BACKGROUND: The structural complexity of terminal myelocystocoele (TMC) precludes a recognizable link to spinal cord development and therefore also a plausible embryogenetic theory. OBJECTIVE: To demystify TMC using clinical, imaging, surgical, electrophysiological and histopathological data, and to propose a theory of embryogenesis. METHODS: Our series consisted of 4 newborns and 6 older children. All had pre-operative MRIs and surgical repair of the myelocystocoele using electrophysiological monitoring. RESULTS: TMC can be deconstructed into "essential" and "non-essential" features. "Essential features" are present in all TMCs and constitute the "core malformation", comprising an elongated spinal cord extending extraspinally into a CSF-filled cyst that is broadly adherent to the subcutaneous fat. The functional conus resides in the proximal cyst or within the intraspinal cord, and the caudal myelocystocoele wall is non-functional fibroneural tissue. "Non-essential features" include variable measures of hydromyelia, caudal "meningocoele" and fat, present in only some patients. TMC's core structure strikingly resembles a transitory stage of late secondary neurulation in chick in which the CSF-filled bleb-like distal neural tube bulges dorsally to fuse with the surface ectoderm, before focal apoptosis detaches it from the surface and undertakes its final dissolution. We theorize TMC results from a time-specific paralysis of apoptosis just before the dehiscence of the cystic distal cord from the future skin, thereby preserving the embryonic state. CONCLUSION: Besides tethering, the myelocystocoele may show early rapid expansion causing precipitous deterioration. We recommend early repair with resection of the non-functional caudal cyst wall, reconstruction of the proximal neural placode, and duraplasty.
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