H. Ahmad1, D. R. Halleran1, A. Akers1, V. Alexander1, M. Levitt1, R. J. Wood1 1Nationwide Childrens Hospital,Center For Colorectal And Pelvic Reconstruction,Columbus, OH, USA
Introduction: The sacral ratio (SR) has been used as a tool to evaluate sacral development in patients with anorectal malformations (ARM) and to help (along with the type of ARM and spinal status) to predict future bowel control. Although the ratio can be calculated using images from either the AP or lateral planes, lateral images are believed to produce more reliable ratios, given that the calculation is not influenced by the tilt of the pelvis. The congruency of the sacral ratio in the AP and lateral planes has not been previously investigated. We therefore aimed to assess the variability in the AP and lateral sacral images.
Methods: We reviewed all patients with ARM treated at our institution between 2014 and 2018 who had both an AP and lateral image of their sacrum. The SR was calculated using the ratio of the distance from the sacroiliac joint to the tip of the coccyx to the distance from the top of the iliac crest to the sacroiliac joint. All ratios were calculated by a pediatric radiologist. Variation between the SRs as determined by the AP and lateral images were compared across all patients and by ARM type using sacral ratio categories (0-0.39, 0.40-0.69, >0.70) that were developed for the purpose of counseling families.
Results: 561 patients were included in the study. SRs in the AP plane varied by an average of 17% (IQR 4,25, range 0-154). The AP SR overestimated the lateral SR in 23% (N=128) and underestimated the lateral SR in 63% (N=354) of patients. The variability in measurements decreased with increasing sacral development, as patients with a severe hypodevelopment (SR <0.4, N=39) demonstrated a variation of 27%, patients with moderate hypodevelopment (SR 0.4-0.69, N=193) demonstrated a variation of 18%, and patients with normal sacral development (SR >0.7, N=329) demonstrated a variation of 15%. The difference in these groups was statistically significant (p=0.03).
Conclusion: The SR determined by images in the AP plane varied significantly from that measured using lateral images. These results demonstrate that the AP sacral ratio can lead to a significant misinterpretation of the degree of sacral development which would impair the ability to accurately counsel families on their child’s future continence potential. Based on these data, we recommend the lateral SR to be used as the preferred measure. The AP view remains valuable to assess for hemisacrum.