Low Conus Position on Fetal MRI with Postnatal Correlation

Abnormal low position of the conus medullaris on fetal MRI may portend tethered cord syndrome after birth. However, prenatal identification of low conus is challenging and frequently requires postnatal confirmation. This study aimed to compare two methods for evaluating conus position on fetal MRI by correlating with postnatal outcomes.

This is a retrospective cohort study of fetal MRIs performed between 2011-2022 that reported low conus in the absence of other spinal dysraphism and had available postnatal follow-up. Each case was re-classified as normal or low conus, using two methods: localization relative to lumbar vertebral level versus the inferior border of the kidneys. Postnatal imaging and clinical follow-up data including surgical detethering were also collected.

We found 203 fetal MRI examinations (224 fetuses in total) that reported low conus, 15% (34/224) with available postnatal spinal imaging, 65% (22/34) with confirmed low conus (level L3 or below), and 41% (9/22) who were treated with detethering surgery. There was no statistically significant difference between the accuracies of the two approaches (p = 0.66) in predicting low conus on postnatal imaging.

Both the vertebral level and the inferior renal border landmarks on fetal MRI can be used to predict postnatal low conus in the majority of cases, without a statistically significant difference between the two methods. A combined approach performed in the third trimester may improve diagnostic confidence. Interpretation of isolated low conus on fetal MRI may be influenced by factors including gestational age, transitional lumbosacral anatomy, caudal regression group 2, and occult tethering lesions.