Alternative Names
spina bifida; NTD (NTDs); anencephaly; open neural tube defects; closed neural tube defects
Symptoms & Characteristics
A neural tube defect (NTD) is a congenital birth defect caused by the failure of the neural tube to close properly during embryonic development. The neural tube is the embryonic structure that ultimately develops into the spinal cord and brain. The neural tube begins to develop shortly after conception as a thin ribbon of tissue, and completes development (by folding inward to form a tube) at approximately 28 days after conception. When the neural tube fails to close completely, structural defects in the spine and brain can develop.
A neural tube defect can be isolated or part of an underlying genetic condition. Neural tube defects may cause mild to severe problems depending on the type, location and severity of the defect, as well as if an underlying genetic condition is also present.
There are two types of neural tube defects (NTDs): open NTDs and closed NTDs.
- Open NTDs occur when the brain and/or spinal cord are exposed through a structural defect in the skull or back bones (called vertebrae).
- Closed NTDs occur when the spinal defect is covered by skin.
Open NTDs are more common that closed NTDs. Common open NTDs types include spina bifida, encephalocele and anencephaly.
Anencephaly is the absence of a large part of the brain (cerebellum and cerebral hemispheres) and the skull. It results when the upper part of the neural tube fails to close. There is no current therapy or treatment for anencephaly. This condition usually causes death within a few days of birth.
Encephalocele is characterized by sac-like protrusions of the brain and the membranes that cover it through openings in the skull. This type of open NTD is rare.
Spina bifida is the most common neural tube defect. There are two major forms of spina bifida: meningocele and myelomeningocele.
- Meningocele is a cyst or fluid-filled sac that pokes through the open part of the spine. This sac contains the membranes that protect the spinal cord, but not the spinal nerves.
- Myelomeningocele can be a cyst that holds both the membranes and nerve roots of the spinal cord and, sometimes, the cord itself. However, with a myelomeningocele, there can sometimes be no cyst, but only a fully exposed section of the spinal cord and nerves.
Other congenital disorders may also be present in a person affected with spina bifida. For example, hydrocephalus may affect as many as 90% of children affected with spina bifida. Some of the other problems that can be associated with spina bifida include loss of bladder or bowel control, partial or complete lack of sensation, partial or complete paralysis of the legs, a tethered spinal cord; as well as weakness of the hips, legs, or feet of a newborn.
Treatment
The overall treatment, prognosis and management of a neural tube defect is dependent upon the type, location and severity of the spinal defect. When possible, treatment is aimed at limiting nerve damage and maintaining functioning. For example, treatment for spina bifida may include surgery, bowel and bladder management, medications and physical therapy.
How Common Is It?
In the United States, about 1 in 1000 births are affected with a neural tube defect.
Neural tube defects are the second most common major birth defect, after congenital heart defects.
Genetics & Inheritance
Many cases of neural tube defects occur as isolated defects. An isolated neural tube defect is a multifactorial condition, which means it involves a combination of genetic and environmental factors.
- Low levels of folic acid during pregnancy are associated with an increased risk of a neural tube defect. As such, the Center for Disease Control and Prevention (CDC) and the March of Dimes recommend that all women who can become pregnant take a multi-vitamin containing 400 micrograms of folic acid before and during pregnancy.
- Other factors, such as taking certain anti-seizure medications, as well as hyperthermic events (i.e. saunas and hot tubs) have been proposed to increase the risk for a neural tube defect during pregnancy.
- In addition, females with uncontrolled diabetes are at increased risk to have a child with a neural tube defect as well as other birth defects.
Some cases of neural tube defects occur as a feature of an underlying genetic condition. In these genetic conditions, there are often additional characteristic mental and/or physical defects. For example, neural tube defects can be found in people with certain chromosome abnormalities, Meckal-Gruber syndrome or Walker-Warburg syndrome.
An evaluation by a medical geneticist may be helpful in distinguishing between an isolated NTD versus an NTD caused by an underlying genetic condition. This distinction is important for not only the health and management of the affected person but also for an accurate estimation of recurrence risk within a family.
In the absence of a known genetic cause/condition, the estimated recurrence risk for isolated neural tube defect within a family depends on different factors. These factors may include the number of affected people within the family as well as the degree of relationship (close, distant) of the affected relatives. For example, parents of one child with spina bifida typically have about a 4% chance to have another affected child, while parents of two affected children have about a 10% chance to have another affected child.
Genetic Testing
During pregnancy, a comprehensive ultrasound (a detailed fetal ultrasound) may be helpful in visualizing a neural tube defect. In addition, pregnant women are routinely offered a blood test(s) to help identify fetuses at increased risk of open neural tube defects as well as Down syndrome and Trisomy 18.
If the screening test suggests an increased risk of a neural tube defect (or other abnormality), the availability of prenatal testing can be discussed with a genetic counselor and/or a medical geneticist. For more information about screening options and genetic testing options during pregnancy see: Genetic Testing in Pregnancy.
It is always recommended to see genetic counseling with a trained genetic professional for a complete evaluation, accurate diagnosis, and discussion of the benefits and limitations of testing and recurrence risk. Use our find a genetic professional directory to locate a trained genetic professional in your area.
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