Albert Ang, May Ting Tan, Mohammed Abdi, Joseph Alex, Deepak Pai, Hussein HassanPatient
30 years, female
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.
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 . 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 . 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 .
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 . Most of these cases are asymptomatic due to collateral circulation and remain undetected . 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. 
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% . 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.  Recognition of this anomaly is important in thromboembolic disease as there are implications with planning carotid endarterectomy and trans-sphenoidal hypophyseal surgery. 
Written patient consent was obtained for publication.
 Given II CA, Huang-Hellinger F, Baker MD, Chepuri NB, Morris P. Congenital Absence of the Internal Carotid Artery: Case Reports and Review of the Collateral Circulation. American Journal of Neuroradiology Nov 2001, 22 (10) 1953-1959 (PMID: 11733331)
 Lie TA. Congenital Anomalies of the Carotid Arteries. Amsterdam: Excerpta Medica; 1968:35–51
 Oz I, Serifoglu I, Yazgan O, Erdem Z. Congenital absence of internal carotid artery with intercavernous anastomosis: Case report and systematic review of the literature. Interventional Neuroradiology. 2016;22(4):473-480 (PMID: 27091873)
 Yilmaz C, Utebay B, Kalaycioglu S, Onat G, Solak A. Non-visualization of the internal carotid artery with a normal ipsilateral common carotid artery Doppler waveform: a finding suggesting congenital absence of the ICA on colour Doppler ultrasound. Br J Radiol, 79 (945) (2006), p. e108-11 (PMID: 16940363)
 Li S, Hooda K, Gupta N, Kumar Y. Internal carotid artery agenesis: A case report and review of literature. The Neuroradiology Journal. 2017;30(2):186-191 (PMID: 28424012)
 Afifi AK, Godersky JC, Menezes A, Smoker WR, Bell WE, Jacoby CG. Cerebral hemiatrophy, hypoplasia of internal carotid artery, and intracranial aneurysm: a rare association occurring in an infant. Arch Neurol 1987;44:232-235 (PMID: 3813939)
 Chen CJ, Chen ST, Hsieh FY, Wang LJ, Wong YC (1998) Hypoplasia of the internal carotid artery with intercavernous anastomosis. Neuroradiology 40:252-4 (PMID: 9592798)
 Kishore PR, Kaufman AB, Melichar FA. Intrasellar carotid anastomosis simulating pituitary microadenoma. Radiology 1979;132:381-383 (PMID: 461796)
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