OBJECTIVE: To present two cases of Internal Carotid Artery (ICA) agenesis and conduct a systematic review to assess for associations with other anomalies and intracranial aneurysms.
METHODS: We performed a retrospective review of published cases of patients with ICA agenesis with intercavernous anastomosis in MEDLINE database on August 2022 using search terms \"internal carotid artery\", \"agenesis\" and \"transcavernous anastomosis\". We also included two cases of ICA agenesis with type D collateral that we encountered.
RESULTS: Total of 46 studies that included 48 patients and two of our cases resulted in 50 patients. Only 70% of studies reported the location of a collateral vessel of which more than two-thirds were on the floor of sella. More than half of the vessels connected cavernous segments of ICA. A1 segment ipsilateral to the side of ICA agenesis was absent in most of the cases but was not true for all cases. Aneurysm was seen in more than one-quarter of the patients. It can also mimic microadenoma as in prior reported cases as well as in one of our cases.
CONCLUSIONS: ICA agenesis with type D collateral is a rare anomaly but clinically relevant due to the increased risk of an aneurysm or mimic microadenoma or false alarm for occlusion of ICA but knowledge of this rare variant can help in better management of these patients.

Agenesis of the internal carotid artery (ICA) is a rare congenital vascular disorder of the cerebral circulation. CT scan of the skull base disclosing complete absence of the bony carotid canal helps to differentiate an agenesis from aplasia or hypoplasia. Although most of the patients remain asymptomatic (thanks to the sufficient collateral circulation provided by the circle of Willis) cerebral infarcts, transient ischemic attacks or intracranial aneurysms have been rarely described in association with agenesis of the ICA. Most often, the vascular territory of the involved ICA is supplied by the contralateral carotid artery and from the vertebrobasilar circulation through the anterior and posterior communicating arteries, respectively. However, collateral supply can also be provided thanks to a transcavernous anastomosis, an aberrant vascular communication between the cavernous portions of the ICAs coursing through the sella turcica. We report here the case of a 55-year-old man with right carotid agenesis and associated transcavernous anastomosis revealed by transient ischemic attack. Embryogenesis, imaging findings, possibilities of collateral circulation and potential complications have also been discussed.

Agenesis of the internal carotid artery (ICA) is a rare congenital anomaly. Recently, several cases of ICA agenesis have been associated with cerebral aneurysms. We present the seventh case of ICA agenesis with transcavernous anastomosis associated with cerebral aneurysms. A 47-year-old man presented with transient numbness of his left hand. Magnetic resonance angiography indicated the presence of anterior communicating artery (ACoA) aneurysms. Digital subtraction angiography revealed two ACoA aneurysms, absence of the left ICA, and an anomalous collateral vessel connecting the cavernous portions of both internal carotid arteries, which was considered to be a transcavernous anastomosis. Head bone window computed tomography with contrast medium revealed the absence of the petrous carotid canal on the left suggesting the diagnosis of left ICA agenesis. The ACoA aneurysms were clipped successfully, and the postoperative course was uneventful.