Correlation of thecal sac cross sectional area to total volume

Spinal anesthesia (SA) has been increasingly utilized in lumbar surgery due to its various advantages over general anesthesia (GA), however failure of the first dose requiring intraoperative conversion to GA occurs in as many as 3.6% of SA patients. Some studies have reported that a larger thecal sac volume may dilute the anesthetic and play a role in first dose failure. Unfortunately, easy determination of thecal sac volume has not been reported in the literature. Thus, we sought to determine whether cross-sectional area obtained from MRI accurately predicts the volume of the thecal sac.

We conducted a retrospective review of 80 patients who underwent lumbar surgery with spinal anesthesia. T1 and T2-weighted MRI sequences were used to measure thecal sac area at each level between L1-S1. The volume of the thecal sac was calculated using Horos^TM. A statistical model was derived relating the area at each level to the thecal sac volume. Of the 80 patients, 20% were reserved and utilized to test the accuracy of the statistical model.

The area of the thecal sac positively correlated with volume at each lumbar level. The area of the thecal sac at the L4-L5 level most accurately represented total thecal sac volume (R² = 0.588, RMSE = 2.76).

Cross-sectional area of the L4-L5 spinal level obtained from MRI sequences may be utilized as a proxy for thecal sac volume.