Unilateral cervical and petrosal segment agenesis of the internal carotid artery with rete mirabile

Rare Segmental Agenesis of Internal Carotid Artery without Rete-Like Collaterals: A Case Report

Objective

Internal carotid artery (ICA) agenesis has been classified into six types: A-F. Type F demonstrates distal reconstitution of the ICA via anastomosis with distal branches of the external carotid artery. Herein, we report the ICA agenesis of type F without rete-like collaterals, which has not been previously reported.

Case Presentation

An 80-year-old woman presented with segmental agenesis of the right ICA accompanied by an unruptured intracranial aneurysm. Stent-assisted coil embolization was successfully performed. Digital subtraction angiography showed segmental agenesis of the right ICA from the cervical to the ascending foramen lacerum segment, which was preoperatively supplied with collateral blood flow by a dilated right accessory meningeal artery (AMA) anastomosed with the inferolateral trunk (ILT)-posteromedial branch. Based on the segmental concept, the case was diagnosed with segment 7 (horizontal intracavernous portion until ICA branches off the ILT) agenesis, which may have resulted in secondary regression of the ICA proximal to segment 7. According to the ICA agenesis classification, this was of type F because the case showed collateral flow to the distal ICA via transcranial anastomoses from the AMA without carotid rete-like collaterals.

Conclusion

These findings suggest that the carotid rete-like collaterals did not form because the AMA was first developed during embryonic development.

Composition of encephalic arteries and origin of the basilar artery are different between vertebrates

Intracranial arterial anatomy is lacking for most mammalian and non-mammalian model species, especially concerning the origin of the basilar artery (BA). Enhancing the knowledge of this anatomy can improve animal models and help understanding anatomical variations in humans. We have studied encephalic arteries in three different species of birds and eight different species of mammals using formalin-fixed brains injected with arterial red latex. Our results and literature analysis indicate that, for all vertebrates, the internal carotid artery (ICA) supplies the brain and divides into two branches: a cranial and a caudal branch. The difference between vertebrates lies in the caudal branch of the ICA. For non-mammalian, the caudal branch is the origin of the BA, and the vertebral artery (VA) is not involved in brain supply. For mammals, the VA supplies encephalic arteries in two different ways. In the first type of organization, mostly found in ungulates, the carotid rete mirabile supplies the encephalic arteries, the caudal branch is the origin of the BA, and the VA is indirectly involved in carotid rete mirabile blood supply. The second type of encephalic artery organization for mammals is the same as in humans. The caudal branch of the ICA serves as the posterior communicating artery, and the BA originates from both VAs. We believe that knowledge of comparative anatomy of encephalic arteries contributes to a better understanding of animal models applicable to surgical or radiological techniques. It improves the understanding of rare encephalic variations that may be present in humans.

Revised concept of rete-like collateral formation: Rete mirabile does not exist in humans

The carotid rete is a physiological network between the external and internal carotid arteries (ICA) in lower vertebrates. However, true carotid rete does not exist in humans. This review aimed to contrast the physiological function of human "rete-like collaterals" with that of lower vertebrate "rete mirabile". An explanation for the development of rete-like collaterals in human intracranial arteries was also discussed. The rete mirabile (carotid, vertebral, spinal, and thoracic) in lower vertebrates has a specific physiological role and does not form vasculature for the same purpose in humans. Therefore, the term "rete mirabile" should not be used for cases reported in humans. Instead, "rete-like collaterals" is preferred. In the literature, rete-like or arterial anastomosis was observed in the ICA cavernous portion and the intradural arteries. Based on the hypothesis of the segmental concept, it applies to the ICA and intracranial arteries. Whether in the ICA, middle cerebral artery, posterior cerebral artery, or posterior inferior cerebellar artery, the segmental concept is the same and should be considered to have formed secondary collaterals after segmental regress or dysgenesis of affected arteries. Summarily, the significance of this review lies in its reevaluation of vascular structures previously described as "carotid rete" in humans to a true and preferred term, "rete-like collaterals". It also provides insights into the historical context and potential genetic factors associated with the formation of arteries in humans, contributing to a better understanding of human vascular anatomy.

Proposed new classification for internal carotid artery segmental agenesis based on embryologic and angiographic correlation

BACKGROUND AND PURPOSE

Internal carotid artery (ICA) agenesis is a rare anatomical variant that can involve different segments of the ICA. Although many authors proposed their own classifications of this variant basing on different criteria, none of these allows to include all the cases described in the literature. The aim of the authors is to propose a new classification that allows to include all cases of ICA agenesis described until now.

MATERIALS AND METHODS

The study is based on the review of all the cases of ICA agenesis described in the literature and of the classifications already proposed. After the analysis of these cases, we looked for the limits of each classification to elaborate a new more complete one.

RESULTS

We found 228 cases of ICA agenesis. Among them 40 were not includible in the existing classifications. For this reason, we proposed a new classification based on the type of flow compensation and on the embryological events that determine the different variants. The flow is compensated in Type I by the Circle of Willis; in Type II by the non-regression of an embryonic artery; in Type III by the presence of an arterio-arterial anastomosis and in Type IV by external-internal carotid arteries anastomoses.

CONCLUSION

After the literature review, we proposed a new comprehensive classification based on the detailed analysis of arterial embryology. Even if all the embryological details that determine this complex variant are not completely understood yet, this classification allows to include all the cases described in literature.

Differentiation Between Internal Carotid Artery Hypoplasia and Acquired Narrowing by Neurovascular Ultrasound: Case Series and Literature Review

Uniformly narrowed internal carotid artery (ICA) without proximal steno-occlusion or parietal anomalies is often subject to misdiagnosis due to lack of awareness. We combined our experiences of 4 cases with 29 previously published cases to form a retrospective series including 18 cases of ICA hypoplasia and 15 cases of ICA acquired narrowing. The ultrasonic manifestations of ICA acquired narrowing and ICA hypoplasia are extremely similar, but narrowed ICA without intracranial occlusion or bottle-neck-sign highly indicates ICA hypoplasia, whereas moyamoya vessels favor ICA acquired narrowing, thus promoting the understanding of and discriminability between the two on neurovascular ultrasound.

Hypoplastic internal carotid artery ending as an ophthalmic artery with multiple cerebral aneurysms, fenestrated Acom and triple A2

A rare case of a hypoplastic internal carotid artery (ICA) terminating in the ophthalmic artery with multiple intracranial saccular aneurysms in the contralateral ICA, anterior communicating artery fenestration and triple A2 was identified. The aetiology and pathogenesis of ICA hypoplasia are subjected to certain hypotheses. Developing several collaterals to preserve the blood supply of the ipsilateral cerebral hemisphere could result in aneurysm formation due to flow overload on the contralateral vasculature, but it could also result in hemicranial hypoplasia, cerebral atrophy and deep watershed infarcts, as in our case.