Neural Tube Defects

A neural tube defect (NTD) is a birth defect that occurs when the spine, the brain, or the bone and skin that protect them do not develop properly. The most common type of neural tube defect is spina bifida, in which the spinal cord or spinal nerves may bulge out through an opening in the bones of the spine.

The neural tube is the part of a developing fetus that grows into the spinal cord and brain. Normally, the bones of the skull and spine grow around the brain and spinal cord, and then skin covers the bones, creating the neural tube. A neural tube defect occurs when this process doesn't happen normally.

In encephalocele, a saclike portion of the meninges and brain protrudes through a defective opening in the skull. Usually, it's in the occipital area, but it may also occur in the parietal, nasopharyngeal, or frontal area.

In anencephaly, the most severe form of NTD, the closure defect occurs at the cranial end of the neuroaxis and, as a result, part or all of the top of the skull is missing, severely damaging the brain.

Causes

NTDs may be isolated birth defects, may result from exposure to a teratogen, or may be part of a multiple malformation syndrome (for example, chromosomal abnormalities such as trisomy 18 or 13 syndrome). Isolated NTDs (those not due to a specific teratogen or associated with other malformations) are believed to be caused by a combination of genetic and environmental factors. Although most of the specific environmental triggers are unknown, recent research has identified a lack of folic acid in the mother's diet as one of the risk factors.

The incidence of NTDs varies greatly among countries and by region in the United States. For example, the incidence is significantly higher in the British Isles and low in southern China and Japan. In the United States, North and South Carolina have at least twice the incidence of NTDs as most other parts of the country. These birth defects are also more common in whites than in blacks.

Signs and symptoms

Spina bifida occulta is often accompanied by a depression or dimple, tuft of hair, soft fatty deposits, port wine nevi, or a combination of these abnormalities on the skin over the spinal defect; however, such signs may be absent. Spina bifida occulta doesn't usually cause neurologic dysfunction but occasionally is associated with foot weakness or bowel and bladder disturbances. Such disturbances are especially likely during rapid growth phases, when the spinal cord's ascent within the vertebral column may be impaired by its abnormal adherence to other tissues.

In both myelomeningocele and meningocele, a saclike structure protrudes over the spine. Like spina bifida occulta, meningocele seldom causes neurologic deficit. But myelomeningocele, depending on the level of the defect, causes permanent neurologic dysfunction, such as flaccid or spastic paralysis and bowel and bladder incontinence. Associated disorders include trophic skin disturbances (ulcerations, cyanosis), clubfoot, knee contractures, hydrocephalus (in about 90% of patients), and possibly mental retardation, ArnoldChiari syndrome (in which part of the brain protrudes into the spinal canal), and curvature of the spine.

Clinical effects of encephalocele vary with the degree of tissue involvement and location of the defect. However, surviving infants are usually severely mentally retarded with paralysis and hydrocephalus common.

Diagnosis

At birth, the diagnosis of a neural tube defect is usually obvious based on external findings. Prenatal diagnosis may be made with ultrasound examination after 12-14 weeks of pregnancy. Screening of pregnancies can be carried out at 16 weeks by testing the mother's blood for the level of alpha-fetoprotein. Open neural tube defects leak this fetal chemical into the surrounding amniotic fluid, a small portion of which is absorbed into the mother's blood. During an MSAFP profile, a small amount of blood is taken from your arm and measured for three chemicals (alpha-fetoprotein, estriol, and human chorionic gonadotropin). If the level of these chemicals is higher than normal, it may mean that your baby has a neural tube defect. However, it can also mean that your fetus is older than expected, or that you are carrying more than one fetus. Your doctor will offer additional tests (like an ultrasound or an amniocentesis) to evaluate the health of your baby.

Treatment

Prompt neurosurgical repair and aggressive management may improve the condition of children with some NTDs, but serious and permanent handicaps are likely.

Spina bifida occulta usually requires no treatment. Treatment of meningocele consists of surgical closure of the protruding sac and continual assessment of growth and development. Treatment
of myelomeningocele requires repair of the sac and supportive measures to promote independence and prevent further complications. Surgery doesn't reverse neurologic deficits. A shunt may be needed to relieve associated hydrocephalus.

Treatment of encephalocele includes surgery during infancy to place protruding tissues back in the skull, excise the sac, and correct associated craniofacial abnormalities.

Prevention

Researchers have found that 50-70% of NTDs can be prevented when women supplement their diet with folic acid, a water-soluble B vitamin. The Centers for Disease Control (CDC) recommends all women of childbearing age eat a diet high in folic acid or take a multivitamin with 0.4mg of folic acid each day, especially one month prior to conception through the first three months of pregnancy. This dosage is the amount found in most over-the-counter multivitamins. However, women who have had a previous NTD pregnancy are recommended to take an even higher dosage of folic acid prior to planning a pregnancy. They should increase the daily dose of folic acid from 0.4mg to 4.0mg, one month prior to conception through the first three months of pregnancy. The 4.0mg of folic acid should only be obtained through a prescription from the doctor.