Clinical features: The two main types of neural tube defect, are spina bifida and anencephaly and they are both equally as common. Spina bifida occurs when the spinal column, which usually provides a protective tubing of bones to the spinal cord running through it, does not close properly during the first month of pregnancy. This leads to damage to the nerves, which may be exposed. The effects of spina bifida are different for every affected person and depend upon the site of the lesion and whether the overlying skin remains intact. Approximately 90 per cent of children with the most severe form of spina bifida also have hydrocephalus (fluid on the brain) which requires surgery. The most common features of spina bifida include weakness and paralysis of the legs, bladder and bowel control difficulties and some learning disabilities. In anencephaly a large part of the skull is missing exposing the brain. Because of the severe damage to the structure of the brain, babies with anencephaly die before or soon after birth. Research has shown that if women at the population risk of having a child with a neural birth defect take 400 mcg of folic acid every day at least one month before embarking upon a pregnancy and for two months from the date of the last menstrual period, the risk of having a baby with spina bifida or another neural tube defect is reduced by as much as 70%. Women who are at high risk of having a child with spina bifida because they have already had an affected pregnancy, have a history of spina bifida in their family or have spina bifida themselves, should take a prescription level of 4 � 5 mg of folic acid.
Incidence: The frequency of neural tube defects shows marked geographical variation. In Great Britain it is about 0.8/1000 total births, a marked decline over the last 30 years. Before the use of folic acid supplementation, Eire had the highest incidence in the world at 1%.
Inheritance: Most neural tube defects show multifactorial inheritance. However, they may also be seen in chromosome or rare single gene disorders or be secondary to teratogenic influences. The empiric recurrence risk for any neural tube defect after one affected child is about 5%, and about 4% if a parent is affected, but this figure does depend upon the population. These figures predate the use of periconceptual folic acid supplementation.
Prenatal diagnosis: Prenatal diagnosis: Mid-trimester detailed fetal ultrasonography. Mild defects may be difficult to detect.
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