Background Caudal epidural analgesia is the most common regional anesthetic performed in infants. Dural puncture, the most common serious complication, is inversely proportional to age. Measuring the distance from the sacrococcygeal membrane to the dural sac may prevent dural puncture. This study measures the sacrococcygeal membrane to dural sac distance using ultrasound imaging to determine feasibility of imaging and obtaining measurements.

Methods Sacral ultrasound imaging of 40 preterm neonates was obtained in left lateral decubitus, a typical position for caudal blockade. No punctures were made. The sacrococcygeal membrane and termination of the dural sac were visualized, and the distance measured. The spinal levels of the conus medullaris and dural sac termination were recorded.

Results 20 males and 20 females former preterm neonates with an average weight (SD; range) of 1740 (290; 860–2350) g and average age (SD; range) of 35.0 (1.35; 32.2–39) weeks gestational age at the time of imaging. The average sacrococcygeal membrane to distal dural sac distance (SD; range) was 17.4 (3.1; 10.6–26.3) mm. Overall, the weights correlated positively with the distance but the coefficient of variation was large at 23%. The conus medularis terminated below the L3 level and dural sac below the S3 level in 20% and 10% of subjects respectively with hip flexion.

Conclusion Ultrasound can be used to measure the sacrococcygeal membrane to dura distance in preterm neonates prior to needle insertion when performing caudal block and demonstrates large variability. Ultrasound imaging may identify patients at risk for dural puncture. When ultrasound is not available, needle insertion less than 3 mm/kg beyond the puncture of the sacrococcygeal membrane should prevent dural contact in 99.9% of neonates.