NEURAL TUBE DEFECTS IN THE NEONATAL PERIOD

Neural Tube Defects in the Neonatal Period
Congenital deformities involving the coverings of the nervous system are called neural tube defects (NTDs). NTDs vary in severity. The mildest form is spina bifida aperta, in which osseous fusion of one or more vertebral arches is lacking without involvement of the underlying meninges or neural tissue. A slightly more severe form of spina bifida, which is discussed in detail in this article, is spina bifida cystica or myelomeningocele, in which a saclike casing is filled with cerebrospinal fluid (CSF), spinal cord, and nerve roots that have herniated through a defect in the vertebral arches and dura .

Anencephaly and rachischisis are extremely severe forms of NTD in which an extensive opening in the cranial and vertebral bone exists with an absence of variable amounts of the brain, spinal cord, nerve roots, and meninges. Anencephaly has been studied since antiquity, and an almost dizzying array of synonyms and classifications exists. For a more complete description of anencephaly, see the Bibliography for the seminal work written by Lemire, Beckwith, and Warkany in 1978.

Malformations of the brain and spinal cord may result from genetic mutation or may be acquired deformities. Most malformations, especially those such as NTDs, occur early in embryogenesis and are likely the result of aberrant expression of a yet undefined developmental gene or family of genes. The nervous system develops in a precise temporal embryologic sequence; therefore, an interruption of one part of the developmental sequence often affects remaining development.

The NTD discussed in this article is classified as an embryologic induction disorder. It results in failure to properly form both the mesoderm and neuroectoderm. The primary embryologic defect in all NTDs is failure of the neural tube to close, affecting neural and cutaneous ectodermal structures. The inciting event can be traced to days 17-30 of gestation.

The precise etiology and the specific genes that may be involved during this abnormal neural ontogenesis have not yet been elucidated. These deformities are not only disorders of embryologic induction but also disorders of cellular migration and include the secondary mechanical complications that occur with an unprotected nervous system. Specifically, the amniotic fluid can have a caustic and destructive effect on the open neural structures.

As described, the primary defect is a failure of the neural folds to fuse in the midline and form the neural tube, which is neuroectoderm. However, the subsequent defect is the maldevelopment of the mesoderm, which, in turn, forms the skeletal and muscular structures that cover the underlying neural structures. These NTD defects can be open (neural structures that communicate with the atmosphere) or closed (skin covered). They can be ventral or dorsal midline defects.