J Korean Neurosurg Soc.  2020 May;63(3):327-337. 10.3340/jkns.2020.0018.

Junctional Neural Tube Defect

Affiliations
  • 1Department of Clinical Medicine, Faculty of Medicine and Health Sciences, Macquarie University, Sydney, Australia
  • 2Department of Neurosurgery, Macquarie University Hospital, Sydney, Australia
  • 3Department of Paediatric Neurosurgery, Sydney Children’s Hospital Randwick, Sydney, Australia
  • 4Department of Paediatric Neurosurgery, Great Ormond Street Hospital for Children, NHS Trust, London, UK
  • 5Department of Paediatric Neurosurgery, University of California, Davis, CA, USA

Abstract

Junctional neurulation represents the most recent adjunct to the well-known sequential embryological processes of primary and secondary neurulation. While its exact molecular processes, occurring at the end of primary and the beginning of secondary neurulation, are still being actively investigated, its pathological counterpart –junctional neural tube defect (JNTD)– had been described in 2017 based on three patients whose well-formed secondary neural tube, the conus, is widely separated from its corresponding primary neural tube and functionally disconnected from corticospinal control from above. Several other cases conforming to this bizarre neural tube arrangement have since appeared in the literature, reinforcing the validity of this entity. The cardinal clinical, neuroimaging, and electrophysiological features of JNTD, and the hypothesis of its embryogenetic mechanism, form part of this review.

Keyword

Spinal dysraphism; Embryology; Neurulation; Neural tube defects

Figure

  • Fig. 1. T2-weighted MRI of JNTD. A : Sagittal view shows the “upper spinal cord” (primary neural tube) ending at T12/L1, with a “cut off” stump instead of the usual taper. B : At S1/2 level the sagittal view shows the “lower spinal cord” (secondary neural tube) tapering into the filum, resembling a true conus. C : Axial view at T12 shows the “upper spinal cord”. D : Axial view at L2 shows the connecting band between the “upper” and “lower spinal cords”. E : Axial view at S1 demonstrates the “lower spinal cord” with bilateral ventral and dorsal roots. MRI : magnetic resonance imaging, JNTD : junctional neural tube defect.

  • Fig. 2. Intraoperative pictures during exploratory surgery of JNTD. A : The “upper spinal cord” (primary neural tube) is connected to the “lower spinal cord” (secondary neural tube) by a whitish soft band. B : Close-up view of this bridging band. C : Close-up view of the “upper spinal cord” (primary neural tube) showing a dense leash of nerve roots. JNTD : junctional neural tube defect.

  • Fig. 3. Direct stimulation of the “lower spinal cord” in JNTD elicits strong bilateral EMG responses in the external anal sphincter, indicating it is in fact a functioning conus. EMG : electromyography, L : left, Rec Fem : rectus femoris muscle, Ant Tib : anterior tibialis muscle, Gastroc : gastrocnemius muscle, Anus : sphincter ani muscle, R : right, JNTD : junctional neural tube defect.

  • Fig. 4. Bipolar stimulation of the non-functional, bridging band in JNTD between the “upper” and “lower spinal cord” shows no EMG response. EMG : electromyography, L : left, Rec Fem : rectus femoris muscle, Ant Tib : anterior tibialis muscle, Gastroc : gastrocnemius muscle, Anus : sphincter ani muscle, R : right, JNTD : junctional neural tube defect.

  • Fig. 5. TcMEP in JNTD showing motor responses in bilateral rectus femoris, anterior tibialis, and gastrocnemius muscles. There is no motor response of the abductor hallucis or sphincter ani muscles bilaterally, indicating no functional connection to the conus (S2–S5 segments) from the motor cortex. TcMEP : transcranial motor evoked potentials, L : left, Rec Fem : rectus femoris muscle, Ant Tib : anterior tibialis muscle, Gastroc : gastrocnemius muscle, Anus : sphincter ani muscle, R : right, JNTD : junctional neural tube defect.


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