MAGNETIC-RESONANCE ANGIOGRAPHIC ANALYSIS OF ATLANTOAXIAL ROTATION - ANATOMIC BASES OF COMPRESSION OF THE VERTEBRAL ARTERIES

The aim of this study was to identify the functional anatomic factors involved in the maintenance or disturbance of flow in the vertebral aa . during atlanto-axial rotation. Fourteen healthy volunteers were stud ied by magnetic resonance angiography (MRA) by a three-dimensional seq uence in phase contrast centered on the vertebral aa, at the level of the cranio-cervical junction before and after left rotation of the hea d. A decrease in the signal intensity of the arterial flow was sought for. The results were compared to the posterolateral development of th e loop of the vertebral a. in its atlanto-axial segment in neutral pos ition, and to the measurement of the angular opening between the atlas and axis in dynamic position. Seven subjects also had a three-dimensi onal CT study (3D CT) of the bony relations of C1 and C2 after rotatio n. In 4 subjects a disturbance of flow in the right vertebral a. was o bserved in the transverse foramen of C2. This occurred when two factor s were combined: an under-developed atlanto-axial arterial loop and a C1-C2 angle exceeding 35 degrees in maximal rotation. In the other sub jects a well-developed arterial loop and/or a C1-C2 angle of less than 35 degrees in maximal rotation were factors preserving the arterial f low. The risk factor associated with the C1-C2 angle seemed correlated in 3D CT with loss of the usual asymmetric character of rotation. A c linical application is reported with a case combining chronic rotation al dysfunction of the cranio-cervical junction as shown by 3D CT and c omplete compression of the vertebral a. in MRA, confirmed by conventio nal angiography. A knowledge of this physiopathologic mechanism allows clinical detection and evaluation of the risk of any effect of pathol ogy of the cranio-cervical junction on the vertebral a.