Bilateral congenital absence of the internal carotid arteries: a case report

Abstract

Congenital absence of the internal carotid artery is a rare occurrence. Even more infrequent are cases where the patient has a bilateral absence of the internal carotid arteries. Reported is a case of a 52-year-old woman who presented with optic nerve neuropathy, and was incidentally discovered to have a congenital bilateral absence of her internal carotid arteries. During computed tomography angiography imaging looking for cerebral venous thrombosis, related to her preexisting condition of bilateral elevated optic discs and residual left optic neuropathy, the findings were made. The absence of the arteries is not always recognizably symptomatic, with most findings being incidental through imaging studies only. This is because collateral flow allows for sufficient cerebral circulation. However, this condition puts such patients at higher risk for conditions such as aneurysms and subsequently strokes where the collateral flow exists.

Introduction

Bilateral congenital absence of the internal carotid arteries (ICA) is an uncommon condition, impacting less than 0.01% of the general population [1], [2], [3], [4]. Absence of the ICA most often presents unilaterally in patients, thereby making instances of bilateral hypoplasia even more so intriguing [3]. This natural absence may be consequent of hypoplasia (incomplete development), agenesis (no development), or aplasia (no development despite the presence of developmental precursors). The result of this absence is collateral blood flow, typically from the circle of Willis; for this reason, this abnormality is oftentimes asymptomatic and detected incidentally via imaging such as computed tomography (CT), angiography, or magnetic resonance imaging (MRI) [2], [3]. However, symptoms that may present in some patients include recurrent headache, blurred vision, and convulsions [4]. This collateral flow may also occur through persistent embryonic vessels or through anastomotic branches of the external carotid artery (ECA) [2]. If there is a lack of adequate collateral flow, cerebrovascular accidents (CVA) and/or intracranial hemorrhage could occur and thereby this issue may present as cerebrovascular disease [5], [6]. The presented case involves a 52-year-old female patient with an incidental finding of bilateral absence of the ICAs.

Case report

A 52-year-old female presented for a routine follow-up appointment related to her bilateral elevated optic discs and residual left optic neuropathy. She had constant headache, especially when laying down flat. A computed tomography angiography (CTA) of her head and neck was done (see Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5). While inspecting the major branches of the aorta in the neck, the right common carotid originated from the brachiocephalic trunk and the left common carotid artery arose from the arch of the aorta. However, there was no bifurcation of the common carotid arteries at the expected level superior to the thyroid cartilage (see Fig. 1). The vessels identified in this area were determined to be the external carotid arteries as they followed the appropriate anatomical course and had the expected branches in the neck with no intracranial extension. The expected course of the intracervical and intracranial portions of the ICA failed to demonstrate any visible vessels (see Fig. 2). Additionally, petrous carotid canals were not identified on either side. These findings were representative of bilateral absence of the ICAs. The CTA head revealed patent anterior cerebral circulation, which was found to be from compensatory enlarged feeding posterior communicating arteries (PCOM), prominent basilar artery, posterior cerebral arteries (PCA) and codominant enlarged vertebral arteries (see Fig. 3, Fig. 4, Fig. 5, Fig. 6). This patient also had hypoplastic left internal jugular vein, sigmoid, and transverse sinuses.

Fig. 1.

Agenesis of the bilateral internal carotid arteries. CT angiogram head/neck at the level of the C5 vertebral body, where the internal carotid artery (ICA) is absent. The common carotid arteries continue as the external carotid arteries (orange arrows). Neither intracervical nor intracranial portions of the ICAs were identified. Enlarged bilateral vertebral arteries were identified (blue arrowheads). (Color version of figure is available online.)

Fig. 2.

Empty petrous carotid canals. Axial CT angiogram at the level of the petrous portions of the carotid canals demonstrates absence of carotid canals within the expected portions of temporal bones on both sides (red arrows). The finding favors absence of internal carotid arteries (ICA). (Color version of figure is available online.)

Fig. 3.

