Prepuncture Ultrasound-Measured Distance: An Accurate Reflection of Epidural Depth in Infants and Small Children

Background and Objectives: Epidural cannulation is technically difficult in children who have small anatomic structures. Ultrasound information regarding the distance of skin-to-ligament flavum may be useful, leading to an increase in success rate without dural puncture. This study was performed to assess whether ultrasound-measured, skin-to-ligament flavum distance would reflect the needle depth during epidural puncture in infants and children. Methods: The study compromised 180 children, aged 2 to 84 months, undergoing urologic surgery. After induction of anesthesia, ultrasound images of the longitudinal median and transverse views were acquired from L4-L5 in lateral decubitus position. Measured distance of skin-to-ligament flavum in each view was compared with the perpendicular skin-to-epidural depth, which was obtained from needle depth and angle by use of a trigonometric ratio equation. Additionally, we evaluated the ultrasound visibility of the ligament flavum and dura mater, number of puncture attempts, and complications. Results: The correlation coefficient between measured distance and perpendicular epidural depth was slightly higher in longitudinal median view (R² = 0.848) than in transverse view (R² = 0.788). The visibility of ligament flavum and dura mater was "good" in 91 and 170 of 180 patients, respectively, and "sufficient" in the remaining subjects. The epidural space was located on first puncture attempt in 179 of 180 cases (99.4%). No incidents of dural puncture or bloody tap occurred. Conclusions: Ultrasound, particularly in the longitudinal median view, provides accurate information on the distance of skin-to-ligament flavum in infants and children. With reference to the measured distance, epidural puncture can be performed with minimal risk of dural puncture (upper limit of 95% CI = 1.67%).