Incidental finding of a proatlantal intersegmental artery. A case report.

We describe the case of a 70-year-old woman who had a left proatlantal intersegmental artery disclosed during carotid artery angiography with the findings of carotid-basilar anastomosis which originated from the petrous segment of the ICA and was filling the horizontal segment of the vertebral artery. Awareness of this vascular variation can prevent morbidity and mortality associated with carotid ligation or carotid endarterectomy, especially when an incomplete circle of Willis is present.