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The Neural Tube
Classification and external resources
OMIM 182940 301410
eMedicine neuro/244 ped/2805
MeSH D009436

Neural tube defects (NTDs) are one of the most common birth defects, occurring in approximately one in 1,000[1] live births in the United States. An NTD is an opening in the spinal cord or brain that occurs very early in human development. In about the 3rd or 4th weeks of pregnancy, specialized cells on the dorsal side of the fetus begin to fuse and form the neural tube. When the neural tube does not close completely, an NTD develops.

Contents

Types of NTDs

There are two types of NTDs: open, which are more common, and closed. Open NTDs occur when the brain and/or spinal cord are exposed at birth through a defect in the skull or vertebrae (back bones). Examples of open NTDs are anencephaly, encephaloceles, hydranencephaly, iniencephaly, schizencephaly,and spina bifida. Rarer types of NTDs are called closed NTDs. Closed NTDs occur when the spinal defect is covered by skin. Common examples of closed NTDs are lipomyelomeningocele, lipomeningocele, and tethered cord.

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Anencephaly

Anencephaly (without brain) is a neural tube defect that occurs when the head end of the neural tube fails to close, usually during the 23rd and 26th days of pregnancy, resulting in an absence of a major portion of the brain and skull. Infants born with this condition are born with out the main part of the forebrain-the largest part of the cerebrum. Infants born with this condition are usually blind, deaf and unconscious. The lack of a functioning cerebrum will ensure that the infant will never gain consciousness. Infants are either stillborn or usually die within a few hours or days after birth.

Encephaloceles

Encephaloceles are a neural tube defect that characterize by sac-like protrusion of the brain though through the opening in the skull. The defect is caused by the incomplete closure of the cranium during development.

Hydranencephaly

Hydranencephaly is a condition in which the cerebral hemispheres are missing and instead filled with sacs filled with cerebrospinal fluid.

Iniencephaly

This is a rare neural tube defect that results in extreme bending of the head to the spine. The diagnosis can usually be made on antenatal ultrasound scanning but if not will undoubtedly be made immediately after birth because the head is bent backwards and the face looks upwards. Usually the neck is absent. The skin of the face connects directly to the chest and the scalp connect to the upper back. The infant usually will not survive more than a few hours.

Spina Bifida

Spina Bifida is further divided into two subclasses, Spina Bifida Cystica and Spina Bifida Occulta.

Spina Bifida Cystica

This includes meningocele and myelomeningocele. Meningocele is less severe and is characterized by herniation of the meninges, but not the spinal cord, through the opening in the spinal canal. Myeolomeningocele involves herniation of the meninges as well as the spinal cord through the opening.[2]

Spina Bifida Occulta

In this type of neural tube defect, the meninges do not herniate through the opening in the spinal canal.[2]

Causes

Neural tube defects are cause by multiple genes and environmental factors which are not completely understood.[3] Although more than 200 genes are known to cause NTDs in mice, the cause of NTDs in human is still poorly understood.[4] The failure to identify a single gene factor suggests that the major causes of human NTDs are from the summation of genetic, environmental and nutritional factors.[5] [6] Research has shown that women with folic acid deficiencies also have a higher chance of having a child with a neural tube defect, but this is only one factor. Taking folic acid does not completely negate the risk of neural tube problems, but it does significantly reduce the risk. Folic acid deficiency leads to hyper-homocysteinemia,which in turn results in neural tube defects.[7]

Folic acid pathway

Folic acid and vitamin B12 are very important in reducing the occurrences of NTDs. [8] Folate is required for production and maintenance of new cells., for DNA synthesis and RNA synthesis. Folate is needed to carry one carbon groups for methylation and nucleic acid synthesis. Vitamin B12 is also an important receptor in the folic acid biopathway such that studies have shown deficiency in vitamin B12 contributes to risk of NTDs as well. [9]

Other Causes

Other potential causes can include folate antimetabolites (such as methotrexate), maternal diabetes, maternal obesity, mycotoxins in contaminated corn meal, arsenic, and hyperthermia in early development[10].[11][12] Studies have shown that both maternal cigarette smoking and maternal exposure to secondhand smoke increased the risk for neural tube defects in offspring. A mechanism by which maternal exposure to cigarette smoke could increase NTD risk in offspring is suggested by several studies which show an association between cigarette smoking and homocysteine levels. The study suggests that cigarette smoke including secondhand exposure is not only hazardous to the mother but also mayalso interfere with neural tube closure in the developing embryo.[13]

Detection

Tests for neural tube defects include ultrasound examination and measurement of maternal serum alpha-fetoprotein(MSAFP). Amniotic fluid alpha-fetoprotein(AFAFP) and amniotic fluid acetylcholinesterase(AFAChE) tests are also used to confirming if ultrasound screening indicates a positive risk.[14] Often, these defects are apparent at birth, but occult defects may be not diagnosed until much later in life. An elevated MSAFP measured at 16-18 weeks gestation is a good predictor of neural tube defects.

Prevention

In 1996, the United States Food and Drug Administration published regulations requiring the addition of folic acid to enriched breads, cereals, flour and other grain products. [15]It is important to note that during the first four weeks of pregnancy (when most women do not even realize that they are pregnant), adequate folic intake is essential for proper operation of the neurulation process. Therefore, women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate-rich foods to reduce the risks of serious birth defects. [16][17][18] In Canada, mandatory fortification of selected foods with folic acid has been shown to reduce the incidence of neural tube defects by 46%[19].

