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Table of Contents    
Year : 2021  |  Volume : 69  |  Issue : 6  |  Page : 1824-1827

Segmental Agenesis of External Carotid Artery from Common Carotid Artery with Anomalous Reformation through Occipital Artery Anastomoses

Department of Neurosurgery, Ramaiah Medical College and Hospital, Bangalore, Karnataka, India

Date of Submission26-Apr-2019
Date of Decision22-Jun-2019
Date of Acceptance11-Jul-2020
Date of Web Publication23-Dec-2021

Correspondence Address:
Dr. Sunil V Furtado
Department of Neurosurgery, Ramaiah Medical College and Hospital, MSRIT Post, Bangalore - 560 054, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0028-3886.333480

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 » Abstract 

Anomalous origin of extracranial and intracranial carotid system is subject to permutations of embryonal regression of the developing primitive embryonal vasculature. We present a case of segmental agenesis of left external carotid artery from the cervical carotid circulation. The entire left external carotid artery was supplied by the vertebral artery through muscular branches via the occipital artery. This anomaly of proximal external carotid agenesis with the entire vascular tree dependent on the posterior circulation via occipital artery muscular collaterals is unique and has not been described before. We discuss this anomaly with its embryology, clinical implications, and its relationship to a Proatlantal artery.

Keywords: External carotid artery, moyamoya disease, occipital artery, proatlantal artery, vertebral artery
Key Message: Agenesis of the external carotid artery from the common carotid artery and its reformation through posterior circulation collaterals is a rare anomaly. The clinical implications are discussed especially in conditions associated with poor cerebrovascular reserve, cerebral revascularisation and skull-base surgeries.

How to cite this article:
Furtado SV, Quryshi SA, Jagannatha AT, Hegde AS. Segmental Agenesis of External Carotid Artery from Common Carotid Artery with Anomalous Reformation through Occipital Artery Anastomoses. Neurol India 2021;69:1824-7

How to cite this URL:
Furtado SV, Quryshi SA, Jagannatha AT, Hegde AS. Segmental Agenesis of External Carotid Artery from Common Carotid Artery with Anomalous Reformation through Occipital Artery Anastomoses. Neurol India [serial online] 2021 [cited 2022 Jan 26];69:1824-7. Available from:

Congenital anomalies of the carotid arterial system vary from non-bifurcation of the common carotid artery to aortic arch origin of the external carotid artery (ECA) or internal carotid artery (ICA).[1],[2],[3] Gottschau in 1885 provided the first description of a persistent primitive proatlantal artery, arising from the ICA.[4],[5] We present a case of segmental aplasia of the left external carotid artery from the cervical carotid circulation and discuss the embryology, clinical implications and its relationship to a Proatlantal artery.

 » Case Presentation Top

A 36-year-old lady, with insignificant past medical history presented with sudden onset left hemiparesis. Magnetic resonance imaging (MRI) brain and MR angiogram (MRA) showed watershed infarcts in the right middle cerebral artery (MCA) territory with narrowing of bilateral supraclinoid ICAs. Routine hematological, coagulation tests were normal and workup for vasculitis returned negative. Digital subtraction angiography (DSA) of the cerebral vasculature showed narrowing of bilateral supraclinoid segment of the ICAs, right anterior, and MCAs suggestive of moyamoya disease (MMD). The left common carotid artery (CCA) continued intracranially as the ICA. The ECA was not visualized on left CCA injection [Figure 1]a and [Figure 1]b. The left ECA was reformed by muscular branches of the vertebral artery (VA) in the early phase [Figure 2]a. The distal occipital artery (OA) was also reformed by robust muscular branches of the VA in the later phase. This occipital artery reformed the ECA in its entirety [Figure 2]a,[Figure 2]b,[Figure 2]c. The V4 segment of the left VA was hypoplastic and terminated as the posterior inferior cerebellar artery [Figure 2]c and [Figure 2]d. The right VA was dominant and continued as the basilar artery.
Figure 1: DSA left carotid artery (a) AP (b) lateral showing absence of left external carotid artery with the carotid artery continuing as the ICA

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Figure 2: DSA left vertebral artery (a) lateral early phase, (b) lateral late phase and (c), (d) AP showing muscular arteries (triangle), reformed occipital artery (full arrow), hypoplastic left VA (open arrow), superior thyroid artery with no proximal ECA (curved arrow)

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Treatment and outcome

She underwent a right superficial temporal artery-MCA bypass. One year follow-up with MR Imaging and cerebral DSA showed a patent bypass with no further infarcts.

