Title: Predicting the the safe depth of the caudal epidural space in caudal block for children using magnetic resonance imaging

Background :Caudal block is often performed through the sacral hiatus to provide pain control to children undergoing urologic surgery or lower abdominal surgery. In order to prevent unintended dural puncture during the caudal block, the needle must not be advanced further after penetrating the sacrococcygeal ligament. However, the penetration of the sacrococcygeal ligament may not always be identified, and the dural sac of small children is located at a lower level than that of adults. The aim of this study was to establish simple guidelines for predicting the safe depth of the caudal epidural space in children based on demographic data.
Materials and methods: The study's participants were children under 12 years old who had undergone lumbar-sacral magnetic resonance imaging. The T2 sagittal image of showing the best view of the sacrococcygeal membrane and the dural sac was chosen. We measured the distances using the centricity web PACS viewer. Distance ´a´ represented the length from the upper margin of the sacrococcygeal ligament to the dural sac, distance ´b´ was the length of the sacrococcygeal ligament, distance ´c´ represented the maximum depth of the caudal epidural space including the sacrococcygeal ligament, and distance ´d' was defined as the length from the central point of the sacrococcygeal ligament to the skin. fig 1
Results and discussion: A total of 141 magnetic resonance images were analyzed. The study's participants were 20.0 months(1.00-154.00) when MRI imaging was taken. Their median weight and height were 11.5kg (3.40-60.00) and 82.7cm(50.00-159.50). The median BSA was 0.49 m2(0.21-1.60). The age, height, weight, and BSA were highly correlated. Distances a, b, and c showed a high correlation with the child's age, weight, height and BSA. Distance a showed the highest R2correlation with the BSA. Based on those results, a simple formula was developed: distance a (mm) = 25 x BSA. fig2 When this formula was applied to the children in this study, the proportion of the needles predicted to be safely located in the caudal canal was 100% (95% CI 100-100).
Conclusion(s): The simple formula, 25 x BSA, can be recommended to calculate the safe depth of the caudal epidural space in order to prevent unintended dural puncture after puncture of the sacrococcygeal ligament during caudal block in children. However, further clinical studies based on this formula are needed.