Posterior inferior cerebellar artery with an extradural origin from the V 3 segment: higher incidence on the nondominant vertebral artery.

OBJECTIVE The posterior inferior cerebellar artery (PICA) and the vertebral artery (VA) often exhibit anatomical variations at the craniovertebral junction (CVJ). An example of this is the PICA originating extradurally from the V3 segment of the VA. To date, some cadaveric investigations have been reported, but the incidence and relationship of this variation to the VA and the atlas as observed on clinical imaging have not been discussed. This study evaluated the prevalence of PICAs originating from the V3 on CT scanning. Other variations of the atlas and VA were also analyzed. METHODS CT images from a series of 153 patients who underwent 3D CT angiography (CTA) were analyzed, and variations of the PICA, VA, and atlas were investigated. RESULTS A total of 142 patients (284 sides) were analyzed; 11 patients (7.2%) were excluded due to poor image quality. The most common VA variation was the PICA originating from V3 (9.5% of 284 sides), which was more frequently observed on the nondominant VA than the dominant VA (22.5% vs 6.25%, p = 0.0005). A VA with a PICA end was identified in 4 sides (1.4%), which is the same incidence as observed in the persistent first intersegmental VA (1.4%). VA fenestration was only found in 1 side (0.35%). Regarding the atlas, ponticulus posticus was observed in 24 sides (8.5%). There was no relationship between the incidence of ponticulus posticus and the variations of the VA. CONCLUSIONS A PICA originating from V3 was the most common VA variation at the CVJ and was more common on the nondominant VA. Three-dimensional CTA is useful for the evaluation of this variance. Surgeons should be mindful of this variation during operations.