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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.
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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.
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Cranioraschisis
and Anencephaly
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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.
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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 reflexive ...