Dynamics of the junction between the medulla and the cervical spinal cord: an in vivo study in the sagittal plane by magnetic resonance imaging.

Sagittal sections of the brain-stem made by MRI reveal differences in the angle formed by the medulla and the cord. In order to study the normal mobility of this region of the CNS during flexion and extension of the head, sagittal MRI studies were made in the sagittal plane in 18 young volunteers. The volunteers were in dorsal decubitus with the cervical spine first flexed and then extended, with the movement localized to the cranio-cervical junction as far as possible. T1-weighted sequences were used, with body coils in 16 cases and surface coils in two. Measurements were related to global cranio-cervical range of movement, movement at the cranio-cervical junction and spino-medullary movement. Variations in the depth of the free space in front of the medulla, pons and spinal cord during movement were also noted. We also checked for downward shift of the lower part of the 4th ventricle and modification of the shape of the ventricle during flexion-extension. The global range of cranio-cervical movement was between 31 and 100 degrees (average 63 degrees). The range between the cranium and C1C2 was 4 to 39 degrees (average 19 degrees) and the spino-medullary range was from 1 to 32 degrees (average 14 degrees). During flexion, the free space narrowed in front of the pons 11 times, in front of the medulla 14 times and in front of the cervical cord 11 times. There was a downward shift of the lower part of the 4th ventricle during flexion in 4 cases but no change in shape was noted. Though this study is open to criticism from several aspects, it may be concluded that variations of the spino-medullary angle in the sagittal plane during flexion-extension do occur, that they are closely correlated with movements at the cranio-cervical junction, moves forward during flexion.