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Most distal anterior cerebral artery (ACA) aneurysms arise at the pericallosal-calloso-marginal (PC-CM) junction, which is usually located in the A3 segment of the ACA around the genu of the corpus callosum. PC-CM aneurysms are divided into 2 types according to their location: supracallosal and infracallosal. Infracallosal distal ACA aneurysms are defined as those located in the lower half of A3, thus making it more difficult to gain proximal control. In this study, we examined the microsurgical anatomy of the distal ACA region, focusing especially on the relationship between the pericallosal and callosomarginal arteries located in the lower half of the A3 (infracallosal) segment, and present the surgical strategy for dealing with distal ACA aneurysms. The microsurgical anatomy of the distal ACA region was examined in 22 adult cadaveric cerebral hemispheres after perfusion of the arteries and veins with colored silicone. The relationships of the infracallosal segment of the pericallosal to the callosomarginal was examined. The distance between the nasion and the site at which a parallel line directed along the long axis of the infracallosal pericallosal artery just proximal to the origin of the callosomarginal artery crosses the forehead (PC point) was also measured. Surgical approaches to distal ACA aneurysms were examined in stepwise dissections and applied to 2 clinical cases. The PC-CM junctions were located in the supracallosal and infracallosal segments of A3 in 36% and 55% of cases, respectively. In the infracallosal region, it is difficult to identify the proximal pericallosal artery and to establish proximal control of the vessel. The infracallosal part of the proximal pericallosal artery courses almost parallel to the frontal cranial base, and the PC point was 42.2 ± 15.9 mm from the nasion. These findings indicate that there is only a limited space to access an infracallosal distal ACA aneurysm below the PC point and establish proximal control by the anterior interhemispheric approach. When the approach is made above the PC point, an anterior callosotomy prior to final aneurysm dissection and clipping may be necessary to establish proximal control. The PC point is an important surgical landmark in planning the surgical strategy for infracallosal distal ACA aneurysms.
