What are neural tube defects?
Neural tube defects (NTDs) are common birth defects of the brain and spinal cord that include anencephaly and spina bifida (meningomyelocele). Deficiency of the vitamin folic acid raises the risk of neural tube defects, as does mutation of a gene for an enzyme that processes folic acid. The mutation is called the C677T variant, or T for short. Incomplete formation and protection of the brain or spinal cord with bony and soft tissue coverings that occur during the fourth week of
embryo formation are known collectively as neural tube defects. These lesions may occur anywhere in the midline of the head or spine. Neural tube defects are among the most common serious birth defects, but they vary considerably in their severity. In some cases, the brain or spinal cord is completely exposed; in some cases it is protected by a tough membrane (meninges); and in other cases it is covered by skin.
Spina bifida is a congenital defect that accounts for about two-thirds of all neural tube defects. Its name comes from two Latin words that mean "cloven backbone." The spinal defect may appear anywhere from the neck to the buttocks. In its most severe form, termed "spinal rachischisis," the entire spinal canal is open, exposing the spinal cord and nerves. More commonly, the defect appears as a localized mass on the back that is covered by skin or by the meninges.
Anencephaly, the second most common neural tube defect, accounts for about one-third of cases. Two major subtypes occur. In the most severe form, all of the skull bones are missing and the brain is exposed in its entirety. The second form, in which only a part of the skull is missing and a portion of the brain exposed, is termed "meroacrania."
Encephaloceles are the least common form of neural tube defects, comprising less than 10% of the total. With encephaloceles, a portion of the skull bones is missing, leaving a bony hole through which the brain and brain coverings herniate, or protrude abnormally. Encephaloceles occur in the midline from the base of the nose to the junction of the skull and neck. As with spina bifida, the severity of encephaloceles varies greatly. At the mildest end of the spectrum, an encephalocele may appear as only a small area of faulty skin development with or without any underlying skull defect. At the severe end of the spectrum, most of the brain may be herniated outside of the skull into a skin-covered sac.