Neural tube defects (NTD) are significant birth deformities of the central nervous system that occur due to a defect in the neurulation process of embryogenesis. They are among the most common type of birth defects and are thought to have multifactorial etiology, including multigenetic and environmental influences. Because the neural tube is ultimately formed from the migration and fusion of the neural plate, the type and severity of malformation varies based on the location of the defect. This includes both cranial and spinal cord malformations. Since rostral and caudal neuropore closure is the last phase of neurulation, they are particularly vulnerable to defects. Consequently, a majority of NTDs arise in these areas.
NTDs can be classified as “open” or “closed” types, based on embryological considerations and the presence or absence of exposed neural tissue (i.e., failure of incomplete fusion of the neural plate).
Open NTDs frequently involve multiple aspects of the CNS (e.g., associated hydrocephalus, Chiari II malformation) and are due to failure of primary neurulation, thus the neural tube fails to appropriately close along the dorsal midline. Neural tissue is completely exposed, or covered by a membrane, with associated cerebrospinal fluid (CSF) leakage.
Open NTD’s represent roughly 80% of all NTD’s, with the most common being meningocele (spina bifida), myelomeningocele, encephalocele, and anencephaly.
Closed NTDs are localized and confined to the spine (the brain is rarely affected) and result from a defect in secondary neurulation. Neural tissue is not exposed and the defect is fully covered by epithelium, although the skin covering the defect may be dysplastic (i.e., tuft of hair, dimple, birthmark, or other superficial abnormality).
Cranial manifestations include the following:
Encephalocele (meningocele or meningomyelocele)
Congenital dermal sinus
Spinal presentations include the following:
Spina bifida occulta
Spina bifida in relation to a dermoid cyst
Spina bifida aperta (meningocele, myelomeningocele (see images below), meningomyelocele, myeloschisis)
Lipomatous malformations (lipomyelomeningocele)
Myelomeningocele in a newborn.
Myelomeningocele in a newborn – Lateral view.
For more information on the classification of neural tube defects, see Medscape Reference article Imaging in Spinal Dysraphism and Myelomeningocele.