Background and Aims Measuring the distance from the skin to posterior dura (SPD) before lumbar epidural placement may prevent accidental dural puncture.¹ This study compares the accuracy of measurement of SPD using ultrasound versus body mass index (BMI) in obstetric patients.

Methods An anonymous retrospective data of pre-procedural ultrasound and intraprocedural Tuohy needle measurements of SPD were collected from anaesthetic charts of thirty parturient with varying gestational stages, maternal age and ethnicity, all had received lumbar epidurals for labour analgesia using 18G Tuohy needle and loss of resistance to saline. Then, using the documented patient’s BMI the SPD was calculated using the general regression equation (Y = 2.52 + 0.11 BMI).² The accuracy of measurement of SPD for the two groups: Ultrasound SPD versus BMI SPD compared to the Touhy needle SPD was analysed using Wilcoxon signed-rank test. Ethics approval is not required as per NHS Greater Glasgow and Clyde policy.

Results Ultrasound measurement of SPD was equal to Touhy needle in 23/30 (76.7%), while overestimated in 1/30 (3.3%) by 0.2 cm and underestimated in 6/30 (20%) by a mean of 0.5 cm. BMI measurement of SPD was equal to Touhy needle in 0/30 (0%), while overestimated in 25/30 (83.3%) by a mean of 1.14 cm and underestimated in 5/30 (16.7%) by a mean of 0.29 cm. Ultrasound was more accurate than BMI in the measurement of SPD based on Wilcoxon signed-rank test (non-parametric) with a p-value <0.00001.

Conclusions Compared to BMI, ultrasound offers more accurate measurement of SPD.