Cerebellar herniation demonstrated by the occipitum‐dens line: Ultrasonography assessment of normal fetuses, fetuses with myelomeningocele, and fetuses that underwent antenatal myelomeningocele surgery

OBJECTIVE: To establish a method to quantify the position of the cerebellum by ultrasonography in normal fetuses, fetuses with myelomeningocele (MMC), and fetuses that underwent in utero MMC repair. METHODS: Reference points identifiable on ultrasound were established. The basilar portion of the occipital bone and upper portion of the odontoid process were considered as the external and internal limits, respectively, of the level zero of a line that was designated the occipitum-dens line (ODL). Eighty-three normal fetuses were assessed 1 occasion (cross-sectional study) in addition to 25 fetuses with MMC and 25 fetuses that underwent in utero MMC repair. The groups were compared using analysis of variance or the Kruskal-Wallis test. The intraclass correlation coefficient (ICC) was used to determine intra- and interobserver reproducibility. RESULTS: The analysis of ODL indicated that all normal fetuses had the cerebellar tonsil above level zero at a mean distance of 2.8 ± 1.1 mm (P = 0.125). The fetuses with MMC had the cerebellar tonsil below level zero, except in 2 cases of myelocystocele. All the fetuses that underwent in utero MMC repair exhibited regression of the cerebellar herniation (0.5 mm per week, P < 0.005). Herniation increased gradually (1.0 mm per week, P < 0.005) in fetuses that did not undergo MMC repair. ICC indicated good intra- and interobserver reproducibility (0.996 and 0.983, respectively). CONCLUSION: The reference points for ODL may be used to assess cerebellar height. ODL allowed the demonstration of the regression of cerebellar herniation in fetuses that underwent in utero MMC repair.