Limited Dorsal Myeloschisis: Reconsideration of its Embryological Origin.

BACKGROUND: Limited dorsal myeloschisis (LDM) is postulated to be a result of incomplete dysjunction in primary neurulation. However, clinical experience of LDM located below the first-second sacral (S1-S2) vertebral level, which is formed from secondary neurulation (S2-coccyx), suggested that LDM may not be entirely explained as an error of primary neurulation. OBJECTIVE: To elucidate the location and characteristics of LDM to investigate the possible relation of its pathoembryogenesis to secondary neurulation. METHODS: Twenty-eight patients were surgically treated for LDM from 2010 to 2015. Since the level where the LDM stalk penetrates the interspinous ligament is most clearly defined on the preoperative MRI and operative field, this level was assessed to find out whether the lesions can occur in the region of secondary neurulation. RESULTS: Eleven patients (39%) with typical morphology of the stalk had interspinous defect levels lower than S1-S2. These patients were not different from 17 patients with classic LDMs at a level above or at S1-S2. This result shows that other than the low level of the interspinous level, 11 patients had lesions that could be defined as LDMs. CONCLUSION: By elucidating the location of LDM lesions (in particular, the interspinous level), we propose that LDM may be caused by errors of secondary neurulation. The hypothesis seems more plausible due to the supportive fact that the process of separation between the cutaneous and neural ectoderm is present during secondary neurulation. Hence, incomplete disjunction of the two ectoderms during secondary neurulation may result in LDM, similar to the pathomechanism proposed during primary neurulation.