Differentiation of Aneurism from Infundibulum of the Posterior Communicating Artery

A ARTERIOGRAPHIC demonstration of aneurisms encountered in the lower portion of the C1 segment of the internal carotid artery, below the origin of the anterior choroidal artery and at, or near, the junction with the posterior communicating artery, has remained a vexatious problem because of the danger of interpreting a benign projection of contrast material as representing an aneurismal dilatation. The subject has received little attention although discussions with colleagues reveal an acute awareness of the problem gained, in part, at the expense of negative surgical explorations. The radiographic anatomy of this site has been discussed by Saltzman ~ in noting that the posterior communicating aI'tery is often dilated and funnel-shaped at its point of origin at the carotid siphon, an area which may be termed the infundibulum. Although the infundibulum can fill with contrast material from the carotid artery, the subsequent course of a posterior communicating artery with a small lumen may not be visualized because of failure to fill or because of the low density of contrast material in the small-caliber vessel. The resultant saccular projection of contrast material arising from the supraclinoid internal carotid artery may be readily confused with an aneurism. Five cases are presented in which the apparent aneurism was actually the infundibulum of the posterior communicating artery. The problems encountered in differentiating between the two alternatives are noted and suggestions are offered for establishing the correct diagnosis.