Women who could become pregnant are advised to get 400 micrograms of folic acid daily. Women who are pregnant should receive 1.0 mg daily, and women who have previously given birth to a child with a neural tube defect should get 4.0 mg daily.[20]

Treatment

Treatments of NTDs depends on the severity of the complication. No treatment is available for anencephaly because the infants usually do not survive more than few hours. Aggressive surgical management has improved survival and functions of infants with spina bifida and meningoceles and mild myelomeningoceles. The success of surgery often depends on the amount of brain tissue involved in the encephalocele. The goal of treatment for NTDs is to allow the individual to achieve the highest level of function and independence.

See also

References

  1. ^ http://www.nichd.nih.gov/health/topics/neural_tube_defects.cfm/ through http://www.nichd.nih.gov.
  2. ^ a b Le, Tao; Bhushan, Vikas; Vasan, Neil (2010). First Aid for the USMLE Step 1: 2010 20th Anniversary Edition. USA: The McGraw-Hill Companies, Inc.. pp. 127. ISBN 978-0-07-163340-6. 
  3. ^ Greene, N. D. E., Stanier, P., & Copp, A. J. (2009). Genetics of human neural tube defects. [Review]. Human Molecular Genetics, 18, R113-R129.
  4. ^ Dunlevy, L. P. E., Chitty, L. S., Burren, K. A., Doudney, K., Stojilkovic-Mikic, T., Stanier, P., et al. (2007). Abnormal folate metabolism in foetuses affected by neural tube defects. [Article]. Brain, 130, 1043-1049.
  5. ^ Finglas, P. M., Wright, A. J. A., Wolfe, C. A., Hart, D. J., Wright, D. M., & Dainty, J. R. (2003). Is there more to folates than neural-tube defects?
  6. ^ Dunlevy, L. P. E., Chitty, L. S., Burren, K. A., Doudney, K., Stojilkovic-Mikic, T., Stanier, P., et al. (2007). Abnormal folate metabolism in foetuses affected by neural tube defects. [Article]. Brain, 130, 1043-1049.
  7. ^ Mosley, B. S., Cleves, M. A., Siega-Riz, A. M., Shaw, G. M., Canfield, M. A., Waller, D. K., et al. (2009). Neural Tube Defects and Maternal Folate Intake Among Pregnancies Conceived After Folic Acid Fortification in the United States. [Article]. American Journal of Epidemiology, 169(1), 9-17.
  8. ^ Molloy, A. M., Kirke, P. N., Troendle, J. F., Burke, H., Sutton, M., Brody, L. C., et al. (2009). Maternal Vitamin B-12 Status and Risk of Neural Tube Defects in a Population With High Neural Tube Defect Prevalence and No Folic Acid Fortification. [Article]. Pediatrics, 123(3), 917-923.
  9. ^ Li, F., Watkins, D., & Rosenblatt, D. S. (2009). Vitamin B-12 and birth defects. [Review]. Molecular Genetics and Metabolism, 98(1-2), 166-172.
  10. ^ eMedicine url=http://www.emedicine.com/NEURO/topic244.htm
  11. ^ Suarez, L., Brender, J. D., Langlois, P. H., Zhan, F. B., & Moody, K. (2007). Pregnant women taking medication for epilepsy have a higher chance of having a child with a neural tube defect. Maternal exposures to hazardous waste sites and industrial facilities and risk of neural tube defects in offspring. [Article]. Annals of Epidemiology, 17(10), 772-777.
  12. ^ Zhou, F. C., Fang, Y., & Goodlett, C. (2008). Peptidergic agonists of activity-dependent neurotrophic factor protect against prenatal alcohol-induced neural tube defects and serotonin neuron loss. [Article]. Alcoholism-Clinical and Experimental Research, 32(8), 1361-1371.
  13. ^ Suarez, L., Felkner, M., Brender, J. D., Canfield, M., & Hendricks, K. (2008). Maternal exposures to cigarette smoke, alcohol, and street drugs and neural tube defect occurrence in offspring. [Article]. Maternal and Child Health Journal, 12(3), 394-401.
  14. ^ Milunsky A, Alpert E. Results and benefits of a maternal serum alpha-fetoprotein screening program. JAMA. 1984; 252: 1438–1442. [PubMed]
  15. ^ Daly S, Mills JL, Molloy AM, Conley M, Lee YJ, Kirke PN, Weir DG, Scott JM (1997). "Minimum effective dose of folic acid for food fortification to prevent neural-tube defects". Lancet 350 (9092): 1666–9. doi:10.1016/S0140-6736(97)07247-4
  16. ^ Greene, N. D. E., Stanier, P., & Copp, A. J. (2009). Genetics of human neural tube defects. [Review]. Human Molecular Genetics, 18, R113-R129.
  17. ^ Milunsky A, Jick H, Jick SS, Bruell CL, MacLaughlin DS, Rothman KJ, Willett W (1989). "Multivitamin/folic acid supplementation in early pregnancy reduces the prevalence of neural tube defects". Journal of the American Medical Association 262 (20): 2847–2852. doi:10.1001/jama.262.20.2847
  18. ^ Goh YI & Koren G (2008) "Folic acid in pregnancy and fetal outcomes", J. Obstet. Gynaecol. 28:3–13.
  19. ^ De Wals P, Tairou F, Van Allen MI, et al. (2007). "Reduction in neural-tube defects after folic acid fortification in Canada". N Engl J Med 357 (2): 135–142. doi:10.1056/NEJMoa067103. PMID 17625125. http://content.nejm.org/cgi/content/short/357/2/135?query=TOC. 
  20. ^ Recommendations for the Use of Folic Acid... Accessed 22 Nov 2009.

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