Image characteristics of retained medullary cord in secondary neurulation arrest: an observational study

Retained medullary cord (RMC) is a recently defined term denoting closed spinal dysraphism arising from the failure of regression in secondary neurulation. Despite the acknowledgment of this condition, there needs to be more literature elucidating the radiologic manifestations of RMC. This study aimed to describe the MR imaging findings of RMC. A retrospective analysis was conducted on spinal MRI scans of pediatric patients with surgically confirmed RMC, from January 2014 to August 2023. The investigation focused on evaluating the following image characteristics: level and morphology of the cord-like structure (C-LS), the signal intensity in the far distal C-LS, nerve root-like structures originating from the C-LS, intradural fatty lesions, and arachnoid cysts. This study included 38 patients (19 girls, mean age 7.3 months) who showed a low-lying cord in all cases. The morphology of C-LS was either smooth tapered (50%) or hourglass with fusiform cystic dilatation (50%). The C-LS exhibited aberrant T2 hypointense signal compared to the juxta-proximal level. T2 hypointense signal vestigial nerves emanating from the C-LS and intradural fatty masses were observed (89.5%). Sacral arachnoid cysts in extradural location were identified in eight patients. The characteristic MR features of RMC revealed an extremely low-lying distal C-LS with smooth tapering or hourglass-shaped cystic dilatation of the caudal part, accompanied by an intradural fatty stalk and the aberrant signal vestigial nerve. This study suggests those radiologic findings can be RMC instead of the previously called terminal syrinx with low-lying conus.