Embryopathological relationship between retained medullary cord and caudal spinal lipoma

Purpose: Retained medullary cord (RMC) is a condition in which the cord-like structure (C-LS) is continuous from the conus and extends to the dural cul-de-sac, causing spinal cord tethering, which is thought to originate from the secondary neurulation failure. Although diagnostic criteria for RMC have not yet been fully established, the following three items are generally considered important: (1) Typical morphological features on neuroimaging and intraoperative view. (2) The electrophysiological border between the conus and C-LS. (3) Ependyma-lined canal with surrounding neuroglial tissues (E-LC w/NGT) on histopathological examination of C-LS. To clarify the differences with caudal lipoma, which are also considered to be secondary neurulation disorders, in their embryological backgrounds, the clinicopathophysiological findings were compared.Methods: Five patients with RMC and 11 with caudal lipomas participated in this study. Results: All five patients with RMC naturally satisfied all three items. Four patients histopathologically showed a small amount of fibroadipose tissue. Regarding the first item in caudal lipoma, neuroimages demonstrated C-LS running parallel with or in the lipoma in all patients. In terms of the second item, the electrophysiological border was identified in 3 out of 4 patients in whom a sufficient operative field was obtained. Concerning the third item, E-LC w/NGT was noted in 6 of 7 patients in whom the lipoma was resected as a column. Areas of fibroadipose tissue histopathologically accounted for more than half of the total area. Conclusion: The caudal lipomas fulfilled most of the three items, and the only prominent difference between these two is the difference in the proportions of fibroadipose and fibrocollagenous tissues. These findings provide further evidence for the idea that RMC and caudal lipoma can be considered consequences of a continuum of regression failure during secondary neurulation, and it may not be possible to distinguish between the two clearly.