Neural Tube Defects

Neural tube defects (NTDs) are a collection of congenital disorders related to the failure of neurulation or the process by which the neural tube is formed, closed, and subsequently encapsulated. Although they can occur as single anomalies, neural tube defects are usually one component of multifaceted genetic disorders. Environmental factors, such as a deficiency in [Page 1222]folic acid or the presence of teratogen valproic acid, are also associated with NTDs.

The neural tube is an ectoderm-derived tissue that develops into the central nervous system (CNS) including the brain and spinal cord. Specified by the notochord (the predecessor of the vertebral body), neural tube formation begins at the fourth week of development through a process called neurulation. Neurulation involves the evagination of two neural folds in the neural plate, which grow to fuse at the midline. Fusion or closure of the neural tube proceeds in a rostral to caudal direction, producing a hollow tube with a central lumen called the neural canal. While the neural tube gives rise to the brain and spinal cord, the neural canal forms the ventricular network through which cerebral spinal fluid (CSF) flows. Neurulation is followed by the encapsulation of the CNS within the skull and spinal column. Proper neurulation is necessary for proper function of the CNS as well as proper protection.

NTDs are classified according to which combination of tissues are involved, whether it be the neural tube, meninges (three layers of connective tissue which encapsulate the CNS), and overlying bone or soft tissue. Although there is a whole array of types of NTDs, the cause of NTD is unknown and likely to be a combination of genetics and environmental factors.

Anencephaly is a malformation disorder resulting in the absence of the brain and calavarium. Anencephaly is a result of interrupted forebrain development.

Microcephaly, or small head, results from inefficient cortical expansion.

Holoprosencephaly is defined by the failure of the forebrain to be divided into two separate hemispheres, giving rise to a single continuous cerebral cortex. This is a consequence of defective dorsal midline induction by the roofplate for which bone morphogenetic proteins (BMPs), a group of signaling molecules, are known to be responsible. Schizencephaly is a full-thickness defect or cleft in the cerebral wall. Mutations in the Emx2 gene, a transcription factor responsible for cortical patterning with other proteins, has been found responsible.

Defects in the calavarium allow for parts of the brain to herniate through the skull, resulting in a condition termed encephalocele. This most often occurs in the occipital region of the skull. Spina bifida describes a group of disorders related to incomplete fusion of the vertebral arches. The vertebral arches are part of the spinal column and serve to protect the spinal cord from pinching while providing flexibility for movement. Spina bifida occulta, the most common spinal bifida, occurs in vertebral segments L1 to S1. Spina bifida may often be asymptomatic, presenting as a small tuft of hair on the overlying skin. Spina bifida cystica are severe forms of this malformation, involving protrusions of the spinal cord through the vertebral arches and skin. A meningocele is spina bifida cystica involving only the meninges. Alternatively, meningomyelocele is spina bifida cystica involving the meninges as well as part of the neural tube. Spina bifida cystica is suspected if elevated levels of alpha-fetoprotein are found in amniotic fluid.

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