Persistent primitive trigeminal artery with carotid-dependent posterior circulation: An incidental but clinically relevant discovery.

The persistent primitive trigeminal artery (PPTA) represents the most common embryonic carotid-basilar anastomosis persisting into adulthood. Although rare, with a reported prevalence ranging from 0.061% to 0.6%, its presence may substantially modify posterior circulation anatomy and hemodynamics. Awareness of this vascular variant is essential for radiologists and neurosurgeons, particularly in patients with carotid artery disease or undergoing skull base surgery. We describe the incidental identification of a right-sided PPTA in a 69-year-old woman who underwent computed tomography angiography (CTA) for suspected internal carotid artery (ICA) stenosis. The PPTA originated from the petrous-cavernous junction of the right ICA, exhibited the characteristic "tau sign," and supplied the distal basilar artery, posterior cerebral arteries, and cerebellar arteries. The proximal basilar artery and the left posterior communicating artery were hypoplastic, while both vertebral arteries terminated as posterior inferior cerebellar arteries. This vascular configuration was consistent with a mixed Saltzman type I-II pattern and a medial subtype according to the Salas classification. No aneurysms or arteriovenous malformations were detected. Although frequently incidental, PPTA may critically alter posterior circulation anatomy and flow dependence. Accurate identification and classification are essential to avoid diagnostic pitfalls and to guide the management of cerebrovascular disease, endovascular procedures, and skull base surgery.