Changing body position alters the location of the spinal cord within the vertebral canal: a magnetic resonance imaging study

Background. The influence of changes in body position relevant to neuraxial blockade on the location of the spinal cord and related neural structures has not been fully quantified. Our aim was to determine the changes, if any, that occur in the location of the spinal cord tip [equivalent to the tip of the conus medullaris (CM)] and nerve roots when an individual moves from the supine to the left lateral position with knees and hips flexed. Methods. We used magnetic resonance imaging to determine movement of the spinal cord tip and associated structures in 30 adult volunteers. Results. The tip shifted both anteriorly [average 6.3 mm, standard deviation (SD) 2.15 mm; P < 0.001] and laterally towards the dependent side (average 1.63 mm, SD 1.19 mm; P < 0.001). Although we observed anterior shift in all 30 volunteers, lateral movement did not occur in seven. Movement along the cranio-caudal axis was not statistically significant. Conclusions. Both the CM and associated nerve roots shift consistently and significantly anteriorly when moving from the supine to the lateral position with knees and hips flexed, which may provide a greater margin of safety during neuraxial blockade than might be predicted. However, the absence of significant cranial movement of the CM along the cranio-caudal axis still makes the spinal cord vulnerable to injury during lumbar neuraxial blockade.