MAGNETIC-RESONANCE-IMAGING OF CEREBROSPINAL-FLUID VOLUME AND THE INFLUENCE OF BODY HABITUS AND ABDOMINAL PRESSURE

Background: Although the cerebrospinal fluid (CSF) is the pathway of a nesthetic delivery and the diluent for neuraxially administered drugs, little is known about its volume, including variability among individ uals, longitudinal distribution, or influence of body habitus, Models made to investigate subarachnoid anesthetic distribution lack valid di mensions, CSF volume was measured in volunteers, and the effect of obe sity and abdominal compression on CSF volume was evaluated using magne tic resonance imaging. Methods: Low thoracic and lumbosacral axial mag netic resonance images of 25 healthy volunteers were obtained at 8-mm intervals by fast spin-echo sequence, which highlights CSF. A repeat i mage series was performed in 15 subjects during external abdominal com pression, In two subjects, images were obtained without compression fo r the entire vertebral column. Dural sac and spinal cord areas were de termined in a blinded fashion for each image using video/digital analy sis, Area of the sac minus area of the cord constituted area of CSF an d roots (''CSF/root''); this area multiplied by 8 mm resulted in CSF/r oot volume per section. Results: There is great interindividual variab ility in CSF/root volume. from the T11-T12 disc to the sacral terminus of the dural sac, the mean volume for all subjects is 49.9 +/- 12.1 m l (mean +/- SD; range 28.0-81.1 ml). This volume was significantly les s in relatively obese subjects (42.9 +/- 9.5 ml) than in nonobese subj ects (53.5 +/- 12.9 ml). Abdominal compression decreased CSF/root volu me by 3.6 +/- 3.2 ml. Sections through intervertebral foramina showed the biggest decrease with abdominal compression, with a lesser change in sections with veins and no change in the absence of these anatomic features. Total vertebral CSF/root volume in two subjects was 94.84 an d 120.01 ml, respectively.Conclusions: CSF volume is widely variable b etween individuals, The decreased CSF volume that results from increas ed abdominal pressure, such as with obesity or pregnancy, may produce more extensive neuraxial blockade through diminished dilution of anest hetic, The mechanism by which increased abdominal pressure decreases C SF volume is probably inward movement of soft tissue in the interverte bral foramen, which displaces CSF.