Cystic retained medullary cord in an intraspinal J-shaped cul-de-sac: a lesion in the spectrum of regression failure during secondary neurulation.

Introduction: Retained medullary cord (RMC) is thought to be a product of arrested secondary neurulation during the regression phase. A cord-like structure with a caudal non-functional part ends at the cul-de-sac. If the arrest occurs at the cavitation phase of secondary neurulation, the medullary cord has a cystic portion making "RMC of cystic type."
Clinical Presentation: We report a case of a 4-month-old girl who had a low-lying conus with an extradural-looking dorsal cyst. Preoperative diagnosis was RMC with a lumbosacral extradural cyst such as an arachnoid cyst. At surgery, we found that the extradural cyst was an extension of dural sac with the caudal portion of the cystic RMC inside. The RMC was untethered and the dural sac was reconstructed. The histopathologic examination revealed findings compatible with cystic RMC attached to the cul-de-sac.
Conclusions: We regard this case as an intermediate form between the typical RMC in which is regarded as regression arrest occurred after the terminal balloon collapsed and the medullary cord detached from the skin to the normal cul-de-sac, and the terminal myelocystocele which is considered the result of arrest at the phase of the persisted terminal balloon attached to the skin.