A Silent Threat: Internal Carotid Artery Hypoplasia and Its Role in Basilar Artery Aneurysm Formation—A Case Study

<b>Background and Clinical Significance:</b> Hypoplasia of the internal carotid artery (ICA) is a rare vascular anomaly, with an estimated incidence of less than 0.01%. This condition can result in reduced blood flow to the anterior circulation, often compensated by collateral circulation. Radiological imaging, particularly computed tomography angiography (CTA), digital subtraction angiography (DSA), magnetic resonance angiography (MRA), and ultrasound, plays a crucial role in diagnosing this condition, revealing structural abnormalities in the arterial system. <b>Case Presentation:</b> This case is about a 75-year-old woman who lived her entire life unaware of ICAH until a basilar artery aneurysm ruptured, leading to a large, centrally localized SAH. Further diagnostic workup, including CTA and DSA, confirmed left ICA hypoplasia, with the artery terminating as the ophthalmic artery, along with a developmental anomaly of the left middle cerebral artery from posterior circulation territory. Due to the high risk associated with surgical and endovascular intervention, conservative management was pursued, and the patient's condition stabilized, though she continued to have significant neurological deficits. <b>Conclusions:</b> This case report supports the hypothesis that ICAH may be associated with aneurysm formation. This case demonstrates that if ICAH is not diagnosed early, it can lead to severe complications with permanent neurological deficits. Additionally, it highlights the critical importance of advanced imaging techniques, such as CTA and DSA, in diagnosing complex vascular conditions.