American Medical Association Family Medical Guide - American Medical Association [176]
Neural Tube Defects
The central nervous system (see page 666), which includes the brain and spinal cord, develops in the fetus within the first 2 months of pregnancy from a strip of cells that runs along the back of the embryo. The edges of the strip gradually curl inward to form a tube of cells. The front part of this tube expands to form the brain; the back part of the tube forms the spinal cord. A liquid (called cerebrospinal fluid) produced by the brain forms inside and around the brain and spinal cord to cushion them. Bone cells develop into the skull and spinal column, which provide more protection for the brain and spinal cord.
Neural tube defects—in which the brain, spinal column, or spinal cord is malformed—are disorders of the central nervous system that are present at birth (congenital). Neural tube defects are linked to a deficiency of the B vitamin folic acid in a pregnant woman. During the first few weeks of pregnancy, which are crucial to the development of the fetus’s central nervous system, many women do not know they are pregnant. Most cases of neural tube defects could be prevented if women of child-bearing age who might become pregnant would take 400 micrograms of folic acid (in a supplement or in a multivitamin) every day before they become pregnant and throughout their pregnancy (especially during the early weeks).
Neural tube defects can run in families. If a couple has one affected child, or relatives who have a child with a neural tube defect, they should talk to a genetic counselor (see page 952) about their risks of having another child with the abnormality and to learn how they can prevent it.
Spina Bifida
In a neural tube defect called spina bifida, part of the spinal column does not develop fully, leaving the nerves of the spinal cord in that area exposed and unprotected. The disorder usually affects the bones (vertebrae) in the lower spine and the meninges (the membranes covering the spinal cord). The spinal nerves in that area control the leg muscles, bladder, and intestines. The extent of physical disability depends on the severity of the spinal defect.
Symptoms
The defect in the lower spine and the resulting damage to the spinal cord and symptoms can vary widely. The mildest form of spina bifida, called spina bifida occulta, is not technically a neural tube defect because only the vertebrae are malformed—the spinal cord and meninges remain inside the spinal column. The only visible sign of spina bifida occulta may be a small dimple in the skin, a birth-mark, or a wisp of hair over the lower spine. In some children, the spine has an abnormal curve. In rare cases, a child with spina bifida occulta has a physical impairment.
In the most severe and most common form of spina bifida (meningomyelocele), the meninges and spinal cord are exposed on the child’s back. In another form, known as a meningocele, a bulging sac containing the meninges appears on the child’s back. This sac may or may not be covered by skin but it is fragile and can easily be damaged. An infectious organism can enter the cerebrospinal fluid through a damaged area, causing meningitis (see page 692). When the spinal cord is involved, the cord often is damaged before birth, causing weakness, paralysis, or loss of sensation in the legs and lack of normal bowel and bladder control. Bladder infections are common in a person with spina bifida; they sometimes can lead to kidney damage.
Diagnosis
Spina bifida can be detected before birth by measuring the level of a protein called alpha-fetoprotein (AFP) in a pregnant woman’s blood early in pregnancy. If the woman’s AFP level is elevated, the doctor will recommend an ultrasound (see page 509) of the fetus and, possibly, amniocentesis (see page 510) to confirm and evaluate the abnormality in the fetus. After birth, doctors can recognize more severe forms of spina bifida during a physical examination. Spina bifida occulta usually is detected by chance on an X-ray of the