranial arteries is required with advances in endovascular and surgical interventions.[1-4] Multidetector computed tomographic (MDCT) angiography is a three-dimensional (3D) technique that can provide detailed information about the vessel of interest from different orientations and facilitates the assessment, planning and quantification of disease prior to intervention. Because of the improvements in the image quality of MDCT angiography offered by the 3D high spatial resolution technique, these variations now appear incidentally and frequently during routine MDCT angiography.
Digital subtraction angiography is the gold standard for detection of extracranial vascular anomalies. However, the sensitivity and specificity of MDCT angiography are reported to be high (65–100% and 63–100%, respectively).
With advances in MDCT technology, it is now possible to evaluate the extracranial arteries and their branches with fast acquisition times, reduced contrast media administration and with minimal motion artefacts. This technique is becoming the examination of choice, having fewer complications than invasive angiography. In this paper, we have gathered a number of the variations of the extracranial arteries we have recently encountered and highlight the embryological development of these variational patterns.
Aortic arch anomalies
Vascular rings are developmental anomalies of the aortic arch system. The most common anomalies of the aortic arch can be divided into five groups: double aortic arch, right aortic arch with normal branching, right aortic arch with aberrant branching, cervical aortic arch and left aortic arch with aberrant branching.
Double aortic arch
The double aortic arch is the most common vascular arch anomaly with an incidence of 50–60% of vascular rings,[6, 7] and is due to failure of regression in the fourth right dorsal aorta, which causes both dorsal aorta to remain. The double aortic arch usually has a dominant arch, where the left common carotid (LCC) and left subclavian arteries (LSA) branch from the left arch and the right common carotid (RCC) and right subclavian arteries (RSA) branch from the right aortic arch (Fig. 1). The double aortic arch may be classified according to: the side in which the aorta descends, the relative size of the two arches, the position of the ductus arteriosis and the extent of atresia between each segment. The dominance and prevalence of the double aortic arch segments include; right aortic arch (75%), left aortic arch (20%) and equal size arches (5%) in patients.[8, 9]
Right aortic arch
The right aortic arch is the second most common cause of vascular arch anomalies with an incidence of 0.1%. A right aortic arch is formed when the right dorsal aorta remains patent and either the left fourth arch or the left dorsal aorta regress abnormally. The setting of a right aortic arch depends on the branching of the brachiocephalic vessels and the location of the ductus arteriosis. These vessels originating from the arch are characterised into two subgroups: mirror image or aberrant great vessels of the aortic arch.
Mirror image occurs when there is persistence of the right fourth aortic arch and regression of the left fourth aortic arch. The great vessels that originate from the right aortic arch are; LCC and LSA, RCC and RSA. The vascular ring, is completed by the ligamentum arteriosum (LA) arising from the LCC and LSA (innominate) and bridging over to the origin of the left pulmonary artery (LPA) (Fig. 2a,b). The incidence of right aortic arch with mirror image branching has an association with other heart defects (98%), most commonly Tetralogy of Fallot (47%).
Aberrant great vessels of the aortic arch occur in 12–25% of vascular ring anomalies.[6, 7] This type of anomaly occurs when the right fourth aortic arch persists while regression of the left fourth aortic arch distal to the LCC is evident. The great vessels that originate from the right aortic arch are as follows; the LCC, RCC, RSA and LSA. The LSA arises from the descending aorta (DA; Kommerell's Diverticulum) and travels posteriorly to the oesophagus. The vascular ring is completed the left ligamentum arteriosum (LLA) between the LSA and LPA (Fig. 2c,d).
Cervical aortic arch
Cervical aortic arch is a rare anomaly in which the aortic arch is located over the clavicle and is usually right sided which is unusually long, forms a loop and ascends into the right supraclavicular regions, then descends and crosses behind the oesophagus to continue as a left DA. The prevalence of cervical aortic arch is less than 0.0001% and less than 50 cases been reported.[15, 16] The embryogenesis of the cervical aortic arch may represent persistence of the third primitive aortic arch.[17, 18] Anomalous patterns of branching are common in cases of cervical aortic arch and include the aberrant RSA or LSA and separate origins of the internal and external arteries from the arch of aorta (Fig. 3).
Left aortic arch
The most common congenital aortic arch anomaly is the left aortic arch with aberrant RSA, with an incidence of 0.4–2.3%. Regression of the right dorsal fourth aortic arch results in aberrant origin of the RSA from a left (normal) aortic arch, typically distal to the LSA (Fig. 4a,b).
There are three passages in which the aberrant subclavian artery may travel: firstly behind the oesophagus (80%), secondly between the oesophagus and trachea (15%) and finally anterior to the trachea (5%).[20-22] With 16 cases reported worldwide, a rare form of the left aortic arch consists of a retro-oesophageal aortic segment, a right DA and right ligamentum arteriosum.
Extracranial artery anomalies
Congenital variations of the carotid-vertebral arterial system are well recognised in their common forms, being developmental anomalies associated with the aortic arch system. Anomalies of the carotid-vertebral system can be divided into four groups: Common Carotid Artery (CCA), Internal Carotid Artery (ICA), External Carotid Artery (ECA) and Vertebral Artery (VA) anomalies. In each group, we review the embryological development and anomalous configurations.
Common carotid anomalies
Configuration of the aortic arch and its branches is related to differential growth rates in the various arteries and the associated ‘migration’ and ‘merging’ of the great vessels. The most common variant of aortic arch branching occurs when the LCC artery has a common origin with the innominate artery (Fig. 5a). The incidence of this type of variant is seen in approximately 13% of patients. A second, less common variant occurs when the left common carotid artery originates directly from the innominate artery rather than as a common trunk, with a prevalence of 9% in the general population (Fig. 5b).
