CASE 17469 Published on 28.10.2021

Unilateral agenesis of the left internal carotid artery



Case Type

Clinical Cases


Albert Ang, May Ting Tan, Mohammed Abdi, Joseph Alex, Deepak Pai, Hussein Hassan

Radiology Department, Diana Princess of Wales Hospital, Grimsby, United Kingdom


30 years, female

Area of Interest Arteries / Aorta ; Imaging Technique MR, MR-Angiography
Clinical History

A 30-year-old pregnant lady at 31 weeks presented with a sudden onset headache over the left frontal area with changes to her vision. She had nausea and vomiting with no other focal neurology.

Imaging Findings

Initial computed tomography (CT) head did not reveal any abnormality. Images were reviewed again showing absence of left carotid canal on CT. MRI brain/MRA brain was then performed which then revealed an absent intracranial segment of the left internal carotid artery. Other findings included an absent A1 segment of the anterior cerebral artery (ACA) with a hypertrophic anterior communicating artery supplying the left ACA. Hypertrophy of the left posterior communicating artery was seen which supplies the left middle cerebral artery instead of the left ICA.



Congenital absence of the internal carotid artery (ICA) is a rare vascular anomaly occurring in less than 0.01% of the population [1]. Three variations of anomaly include agenesis, aplasia and hypoplasia. These terms are often used interchangeably when the ICA is small or absent. Lie defined agenesis as the total absence of ICA; whereas aplasia and hypoplasia refer to the ICA where a part of the ICA is present proximal to a sudden narrowing [2]. This is usually defined by cases where the bony carotid canal is absent (agenesis) but is present in aplasia and hypoplasia. Agenesis of the ICA is associated with an insult to the ICA during early embryological development of the first and third aortic arches. However, the exact mechanisms that cause this remain unclear [3].

Clinical Perspective

Lie described 6 pathways of collateral circulation associated with the absence of the ICA. The most collateral flow is via the circle of Willis, through the anterior communicating artery [2]. Most of these cases are asymptomatic due to collateral circulation and remain undetected [4]. When patients did present with symptoms, they usually present with non-specific symptoms such as headache, vertigo or tinnitus slightly more often than cerebrovascular events such as stroke, subarachnoid haemorrhage or vision loss. [5]

Imaging Perspective

Depending on the patient’s symptoms, a CT head or MRI brain will be the investigation of choice. Once a missing internal carotid artery is identified, CT is preferred to evaluate the cases where the bony carotid canal is absent (agenesis). MR angiogram of the cerebral vessels will then adequately visualise the circle of Willis and other congenital variations that may be present.

Outcome and Teaching Points

Congenital absence of ICA is also associated with cerebral aneurysms and the reported prevalence of an aneurysm is 24-34% which is significantly higher than the general population which is 2-4% [6]. The likely cause of this is due to increased flow through the collateral vessels and in such cases, the anterior communicating artery is the most frequent site of aneurysm formation. Although there is no treatment to re-establish the ICA, it is an indication for radiological surveillance due to the increased risk of intracranial aneurysm, including a mandatory angiogram. [7] Recognition of this anomaly is important in thromboembolic disease as there are implications with planning carotid endarterectomy and trans-sphenoidal hypophyseal surgery. [8]

Written patient consent was obtained for publication.

Differential Diagnosis List
Unilateral agenesis of the left internal carotid artery
Internal carotid artery thrombosis
Internal carotid artery dissection
Final Diagnosis
Unilateral agenesis of the left internal carotid artery
Case information
DOI: 10.35100/eurorad/case.17469
ISSN: 1563-4086