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neural tube defect (NTD)
any of a group of congenital malformations involving defects in the skull and spinal column that are caused primarily by the failure of the neural tube to close during embryonic development. In some instances the cleft results from an abnormal increase in cerebrospinal fluid pressure on the closed neural tube during the first trimester of development. The defect may occur at any point along the neural axis or extend the entire length of the spinal column, as in holorachischisis. The amount of deformity and disability depends on the degree of neural involvement, the most severe defect being complete cranioschisis, or the total absence of the skull and defective brain development. Other cerebral dysplasias resulting from the failure of the cranial end of the neural tube to fuse are meningoencephalocele and cranial meningocele. These defects, usually accompanied by severe mental and physical disorders, occur most often in the occipital region of the skull but may also occur in the frontal or basal regions. Most neural tube malformations are caused by incomplete fusion of one or more laminae of the vertebral column, with varying degrees of tissue protrusion and neural involvement. The two most common NTDs are spina bifida and anencephaly. Other NTDs include rachischisis, myelocele, myelomeningocele, and meningocele. In all of these conditions there is constant risk of rupture of the saclike protrusion and danger of meningeal infection. Often immediate surgical repair is necessary. Adequate folate levels during the first month after conception are important in preventing neural tube defects; the U.S. Public Health Service recommends that all women of childbearing age increase their folate intake to 400 mg per day. Many of the major neural tube defects can be determined prenatally by ultrasonic scanning of the uterus and by tests for the presence of elevated concentrations of alpha-fetoprotein levels in the amniotic fluid. Such diagnostic tests are preferably performed during the 14th to 16th week of gestation so that termination of the pregnancy is possible. See also anencephaly, Arnold-Chiari malformation, spina bifida cystica.