Absence of the CCA is an extremely rare anomaly. There have been less than 25 cases that have been reported to date, and only 6 cases have been characterised by angiographic studies.
Absence of the RCC is a rare anomaly and can be characterised according to embryological development: persistence of the third embryonic aortic arch instead of the fourth, persistence of the ductus caroticus and aplasia of the ICA. The RCC also known to rise and course above the sternoclavicular articulation in 12% of cases.[25, 26]
Internal carotid arteries
Congenital anomalies of the ICA can be categorised by their morphological course (aplasia/hypoplasia). The reported incidence of congenital ICA anomalies in adults ranges from 4–66% and 16–43% in children. Embryological development of the ICA develops on both sides out of the third aortic arch and the cranial part of the dorsal aorta. A loop is formed at the junction between the two blood vessels, reaching its maximal extension in the fifth and sixth embryonic weeks.[28, 29] Elongation and straightening of the blood vessels occur during development, when the arteries descend into the mediastinal space. However, persistence of embryonic arteries during embryo development has a cervical plexus, which receives blood from the carotid artery through a group of anastomotic arteries – trigeminal, otic and hypoglossal arteries. During developmental maturity, the embryonic vessels become atrophied and then disappear; however, with persistence of embryonic arteries, they maintain their connection between the carotid and basilar artery.
The variations of the ICA are categorised into 4 trajectories; straight course (65–70%), curved course (25–28%) and kinking and coiling (5–7%) (Fig. 6). Incomplete elongation and straightening, with partial development, or accelerated growth of the artery results in persistence of the loop. Paulsen et al., reported incidences of loop persistence between 5–10%.
Hypoplasia of the ICA has been characterised as being unilateral, primal and bilateral. Hypoplasia of the ICA is determined by the diameter of the carotid canal, which is significantly smaller in these patients (Fig. 7). Although a rare anomaly, the radiological appearance of bilateral hypoplasia of the ICA demonstrates filling from the hypoplastic carotid and middle cerebral arteries to both the ophthalmic and anterior cerebral arteries.
External carotid and vertebral artery anomalies
Variations of the ECA range from its origin, level of common carotid bifurcation and height at which it arises from. The branches of the ECA can be variable in number. When increased in number (by two or more), they arise as a common stem, or by the addition of branches not usually derived from this artery, such as the sterno-mastoid branch of the superior thyroid or occipital artery. Common variations of the ECA are as follows: The ascending pharyngeal may arise from the external carotid or the bifurcation of the common carotid (65–80%), the occipital (14–20%), or the common carotid artery (7–9%). The lingual arises from a common trunk with the facial (linguofacial trunk) in 10–20% of cases; a rare combination branch of the external carotid is a thyrolinguofacial trunk.
Vertebral artery anomalies
VA anomalies are only seen in 3.5% of cases. The common anomalies of the VA arise from the aortic arch, separately or by a common trunk, with the subclavian artery (SCA) on the left side or from the common carotid artery on the right side in case of right retro-oesophageal SCA.
The anomalous origins of the right vertebral artery (RVA) are divided into three groups: those originating directly from the aorta, those originating from the carotid arteries or from the innominate and those of duplicated origin. The VA anomaly originating from the RCC is usually accompanied by an aberrant RSA, while the RVA originating from the right internal carotid artery is less common. The most common RVA anomaly is the origin from the aortic arch, which accounts for 2.5–5.8% of all VA anomalies. The RVA usually arises in the proximal or distal portions of the aorta. However, in 40% of cases, the VA can be of different luminal diameter: the minor VA is either hypoplastic (8.6% of cases), if it joins to the contralateral VA to form the basilar trunk, or atretic, if it ends at the posteroinferior cerebellar artery or the occipital artery. The most unlikely variant of the RVA is the proximal RVA, which can be duplicated with its origins from the RSA or may be from the RSA and innominate, aortic arch or thyrocervical trunk. The RVA is also known to enter the cervical foramen at the level of the second to sixth cervical vertebra, with the example in Fig. 8c demonstrating the RVA entering the cervical foramen at the fourth cervical vertebral foramina.
The anomalous origins of the left vertebral artery (LVA) are divided into three categories: those originating directly from the aorta, those originating from the left internal carotid artery (LICA) (Fig. 9) and those of duplicated origin (Fig. 8a,b). An anomalous origin of the LVA from the aortic arch may be present in 5% of cases (Fig. 8b). The origins of the LVA arising from the aortic arch can have 2 separate origins: an origin between the LCC and LSA (6%) (Fig. 8a), and the other distal to the LSA (0.6%). However, there have only been two cases to date with a bifid origin from the LSA. Bilateral aortic origins of the VA are a rare anatomic variant, with fewer than 20 cases reported in the literature.
The VA has many variations with collateral circulations. The VA communicates with multiple intracranial and extracranial arteries and thus provides collateral circulation if stenosis or occlusion occurs.
The LVA is dominant in 50% of normal healthy population while 25% dominant in the RVA. Twenty five percent of healthy adults have equal vessel diameters that contribute to flow in the vertebrobasilar artery.[41-44] These variations have minimal clinical significance, unless there is an associated VA origin or proximal SCA stenosis.
MDCT allows accurate and fast noninvasive characterization of extracranial arteries. It is an alternative tool that is helpful in establishing the initial diagnosis, defining anatomic landmarks, relationships and associated vascular anomalies. A brief review of the major congenital anomalies of the extra-cranial arteries has been provided, with emphasis on morphology, prevalence and imaging.