MRI-derived lumbosacral cerebrospinal fluid volume influences both the extent and persistence of spinal anesthesia: a prospective observational study

Abstract

Considerable interindividual variability exists in the spread and duration of spinal anesthesia despite standardized anesthetic techniques. Lumbosacral cerebrospinal fluid (CSF) volume has been proposed as an important anatomical determinant of neuraxial block characteristics. However, its influence on clinically relevant outcomes beyond peak sensory block level remains incompletely understood. This study aimed to evaluate the relationship between MRI-derived lumbosacral CSF volume and spinal anesthesia characteristics in patients undergoing elective lower extremity orthopedic surgery. Thirty-nine patients undergoing elective lower extremity orthopedic surgery under spinal anesthesia were prospectively evaluated. Lumbosacral CSF volume was measured using semi-automated segmentation of three-dimensional T2-weighted magnetic resonance imaging (MRI) datasets. Correlations between CSF volume and spinal block characteristics were assessed using Pearson or Spearman correlation analyses, as appropriate. Multivariable linear regression analysis was performed to identify independent predictors of peak sensory block level. The mean lumbosacral CSF volume was 12.60 ± 5.16 cm³. Lower CSF volumes were significantly associated with higher peak sensory block levels (ρ = -0.475, p = 0.002), longer sensory block duration (ρ = -0.455, p = 0.006), longer motor block duration (ρ = -0.445, p = 0.007), prolonged two-segment regression time (ρ = -0.343, p = 0.015), and higher sensory block levels at 5 min (ρ = -0.483, p = 0.004). Height demonstrated a moderate positive correlation with CSF volume (ρ = 0.529, p < 0.01). In multivariable regression analysis, lumbosacral CSF volume was identified as the only independent predictor of peak sensory block level (β = -0.563, p < 0.001), explaining 31.7% of the observed variability (R² = 0.317). MRI-derived lumbosacral CSF volume influences both the extent and persistence of spinal anesthesia. Lower CSF volumes were associated with greater cephalad sensory spread, prolonged sensory and motor blockade, and delayed regression. Furthermore, lumbosacral CSF volume emerged as the only independent predictor of peak sensory block level in multivariable analysis. These findings suggest that lumbosacral CSF volume is an important anatomical determinant of spinal anesthesia behavior and may contribute to future individualized neuraxial anesthetic planning.