ABSTRACT
A 67-year-old male presented with sudden headache and decreased consciousness, diagnosed as spontaneous subarachnoid hemorrhage (SAH). Imaging revealed an anterior communicating artery (ACoA) aneurysm and congenital absence of the left internal carotid artery (ICA), confirmed by the absence of the carotid canal. The aneurysm was surgically clipped. Postoperative angiography demonstrated that the left anterior and middle cerebral arteries were perfused via collateral flow from the posterior circulation. The patient made a full recovery. Congenital ICA absence is rare but significantly alters cerebral hemodynamics, especially within the circle of Willis, predisposing patients to aneurysm formation. This case underlines the importance of vascular imaging in patients with SAH and highlights the need for urgent diagnosis and intervention in the presence of congenital vascular anomalies.
Congenital internal carotid artery (ICA) anomalies, such as agenesis or hypoplasia, are extremely uncommon, occurring in less than 0.01% of the population.These anomalies are often clinically silent because of compensatory collateral flow via the circle of Willis but can predispose patients to ischemic events, hemiplegia, and subarachnoid hemorrhage (SAH).1-2 The uniqueness of this case lies in the rare combination of congenital ICA absence with an ACoA aneurysm that subsequently ruptured. This association represents an uncommon but clinically significant vascular presentation that requires special attention during diagnosis and management. This case illustrates a rare presentation involving a ruptured anterior communicating artery (ACoA) aneurysm in the setting of congenital absence of the left internal carotid artery (ICA), and highlights the importance of comprehensive vascular imaging for the diagnosis and management of such uncommon cerebrovascular anomalies.
Case Report
Patient information
A 67-year-old man visited the emergency department because he experienced a severe headache followed by loss of consciousness and neck stiffness. He lacked any previous medical history of hypertension or smoking or cerebrovascular disease.
On examination, the patient had decreased consciousness with a Glasgow Coma Scale score of 11 and exhibited neck stiffness, suggestive of meningeal irritation. He also presented with right-sided hemiparesis.
Diagnostic assessment
Computed tomography (CT) revealed diffuse SAH (Figure 1A). Magnetic resonance angiography (MRA) showed a saccular aneurysm at the ACoA and the congenital absence of the left ICA.
- Patient imaging A) Cranial computed tomography indicated Fisher Grade 3 subarachoid hemorrhage. B-C-D) CT-angiography and MR-angiography showed ACoA aneurysm and absence of left internal caroid artery. E) Postoperative cranial CT. ACoA - Anterior communicating artery
- Timeline and clinical events.
The congenital nature of ICA absence was established based on 2 definitive radiological findings: 1) complete absence of the left carotid canal on high-resolution temporal bone CT, and 2) absence of collateral vessel formation patterns typically seen in chronic occlusion. These findings conclusively differentiated congenital absence from acquired occlusion, which would show a normal but empty carotid canal and different collateralization patterns.
Postoperative digital subtraction angiography (DSA) demonstrated that the left internal carotid artery (ICA) was congenitally absent, and that both the left anterior cerebral artery (ACA) and middle cerebral artery (MCA) received their blood supply entirely through collateral flow originating from the basilar artery within the posterior circulation (Figure 2).
- DSA (Digital substraction anjiography) examination of the patient A) DSA image demonstrated normally positioned right common carotid, ICA and ECA. B-C) DSA image demonstrated left CCA and ECA with absence of ICA. D) DSA image demonstrated clipped aneurysm with no remnant. Left ACA and MCA did not receive blood flow from the right side. E-F) Left ACA and MCA were totally supplied by posterior circulation. DSA - Digital substraction anjiography, ICA - Internal carotid artery, ECA - External carotid artery, CCA - Common carotid artery, ACA - Anterior cerebral artery, MCA - Middle cerebral artery
Therapeutic intervention
The patient received an emergency right frontotemporal craniotomy procedure to clip the ACoA aneurysm (Figure 1B–E). There were no intraoperative complications.
Follow-up and outcomes
The patient received full neurological recovery before being discharged after one week. The follow-up clinical and radiological assessment revealed no remaining aneurysm and normal vascular flow. The patient demonstrated complete neurological function during the one-month follow-up appointment while imaging tests revealed no aneurysm or ischemic complications.
Discussion
The ICA shows three rare congenital anomalies which include agenesis and aplasia and hypoplasia. Agenesis describes the complete absence of ICA structures together with their bony canal while aplasia and hypoplasia describe partial or underdeveloped ICA structures.3-4 These conditions become apparent through incidental findings or when complications like aneurysms or ischemia occur.
Our patient had complete agenesis of the left ICA, confirmed by the absence of the carotid canal on imaging. Naeini et al5 identified three patterns of collateral circulation. The most common involves the ACA on the affected side being supplied by the contralateral ICA, with the MCA supplied by the posterior circulation. The second pattern, observed in our case, showed that both the ACA and MCA on the affected side were supplied by the basilar artery through the posterior communicating artery. The rarest pattern involves collateral circulation from the external carotid system.
Congenital absence of the ICA is typically asymptomatic due to collateral circulation from the circle of Willis but can occasionally be associated with subarachnoid hemorrhage. Patients with this condition have an increased incidence of intracranial aneurysms (25–35%). Agenesis, aplasia, or hypoplasia of the ICA can cause hemodynamic stress, leading to abnormalities in the circle of Willis.6-7
The recent classifications of anterior communicating artery aneurysms have enhanced our knowledge of their pathogenesis and treatment methods. Zhang et al8 proposed a classification system based on the aneurysm projection relative to the axis connecting A1 origin and the base of the aneurysm, categorizing them into straight, superior, inferior, anterior, and posterior projections. In our case, the aneurysm had a superior projection, which according to their findings, is associated with a higher rupture risk despite typically smaller size. This classification system provides valuable insights for both endovascular and surgical approaches to ACoA aneurysms, particularly in cases with associated vascular anomalies like congenital ICA absence.
Conclusion
The absence of the ICA at birth increases the likelihood of developing intracranial aneurysms and subarachnoid hemorrhage in patients. The case highlights the need for complete vascular imaging in SAH patients who may have congenital vascular anomalies. The patient requires prompt detection and appropriate intervention to achieve positive results. Patients should undergo complete imaging of the circle of Willis.
Acknowledgment
The authors would like to thank ENAGO (www.enago.com.tr) for professional English language editing support.
Footnotes
Disclosure. The authors declare no conflicting interests, support or funding from any drug company.
- Received December 8, 2024.
- Accepted May 19, 2025.
- Copyright: © Neurosciences
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