Retained medullary cord associated with terminal myelocystocele and intramedullary arachnoid cyst.

Introduction The retained medullary cord (RMC) is a newly defined entity of closed spinal dysraphism that is thought to originate from regression failure of the medullary cord during the last phase of secondary neurulation. The terminal myelocystocele (TMC) is an unusual type of closed spinal dysraphism, characterized by localized cystic dilatation of the terminal part of the central canal that then herniates through a posterior spinal bifida. The co-occurrence of RMC and TMC is extremely rare. Case Presentation We treated a baby girl with a huge sacrococcygeal meningocele-like sac with two components. Untethering surgery and repair surgery for the sac revealed that RMC, associated with intramedullary arachnoid cyst (IMAC), was terminated at the bottom of the rostral cyst, forming the septum of the two cystic components, and the caudal cyst was TMC derived from the central canal-like ependymal lining lumen (CC-LELL) of the RMC at the septum. IMAC within the RMC, communicated with TMC, and both contained xanthochromic fluid with the same properties. Conclusion We speculated that the mass effect of the coexistent IMAC impeded the flow of cerebrospinal fluid in the CC-LELL within the RMC and eventually formed a huge TMC. In the surgical strategies for these complex pathologies, we stressed the importance to identify the electophysiological border between the functional cord and nonfunctional RMC and severe RMC at this border to untether the cord, as with a typical or simple RMC.