Bilateral persistence of the proatlantal intersegmental arteries in moyamoya syndrome: illustrative case.

backgroundThe persistent proatlantal intersegmental artery (PPIA) arises from the cervical internal carotid artery (ICA) and joins the vertebral artery near the foramen magnum. In moyamoya syndrome (MMS), such embryonic carotid-vertebrobasilar channels may influence collateralization and procedural strategy. The authors report bilateral PPIA identified concurrently with bilateral MMS. OBSERVATIONS: A 28-year-old woman with Down syndrome presented with sudden severe headache and unresponsiveness. CT showed bilateral intraventricular hemorrhage, a small left periatrial hemorrhage, and obstructive hydrocephalus; an external ventricular drain was placed. CT angiography demonstrated bilateral distal ICA and proximal middle cerebral artery/anterior cerebral artery narrowing concerning for MMS. Digital subtraction angiography revealed bilateral PPIA supplying the posterior circulation with extensive leptomeningeal and meningeal collateral vessels. A tortuous left posterior choroidal branch had no definite aneurysm or treatable lesion. The patient improved with CSF diversion and supportive care and was discharged on hospital day 19. Follow-up angiography confirmed advanced bilateral MMS and bilateral PPIA, and the choroidal finding was recharacterized as a vascular loop. LESSONS: Bilateral PPIA in MMS is rare and clinically consequential. Precise angiographic mapping is essential due to the PPIA's potential to reshape anterior-posterior collateralization, affect bypass targeting and competitive flow risk, and influence endovascular access and surveillance planning. https://thejns.org/doi/10.3171/CASE25974.