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Disorders of neural tube formation and closure relate to disturbances in the inductive events involved in primary neurulation, aberrant closure of the anterior or posterior neuropore, or failure of the caudal regression and differentiation events that lead to formation of the lower spinal cord. Correct development of the neural tube is necessary for formation of the dura and other meninges, cranium and vertebrae, and the dermal coverings. Dysraphism refers to persistent continuity between the posterior neurectoderm and cutaneous ectoderm. External manifestations of abnormal cutaneous ectoderm may be the only initial clinical signs of occult dysraphism.

The incidence of neural tube defects varies widely by geography and by type of dysraphism. For example, rates of myelomeningocele as high as 3% have been reported in the United Kingdom, but extremely low in Japan. There are also significant secular trends with a steady decline in incidence for myelomeningocele and anencephaly in Great Britain, northern Europe, and the United States since the late 1980s. Further details of incidence and etiology are discussed below.

Cranioraschisis and Anencephaly

Cranioraschisis and anencephaly represent the most severe failures of primary neurulation. Cranioraschisis is a rare and lethal condition of essentially total failure of neurulation with a neural plate-like structure present, but with no skeletal or dermal elements overlying it. The incidence is unknown. The equally rare disorder of failure of posterior neuropore closure known as myeloschisis represents the inverse of cranioraschisis, where a neural platelike structure without overlying vertebrae or dermis replaces large portions of the spinal cord. This sometimes involves the base of the skull, the condition of iniencephaly. Infants with this condition are frequently stillborn with severe arthrogryphosis multiplex.

Anencephaly is a defect of anterior neural tube closure and comprises failure of formation of elements from the rostral portion of the neural tube anywhere from the foramen magnum caudally. The forebrain and upper brain stem are usually involved; normal neural tissue is replaced by a formless mass of degenerated, hemorrhagic neuronal and glial tissue; and there is an absence of the frontal, parietal, and squamous occipital bones. The disorder varies considerably with geographic location, race, sex and ethnic group, maternal age, and socioeconomic status. The incidence is higher in whites, especially those of Irish ethnicity, in very young and very old mothers, and in those of lower socioeconomic status as well as in those with a history of previously affected siblings. The incidence in the United States declined from 0.5 per 1000 live births in 1970 to 0.2 per 1000 births in 1989. Polyhydramnios is a frequent prenatal finding and the malformation is now commonly diagnosed in the second trimester during obstetric screening, allowing for elective termination of pregnancy. Babies born alive with anencephaly rarely survive beyond 7 days (< 2%) without intensive care, and even with intensive care, rarely beyond 8 weeks. Neurologic findings in live born babies are usually restricted to residual brain stem activity, including primarily ...

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