Embryologic development of the basilar artery occurs along two axis systems: longitudinal fusion and axial fusion. Longitudinal fusion consists of midline fusion of paired ventral arteries and reflects the simplified pattern of arterial anatomy found in the spinal cord. Axial fusion consists of fusion of the distal basilar artery, which arises from the caudal division of the internal carotid artery, to the midbasilar agenesis to the posterior inferior cerebellar artery termination of the vertebral arteries. Persistent longitudinal nonfusion (or complete duplication) of the basilar artery is very rare, and persistent axial nonfusion is even rarer.The trigeminal artery appears embryologically at the 4-mm stage and involutes at the 7–12-mm stage. It arises from the basilar artery, unlike the posterior communicating artery, another of the carotid-basilar anastomosis, which arises from the posterior division of the internal carotid artery . A persistent trigeminal artery is estimated to occur in 0.1–0.2% of the population and, in most cases, is an incidental finding.
Case by Dr MGK Murthy, Sr Consultant Radiologist