 » Discussion Top

Embryology of the anterior circulation

Development of the cerebrovascular system begins with the formation of 6 pairs of primitive branchial arch arteries at the 1.3 mm embryonic stage.[2],[3] The ICA appears during the 3 mm embryonic stage from a combination of the 3rd branchial arch arteries and the distal segments of the paired dorsal aortae [Figure 3]a,[Figure 3]b,[Figure 3]c. The ventral portion of the 2nd branchial arch disconnects from the dorsal aorta close to the origin of the ICA and becomes the ventral pharyngeal artery [Figure 3]d and [Figure 3]e. The ventral pharyngeal artery and the ICA fuse proximally to form the CCA. The distal segment of the ventral pharyngeal artery becomes the ECA [Figure 3]f and [Figure 3]g. The ICA divides into the anterior and the posterior division at the 4 mm stage (28 days) [Figure 3]h and [Figure 3]i.[3] The posterior division produces the fetal posterior cerebral artery (PCA). Branches from the fetal PCA fuse medially to form the distal end of the BA.[3]
Figure 3: Embryology of development of ICA and ECA (a-i: from 3 mm fetus to term)

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Embryology of the posterior circulation

When the embryo is 4–5 mm long, the paired longitudinal neural arteries (LNA), which are precursors of the BA form on the medial edges of bilateral vascular networks on the ventral hindbrain [Figure 4].[2] The proximal small branches from the ICA deliver blood to the developing hindbrain via the LNA. These branches include 3 transient pre-segmental arteries, one permanent presegemental artery, and the first intersegmental artery. The first intersegmental artery is termed the Proatlantal artery (ProA).[4],[5] The fused LNA of a 12-14 mm embryo represents the formed BA.
Figure 4: Embryology of development of posterior circulation

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Bilateral channels develop between the cervical segmental arteries connecting the developing subclavian arteries to the ProA in a 7-12 mm embryo [Figure 4]. When this longitudinal paravertebral anastomosis, representing the future V1 segment of the vertebral artery (VA) is complete, the proximal segment of the ProA regresses.[3] Its remnant is incorporated into the VA. The most common anastomosis between the ECA and posterior circulation is through the OA, a remnant of the ProA, along posterior radicular branches of the horizontal segment of the OA with the VA.[3]


In our case, the ECA was not arising from the CCA. This is possibly due to involution of distal segment of the ventral pharyngeal artery. The other possibility is segmental agenesis of the proximal ECA from the ventral pharyngeal artery, favored by the presence of all branches of the ECA including the superior thyroid artery [Figure 2]. This anomaly is not due to vasculitis, atherosclerosis, or connective tissue disease as robust angiographic embryonal connections are demonstrated. Hypoplasia of the distal left VA can also be explained by non-regression of the proximal ProA and failure of the distal ProA incorporation in the developing VA.

In the early VA injection phase, the ECA is formed by the muscular arteries connecting the V3 segment of the VA with the muscular arteries of the ascending pharyngeal arteries [Figure 2]a. The anomaly would have developed between 4-6 weeks of gestation.[1],[6]

 » Clinical Implications Top

In a setting of MMD as in this case unilateral trandural-pial anastomoses in the anterior and posterior circulation, a direct or indirect ECA to MCA bypass, skull base collaterals are supplied by the subclavian-vertebral system. Endovascular embolization for dural vascular malformation, epistaxis, head, and neck tumors would demand access through the subclavian-vertebral system. Posterior fossa vascular pathologies based on the ipsilateral hypoplastic intracranial VA can only be approached surgically. Development of extracranial carotid atherosclerotic stenotic disease should not occur due to absence of common carotid bifurcation turbulence. Surgical control of ECA would necessitate a preoperative search for the communicating channels and the occipital artery.


We acknowledge Mr Mohammed Mobin for designing the CAD diagrams of the embryology of the cerebral vasculature.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

The authors received no financial support for the research, authorship, and/or publication of this article.

Conflicts of interest

The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

 » References Top

Horowitz M, Bansal S, Dastur K. Aortic arch origin of the left external carotid artery and Type II proatlantal fetal anastomosis. AJNR Am J Neuroradiol 2003;24:323-5.  Back to cited text no. 1
Liechty JM, Weddle RJ, Shutze WP, Smith BL. Occurrence of a type 2 proatlantal intersegmental artery during carotid endarterectomy for symptomatic stenosis. J Vasc Surg 2016;64:807-8.  Back to cited text no. 2
Menshawi K, Mohr JP, Gutierrez J. A functional perspective on the embryology and anatomy of the cerebral blood supply. J Stroke 2015;17:144-58.  Back to cited text no. 3
Vasović L, Mojsilović M, Andelković Z, Jovanović I, Arsić S, Vlajković S, et al. Proatlantal intersegmental artery: A review of normal and pathological features. Childs Nerv Syst 2009;25:411-21.  Back to cited text no. 4
Arráez-Aybar LA, Navia-Álvarez P, Méndez-Cendón JC. A case of a type II proatlantal artery with arteriovenous malformation. Surg Radiol Anat 2011;33:85-9.  Back to cited text no. 5
Kesler WW, Sabat SB. Isolated anomalous origin of the vertebral artery from the common carotid artery. Interact Cardiovasc Thorac Surg 2018;27:615-61.  Back to cited text no. 6


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]


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