Open Neural Tube Defects (spina bifida)

What are open neural tube defects?
Is it possible to test for open neural tube defects during pregnancy?

What are open neural tube defects?
Open neural tube defects, or ONTDs, occur in 1 of every 500 to 1000 births.^1,2 ONTDs occur because the baby's spine or skull does not close during fetal development. Normally, the bones and tissues in the spinal column form around the spinal cord to protect it. In babies with an ONTD, the bones and tissue do not fuse together over the spine, leaving the spinal cord exposed to the surrounding environment. Because the nerves in the spinal cord are not protected by the bones and the skin, damage to important nervous tissue can occur.^1,3 Children born with an ONTD have a wide range of medical consequences. Some children have only minor bladder control problems, while others may not be able to walk. Some children who have an ONTD may have mental retardation. The most severe ONTD is called anencephaly. In anencephaly the brain fails to form and the baby is unable to survive.^1,3

Is it possible to test for open neural tube defects during pregnancy?
Because ONTDs can cause such severe problems for babies, prenatal screening is generally recommended for all pregnant women. The tests are referred to as maternal serum screening tests and detect about 80% of open neural tube defects.^1,2 Therefore, 15% of pregnancies with ONTDs will not be detected.¹ A maternal serum screening test requires a blood sample from the pregnant woman. The best detection rate for ONTDs occurs between 16 and 18 weeks of pregnancy.¹ Ultrasound imaging can also be used to screen for ONTDs. Having both maternal serum screening and ultrasound imaging improves detection.

Amniocentesis can also be performed to look for the presence of an ONTD. Amniocentesis requires a sample of amniotic fluid that is used to measure the amount of two substances: alpha-fetoprotein and acetylcholinesterase.⁴ This test has a 98% detection rate,⁴ although amniocentesis does increase the chance of miscarriage by approximately 0.5% (1/200).⁵

A specific combination of inherited and environmental factors must be present at the same time for an ONTD to occur.¹ Many of the inherited or environmental factors have not been identified.⁶ Deficiencies in folic acid are also associated with ONTDs. The American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics (ACMG) recommend all women contemplating pregnancy take at least 0.4 mg of folic acid per day prior to pregnancy and throughout the first trimester.¹ If you have a family history of ONTD, the recommended dosage may be higher and you should consult your physician or genetic counselor.¹

More information about ONTDs is available through the Spina Bifida Association of America at www.sbaa.org/.

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References

1. American College of Obstetricians and Gynecologists. Maternal Serum Screening. ACOG Educational Bulletin. Washington, DC: ACOG; September 1996:228.
2. American College of Obstetricians and Gynecologists. Prenatal Detection of Neural Tube Defects. ACOG Technical Bulletin. Washington, DC; October 1982:67.
3. March of Dimes Birth Defects Foundation. Folic Acid. Wilkes-Barre, Pa; September 2000.
4. Wald N, Cuckle H, Nanchahal K. Amniotic fluid acetylcholinesterase measurement in the prenatal diagnosis of open neural tube defects: second report of the collaborative acetylcholinesterase study. Prenat Diag. 1989; 9:813-829.
5. Centers for Disease Control and Prevention. Chorionic Villus Sampling and Amniocentesis: Recommendations for Prenatal Counseling. MMWR. 1995;44(No.RR-9)1-11.
6. American College of Medical Genetics. Statement on folic acid: fortification and supplementation: Policy Statement. Bethesda, Md: ACMG; Nov/Dec 1998.

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