Neural tube defects occur when the neural tube, a structure in the developing embryo that precedes the formation of the mature central nervous system, fails to close properly during pregnancy.
During the development of the human fetus during pregnancy, a structure known as the neural tube is formed. This tube later develops into the central nervous system * in the adult. The process of neural tube formation is called neurulation (noo-roo-LAY-shun). Complete neurulation is essential to brain formation and function in adult humans. Closure defects occur during pregnancy, usually around 24 to 28 days after fertilization, or approximately three to four weeks into the pregnancy. If the neural tube does not close completely, the tube is defective. Most neural tube defects involve the ends of the tube. Two of the most common neural tube defects are anencephaly (ann-en-SEFF-ah-lee) and spina bifida (SPY-na BYE-fi-duh). Anencephaly is caused by the improper closure of the anterior (upper) end of the neural tube, resulting in a fetus that lacks some or most of the brain. Spina bifida is caused by the improper closure of the posterior (lower) end of the neural tube, resulting in the complete absence or only partial covering of the end of the spinal cord.
Spina bifida occurred in 1 to 2 cases per 1,000 births per year in the United States as of 2016. This rate is significantly lower than in previous years. The overall incidence *
Internationally, neural tube defects are among the most common types of birth defects, particularly among low-income populations. Studies done on Caucasian populations show the lowest incidence of neural tube defects in Canada and mainland Europe. The highest rates among Caucasians occur in Great Britain, especially in Scotland and Ireland. The highest overall incidence of neural tube defects in the world occurs in Northern China, the United Arab Emirates, and South America. Areas with high incidence specifically of anencephaly include Ireland, Scotland, Egypt, and New Zealand. Japan has the lowest incidence of anencephaly and other neural tube defects. Individuals of Celtic ethnic origin have the highest incidence of spina bifida.
Neural tube defects generally cause physical features that are readily identified. Anencephaly is obvious at birth, because affected babies are missing a good portion of their brain and part of the skull. The area of missing skull may or may not be covered by skin. Facial features are present and may appear relatively normal. Breathing and reflexes are present, but these babies do not have the portion of the brain responsible for consciousness; they are deaf and blind. Anencephaly causes death either during the pregnancy or shortly after birth. By contrast, symptoms of spina bifida may include a tuft of hair at the base of the spine and a visible sac in which the spinal cord protrudes and is exposed.
Outcomes for cases of neural tube defect vary. Anencephaly causes death shortly after birth. Other types of neural tube defects may cause progressive deterioration of the nervous system, resulting in a poor prognosis. Intellectual disability occurs with some types of neural tube defects. Neural tube defects involving the posterior end of the neural tube may result in paralysis of the lower limbs, as well as bladder and bowel incontinence * . Babies with neural tube defects that are severe enough to expose portions of the central nervous system may have a complication known as meningitis, an infection of the protective membranes that cover the brain and spinal cord. The type and severity of the neural tube defect determine whether the individual will survive to adulthood.
Neural tube formation takes place in the third to fourth week of pregnancy. Many pregnant women are not aware of their pregnancy until after this time, and they may engage in activities that harm or interrupt neural tube formation without realizing it. In order to reduce the risk of neural tube defects, sexually active women of childbearing age are encouraged by healthcare providers to take folic acid supplements.
Among women who have a pregnancy affected by a neural tube defect, the recurrence risk is approximately 2 to 4 percent for subsequent pregnancies. If there are multiple occurrences of neural tube defects within a family history, the risk is higher. These women should take even more folic acid than women without a family history of neural tube defects. Folic acid supplementation is responsible for preventing two-thirds of neural tube defects and is an important preventive measure.
Many different environmental factors besides folic acid deficiency play a role in the development of neural tube defects. Maternal type 1 diabetes; maternal obesity * ; environmental toxins, including some found in corn meal; poisons such as arsenic; and excessively high maternal body temperature may all contribute to neural tube defects during pregnancy. Hyperthermia, a term used to describe excessively high body temperature, may be caused by fever or recreational activities such as using a hot tub. Certain prescription drugs have been linked to the development of neural tube defects, including some antihistamines * and anticonvulsants * . Neural tube defects may also have a genetic component that predisposes certain individuals to respond more strongly to environmental factors.
Alpha-fetoprotein (AFP) is a substance made in the liver of the fetus * . AFP can be measured in the blood and amniotic fluid of a pregnant woman. The level of AFP present is used to determine the likelihood of the presence of neural tube defects. In this manner, AFP measurement can generally be used as a screening test for neural tube defects. Values of AFP approximately 2.5 times above normal are considered a sign of neural tube defects. If the mother has type 1 diabetes, the production of AFP during pregnancy is delayed and altered, and so AFP may be less useful as a screening test.
Neural tube defects may be open, meaning that a portion of the nervous system is exposed to the outside environment, placing the child at risk of infection. Because of this risk, some open neural tube defects such as spina bifida with an exposed spinal cord are treated with surgery to close the defect. In this case, surgical closure generally helps to prevent further complications of injury and infection, but it usually does not correct the other symptoms associated with the defect. Neural tube defects that affect the structures of the brain may be treated surgically to reduce physical pressure on specific brain regions. If the neural tube defect blocks the flow of cerebrospinal fluid * , a shunt (a small device containing tubes and a valve) may be surgically placed in the brain; the shunt allows fluid to drain away and thus prevents pressure buildup.
See also Birth Defects: Overview • Pregnancy • Spina Bifida
Klein, Alberich, editor. Neural Tube Defects: Prevalence, Pathogenesis and Prevention. Hauppauge, NY: Nova Science, 2013.
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Eunice Kennedy Shriver National Institute of Child Health and Human Development. “Neural Tube Defects (NTDs): Overview.” (accessed March 10, 2016).
MedlinePlus. “Neural Tube Defects.” U.S. National Library of Medicine, National Institutes of Health. https://www.nlm.nih.gov/medlineplus/neuraltubedefects.html (accessed March 10, 2016).
National Institutes of Health. 9000 Rockville Pike, Bethesda, MD 20892. Telephone: 301-496-4000. Website: http://www.nih.gov (accessed March 10, 2016).
National Library of Medicine. 8600 Rockville Pike, Bethesda, MD 20894. Toll-free: 888-346-3656 Website: http://www.nlm.nih.gov (accessed March 10, 2016).
U.S. Food and Drug Administration. 10903 New Hampshire Ave., Silver Spring, MD 20993. Toll-free: 888-463-6332. Website: http://www.fda.gov/ (accessed March 10, 2016).
* central nervous system (SEN-trul NER-vus SIS-tem) is the part of the nervous system that includes the brain and spinal cord.
* incidence means rate of occurrence.
* incontinence (in-KON-ti-nens) is loss of control of urination or bowel movements.
* incontinence (in-KON-ti-nens) is loss of control of urination or bowel movements.
* DNA or deoxyribonucleic acid (dee-OX-see-ry-bo-nyoo-klay-ik AH-sid) is the specialized chemical substance that contains the genetic code necessary to build and maintain the structures and functions of living organisms.
* obesity (o-BEE-si-tee) is an excess of body fat. People are considered obese if they weigh more than 30 percent above what is healthy for their height.
* antihistamines (an-tie-HIS-tuhmeens) are drugs used to combat allergic reactions and relieve itching.
* anticonvulsants (an-tie-konVUL-sents) are medications that affect the electrical activity in the brain and are given to prevent or stop seizures.
* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.
* cerebrospinal fluid (seh-ree-broSPY-nuhl) is the fluid that surrounds the brain and spinal cord.