Co-existence of persistent primitive trigeminal artery and another primitive carotid-basilar anastomosis associated with segmental agenesis of the ipsilateral internal carotid artery: a case report

Embryologically, the internal carotid artery (ICA) is comprised of seven segments, any of which may be absent. We presented a very rare case of a well-preserved persistent primitive trigeminal artery (PTA), serving as a collateral pathway to the distal internal carotid artery territory, associated with segmental agenesis of the ipsilateral internal carotid artery. A 47-year-old woman with no relevant previous medical history came to our hospital for a brain checkup. Magnetic resonance angiography revealed focal agenesis of the right ICA involving the cervical, ascending, and horizontal intrapetrous segments. The enlarged ipsilateral PTA mainly supplied the distal segments of the ICA. There was also the possibility of the co-existence of another primitive carotid-basilar anastomosis; however, we could not pursue a detailed evaluation. We report a rare case of a well-preserved PTA serving as a collateral pathway to the distal ICA territory associated with segmental agenesis of the ipsilateral ICA. Clinical and radiological follow-up are needed.