In higher vertebrates, the neural tube is generated by the processes that shape, bend, and fuse the neural plate, and fusion in the dorsal midline progressively seals the neural tube as it forms. If closure is not completed, the neuroepithelium remains exposed to the environment and consequently subject to degeneration and neuronal deficit.
Primary (closure) neuralation:
Neural tube formation via bending of the neuroepithelium at the midline to generate neural folds that elevate, meet, and fuse in the dorsal midline to form most of the spinal cord
Secondary (canalisation) neuralation:
Follows on seamlessly from primary neurulation, and is the process by which the neural tube forms in the lower sacral and coccygeal regions. Malformations resulting from disturbance of secondary neurulation are closed (skin covered) and often involve tethering of the spinal cord, with associated ectopic lipomatous material
Failure of cranial neural tube closure
Anencephaly: Failure of cephalic folds to fuse into a neural tube
Cranium bifida (midline skull defects):
Cranium bifidum occultum (mild form): Persistent wide fontanelle, or persistent parietal foramina, which often close over time
Encephalocele (severe form): Failure of the anterior neuropore to close during days 26-28 of gestation; associated with various syndromic malformations
Failure of caudal neuropore closure
Spina bifida cystica (open spinal dysraphism): Originate from defective closure of the primary neural tube, with persistence of a segment of incompletely fused plaque of neural tissue, referred to as the neural placode
Myelocele (herniation of meninges only): Flat appearance
Myelomeningocele (herniation of meninges and neural tissue): Bulging appearance
Closed spinal dysraphisms (associated with subcutaneous mass):and meningocele)
Lipomas with dural defect: Characterized by a subcutaneous fatty mass located above the gluteal crease. The lipomatous mass herniates through the bony defect and attaches to the spinal cord, tethering the cord, and often the associated nerve roots
Meningocele: Herniation of the meninges through the bony defect (spina bifida) without an associated herniation of the spinal cord or nerve roots into the dural sac
Terminal myelocystocele: Large terminal cystic dilatation of the spinal cord resulting from hydromyelia
Spina bifida occulta: Failure of caudal neuropore to close. The spinal cord, meninges, and overlying skin remain intact, with no herniation.
Myeloschisis: Exposed neural tissue without skin/meninges covering.
Anencephaly: Failure of rostral neuropore to close thus the brain and cranial vault are grossly malformed with normal hindbrain development.
Non-genetic risk factors:
Reduced folate intake
Maternal anticonvulsant therapy (interferance with folate metabolism)
Spina bifida oblongata:
May develop neurogenic degradation later in life
May develop hydrocele
Myelomeningocele (M/C, M/severe):
Spinal cord open dorsally, forming a placode on the back of the fetus or newborn baby that frequently rests on a meningeal sac (then named spina bifida cystica). The vertebrae at the level of the lesion lack neural arches, and so are incomplete dorsally.
Loss of sensation
Bowel & bladder problems
Leg & Feet deformities (Clubfeet)
Associated with: Arnold Chiari II Malformation (cerebellar and medulla oblongata slide down into Foramen Magnum)