Compensatory enlarged vertebral arteries. Coronal image from the CT angiogram reveals bilateral co-dominant enlarged vertebral arteries (blue arrows). (Color version of figure is available online.)

Fig. 4.

Enlarged posterior communicating arteries. Axial CTA maximum intensity projection (MIP) image at the level of the termination of the basilar artery demonstrates bilateral enlarged posterior communicating arteries (purple arrows) arising from the posterior cerebral arteries, which supplies the middle and anterior cerebral arteries (not shown). (Color version of figure is available online.)

Fig. 5.

Anterior cerebral blood supply provided by posterior circulation. Coronal CTA MIP image demonstrates an enlarged basilar artery (green arrowhead), posterior cerebral arteries (blue arrows) which were supplies for the anterior cerebral circulation (not shown) via posterior communicating arteries. (Color version of figure is available online.)

Fig. 6.

Absent common carotid bifurcation bilaterally. A 3D rendering of the vasculature of the neck depicts bilateral common carotid arteries continuing as external carotid arteries, these vessels do not enter the carotid canal (not shown). No carotid bifurcation is seen bilaterally. Bilateral vertebral arteries are enlarged.

Discussion

Absences of carotid arteries have been documented as early as 1787 per postmortem examination, with a case in a living patient as early as 1954 via angiography [1], [2]. Interestingly, the most documented cases involving absence of the ICA are unilateral and most commonly reported on the left side [1], [7]. Our case report is of particular interest because it demonstrates bilateral absence of the vessels. The cause of congenital unilateral carotid absence has been attributed to disruption of the embryo by physical and hemodynamic pressure upon the embryo [2]. Such stresses are thought to include amniotic band constriction and folding of the embryo's neck region to one side [1], [2], [8], [9]. However, no specific findings have been linked to the cause for absence of the bilateral ICAs [2], [8].

Identification and detection of the absence of the ICA may be done via imaging studies including CT, MRI, carotid Doppler ultrasound or conventional as well as CT/MR angiography [7]. CTA was the imaging modality used in the presented case. A strong indicator for a congenital cause is the lack of, or a very small/hypoplastic, bony carotid canal observed during such imaging studies [2], [4], [9], [10]. This is because the formation of the skull base begins at weeks 5-6 of fetal life, whereas the formation of the ICA is normally completed around week 6 of fetal life [1], [2]. Thus, no canal is formed for the ICA when the skull forms. In cases where patients present with signs of lacking the ICA, carefully inspecting the carotid canal may assist in determining if the ICA simply is not visible due to other reasons unrelated to agenesis such as blockage. Based on review of other cases, many of the findings for the absence of the ICA have been discovered incidentally, just as in our reported case [7].

As not many cases are reported in pediatric patients, it is thought that there is enough collateral blood flow to the brain during this period, while symptoms leading discovery of this condition in adult population is due to inadequate collateral flow [2]. A reason for insufficient collateral flow could include development of atherosclerosis in vessels previously supporting blood flow in the absence of the ICA [2], [8]. Additionally, the development of aneurysms is thought to be slow in children as this issue is not common [5]. However, in cases where symptoms, such as headache, do occur in adults, aneurysms have been reported in 24%-67% of ICA absent cases, in contrast with the general population which has an aneurysm rate of 2%-4% [1], [11]. Thus, patients with absence of the ICA(s) are at higher risk of aneurysm and should be screened appropriately. In this case report, the patient had not presented with any specific symptoms apart from headache. Other issues are speculated to be associated with abnormalities of the ICA such as cerebrovascular disease [8]. In the case reported, the headaches were thought to be associated with the optic neuropathy, as no intracranial aneurysm was identified.

Conclusion

The reported bilateral absence of the ICA was discovered incidentally on CTA images of a 52-year-old female patient who initially came in for a routine medical follow up for an unrelated issue. Such absences are not common, and oftentimes remain undetected and unexplored due to sufficient collateral blood flow. Origins of such bilateral absence are congenital, with lack of proper channel and vessel development during the fetal period.