Case Report: A case of atypical persistent proatlantal intersegmental artery

The persistent proatlantal intersegmental artery (PPIA) is a rare embryonic anastomosis between the carotid and vertebrobasilar systems. We report an unusual case of PPIA in a 67-year-old male presenting with recurrent dizziness, which cannot be classified into the classic Lasjaunias types. Imaging revealed absence of the left vertebral artery, a right vertebral artery V4 segment aneurysm, and an anomalous artery originating from the left external carotid artery that gave off the occipital artery and ascended directly through the occipitoatlantal space into the foramen magnum without entering any cervical transverse foramen, ultimately joining the right vertebral artery to form the basilar artery. This variant exhibits a type II origin with a type I course, representing a functional composite artery formed by persistent remnants of both type I and type II proatlantal intersegmental artery (PIA) in the setting of ipsilateral vertebral artery agenesis. Our findings support the embryological concept that the occipital artery derives from PIA components and expand the morphological spectrum of persistent proatlantal arteries. Recognition of such "mixed-type" PPIA is clinically crucial as it may serve as the dominant or sole posterior circulation supply and constitutes a potential dangerous anastomosis; preoperative identification via CTA or MRA is essential for surgical and interventional planning to avoid catastrophic complications.