Rotational changes in the morphology of the vertebral artery at a common site of artery dissection.

OBJECTIVE: Given that vertebral artery dissection and transient vertebrobasilar insufficiency occurs commonly at the C1-C2 junction, the objective was to study, in vivo, the normal rotational anatomy at C1-C2 using magnetic resonance angiography (MRA) with 3-dimensional (3D) reconstructions and to correlate these reconstructions with our experience of dissection location. METHODS: 3D phase-contrast MRAs were obtained from the foramen magnum to C3 in 4 normal volunteers in neutral and rotated (45 degrees) positions. The magnitude images were used as source images for 3D reconstruction. The images from only 1 of the volunteers were completely motion free, and these were subjected to arterial length calculations using image analysis wire frame outline of the vessel. All angiograms of vertebral artery dissection obtained at our institution from 1993 to 1997 were also reviewed. RESULTS: 3D reconstructions document elongation (approximately 7% in quantitatively analyzed case) and slight narrowing of the artery contralateral to the direction of head rotation from C2 to the dural entry point. The artery turns most sharply and is subject to the anterior force of the rotating C1 vertebra as it exits the C1 foramen. Of 14 cases of vertebral artery dissection, 50% occurred between the transverse foramen and the posterior lamina of C1. CONCLUSIONS: With head rotation, the contralateral vertebral artery undergoes the greatest anatomical distortion as it exits the C1 transverse foramen. This increased stress may account for the higher frequency of dissections at this location.