Purpose: Approximately 25% of patients with anorectal malformation have tethered cord. The traditional way of determining conus medullaris level on magnetic resonance imaging (MRI) relies on counting vertebrae, which may be challenging due to vertebral numeric variation, segmentation anomalies, as well as transitional vertebral body anatomy. The purpose of this study was to utilize more reliable anatomic landmarks (foramen magnum, conus termination, and the upper limit of the iliac crest) to establish a consistent ratio that may differentiate patients with normal and low-lying conus.
Methods: A single institution database search identified two groups of patients: 255 with normal and 85 with abnormal spinal MRI, who underwent tethered cord release. The conus medullaris ratio was calculated in both groups. The ratio was obtained by dividing the distance between the conus level and the iliac crest by the distance between the foramen magnum and the conus level (IRB # 16-2330).
Results: The mean ratio was significantly higher in the non-tethered group compared to the tethered patients (0.184 [sd 0.03] versus 0.118 [sd 0.09]; P < .0001). The ratio proved to be a good discriminator between normal and abnormal patients, with area under the curve (AUC) equal to 0.749, meaning that at random, there is a 75% chance that the tethered cord patient will have a lower ratio than the non-tethered cord patient.
Conclusion: "The Conus Medullaris Ratio" is a good predictor of low-lying conus level on MRI and offers an easy alternative to counting vertebral body levels, particularly in patients with variant or abnormal vertebral body anatomy.
Level of evidence: II, Study of Diagnostic Test.
Keywords: Ano-rectal malformation; Conus medullaris ratio; Low-lying conus; Tethered cord.
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