Neural tube defects (NTDs) are malformations of the developing brain and spinal cord. In normal development, the closure of the neural tube occurs over a 4- to 6-day period with completion around the 29th day postconception, often before a woman has realized that she is pregnant. Most current hypotheses consider NTDs to be defects from failure of a neural tube closure rather than the reopening of a previously closed tube. The nomenclature for NTDs is not standardized; frequently used terms are as follows:
Anencephaly. The defective closure of the upper or rostral end of the anterior neural tube. Hemorrhagic and degenerated neural tissue is exposed through an uncovered cranial opening extending from the lamina terminalis to the foramen magnum. Infants with anencephaly have a typical appearance with prominent eyes when viewed face on. Craniorachischisis totalis (a neural platelike structure without skeletal or dermal covering resulting from complete failure of neural tube closure) and myeloschisis or rachischisis (in which the spinal cord is exposed posteriorly without skeletal or dermal covering because of failure of posterior neural tube closure) are other, less frequent open lesions.
Encephalocele. Herniation of brain tissue outside the cranial cavity resulting from a mesodermal defect occurring at or shortly after anterior neural tube closure; usually a closed lesion. Approximately 80% of encephaloceles occur in the occipital region.
Myelomeningocele. Often also referred to as spina bifida (protrusion of the spinal cord into a sac on the back through a deficient axial skeleton with variable dermal covering). Considering that, strictly speaking, “spina bifida” only describes the bony defect, the term spinal dysraphism is considered more accurate by some. More than 80% of defects in this category occur in the lumbar region, and approximately 80% are not covered by skin. In contrast to myelomeningoceles, meningoceles (closed lesions involving the meninges only) usually do not result in neurologic deficits.
Spina bifida occulta and occult spinal dysraphism. Disorders of the caudal neural tube that are covered by skin (skin dimples or only very small skin lesions are present). These dysraphic disturbances range from cystic dilation of the central canal (myelocystocele), over bifida spinal cords with or without a separating bony, cartilaginous, or fibrous septum (diastematomyelia or diplomyelia), to a tethered cord with a dermal sinus or other visible changes such as hair tufts, lipomas, or hemangiomas. The term spina bifida occulta is used incorrectly when it is applied to an incomplete ossification of the posterior vertebral arch, a frequent and insignificant finding that is neither clinically nor developmentally related to NTDs.
Ninety-five percent of children with NTDs are born to couples with no family history of such defects.
Statistics related to neural tube defects are to be interpreted with caution and in the context of population, location, and time because occurrence of NTDs is affected by many epidemiologic and medical factors (see later sections).