Unjoined primary and secondary neural tubes: junctional neural tube defect, a new form of spinal dysraphism caused by disturbance of junctional neurulation

INTRODUCTION

Primary and secondary neurulation are the two known processes that form the central neuraxis of vertebrates. Human phenotypes of neural tube defects (NTDs) mostly fall into two corresponding categories consistent with the two types of developmental sequence: primary NTD features an open skin defect, an exposed, unclosed neural plate (hence an open neural tube defect, or ONTD), and an unformed or poorly formed secondary neural tube, and secondary NTD with no skin abnormality (hence a closed NTD) and a malformed conus caudal to a well-developed primary neural tube.

METHODS AND RESULTS

We encountered three cases of a previously unrecorded form of spinal dysraphism in which the primary and secondary neural tubes are individually formed but are physically separated far apart and functionally disconnected from each other. One patient was operated on, in whom both the lumbosacral spinal cord from primary neurulation and the conus from secondary neurulation are each anatomically complete and endowed with functioning segmental motor roots tested by intraoperative triggered electromyography and direct spinal cord stimulation. The remarkable feature is that the two neural tubes are unjoined except by a functionally inert, probably non-neural band.

CONCLUSION

The developmental error of this peculiar malformation probably occurs during the critical transition between the end of primary and the beginning of secondary neurulation, in a stage aptly called junctional neurulation. We describe the current knowledge concerning junctional neurulation and speculate on the embryogenesis of this new class of spinal dysraphism, which we call junctional neural tube defect.

[Microanatomic and three-dimensional reconstruction study of lateral wall and related structures of optic canal]

OBJECTIVE

To investigate the key microanatomic and radiological structures of optic canal comprehensively so as to provide anatomic parameters and procedural flows for the decompression of optic canal.

METHODS

Gross observations and microscopic measurements were applied on 10 (20 sides) formalin-treated cadaveric specimens and 15 (30 sides) adult skulls. Using multislice helical CT (computed tomography)-aided three-dimensional reconstruction in combination with direct anatomic measurement, the investigators dissected, photographed, measured and analyzed the shape of optic canal and analyze its anatomic relationship with the adjoining structures.

RESULTS

Optic canal was formed by the superior, inferior, medial and external walls and distal proximal opening. The lateral wall of optic canal was formed by anterior clinoid process with a length of (9.87 ± 1.34) mm, a width of (11.66 ± 2.35) mm, a base thickness of (5.35 ± 1.07) mm and a middle thickness of (4.50 ± 1.06) mm. Optic strut separating the optic canal from the superior orbital fissure was located inferiorly. And the distance between the apex of anterior clinoid process and the middle of ICA (internal carotid artery) groove was (4.25 ± 2.30) mm. The CSF (cerebrospinal fluid) leakage and secondary injury of optic nerve and injury of ICA, ophthalmic artery might occur during the surgical procedures due to the variation of anterior clinoid process. The microanatomic figures and radiological measurements had a mean difference very close to each other at (0.08 - 0.48) mm. No statistical difference was found (P > 0.05).

CONCLUSION

Optic nerve, ophthalmic artery and ICA may be exposed by a high-speed drilling of the lateral wall of optic canal. The drilling dissection of lateral wall plays a vital role during a successful optic canal decompression. Radiological measurement and three-dimensional reconstruction of skull base may be of great clinical significance in lesion visualization. And it helps to make a better choice of surgical approaches. The measurements provide valuable references for surgeons and researchers.