The proatlantal intersegmental artery: a case report and review of the literature.

A case of the proatlantal intersegmental artery (PIA) arising from the external carotid artery (ECA) is presented. The PIA gave rise to the occipital artery, a finding which may support the hypothesis that the distal part of the occipital artery is derived from it. The possibility that the PIA of ECA origin might be a hypertrophied collateral occipital artery is also suggested. The importance of the anteropasterior view in differentiating the PIA of ECA origin from the first cervical intersegmental artery is stressed.