Anencephaly is a disorder involving the incomplete development of major parts of the brain. Anencephaly is classified as a neural tube defect (NTD), and that term refers to the incomplete development of the brain, spinal cord, and/or their protective coverings. The neural tube is a narrow sheath that is supposed to fold and to close during the third and fourth weeks of pregnancy, in order to form the brain and spinal cord of the embryo. Anencephaly occurs when the head (cephalic) end of this neural tube fails to close, resulting in the failure of major portions of brain, skull and scalp to form.
Infants with anencephaly are born without both a forebrain (the front part of the brain) and a cerebrum (the thinking and coordinating part of the brain). Often the remaining rudimentary brain tissue may be exposed, without the protective covering of either bone or skin. Although reflex actions such as breathing and responses to touch or sound may occur, gaining consciousness is almost invariably ruled out.
During pregnancy, the presence of excess fluid in the uterus with the fetus (polyhydramnios) may be a nonspecific sign of a problem pregnancy. Anencephaly can be seen before birth using any one of several imaging techniques. In addition, affected pregnancies show high levels of a specific fetal protein (called alpha fetoprotein) in maternal serum or amniotic fluid.
The condition is characterized by the absence of the skull and parts of the brain (cerebral hemispheres and cerebellum). Abnormalities of facial features secondary to the absent skull are common and variable. Anencephaly is usually (~80%) an isolated birth defect not associated with other malformations or anomalies.
Anencephaly as well as other types of neural tube defects is inherited as a multifactorial condition suggesting that multiple genes are involved interacting with environmental agents and chance events. Anencephaly is also a feature of some chromosomal disorders such as trisomy 18. These are usually sporadic and not familial.
The recurrence risk for individuals with one prior pregnancy affected with a neural tube defect estimated to be 3-4%, and to a large degree the recurrent lesion, whether anencephaly or spina bifida, tends to be concordant with the first.
Numerous epidemiologic studies have demonstrated that folic acid supplementation prior to conception and during the first trimester can reduce the prevalence of spina bifida and anencephaly by 50-70%.
Although more females than males are born with anencephaly, it is likely that this is the result of greater loss of affected male fetuses early in pregnancy. A significant proportion of anencephalic fetuses are stillborn or are spontaneously aborted.
Beginning in January 1998, the US Food and Drug Administration mandated the addition of folic acid to all enriched cereal grain products. Post-fortification, in the United States, the average birth prevalence of anencephaly is about 1 per 5000 births. Internationally, the birth prevalence varies and is greatly influenced by the use of prenatal diagnosis and pregnancy termination.
Symptoms of the following disorders can be similar to those of Anencephaly. Comparisons may be useful for a differential diagnosis:
"Spina Bifida" is a term meaning "open (or nonfused) spine". Different forms of the disorder vary in severity, ranging from mild to severe. In spina bifida, the neural tube that will form the spinal cord fails to close completely. Part of the contents of the spinal canal can protrude or herniated through areas where the bony spine (vertebrae) are absent. This produces a sac filled with covering membranes with or without nerve tissue (meningocele or meningomyelocele). Children with spina bifida can often be helped by surgery, and intellectual ability is not usually affected. (For more information, choose "Spina Bifida " as your search term in the Rare Disease Database.)
The U.S. Public Health Service advises women of childbearing age to take 0.4 mg of folic acid daily, either through diet or low dose supplements. Women are urged not to take more than 1.0 mg of folic acid daily unless advised by a physician because high doses of folic acid may mask other vitamin deficiencies. Women who have had a previous NTD-affected pregnancy are advised to consume 4 mg of folic acid beginning 30 days before conception through the first trimester under the care of their physician.
Usually infants with anencephaly do not survive more than a few days or weeks. Treatment is symptomatic and supportive
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
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REVIEW ARTICLES
Barr M Jr, The long arms of anencephaly: A refutation. Birth Defects Res A Clin Mol Teratol. 2009 May 13.
Norem CT, Schoen EJ, Walton DL, et al. Routine ultrasonography compared with maternal serum alpha-fetoprotein for neural tube defect screening. Obstet Gynecol. 2005;106:747-52.
Drugan A, Weissman A, Evans MI. Screening for neural tube defects. Clin Perinatol. 2001;28:279-87.
Sen C. The use of first trimester ultrasound in routine practice. J Perinat Med. 2001;29:212-21.
Kooper AJ, de Bruijn D, van Ravenwaaij-Arts CM, et al. Fetal anomaly scan potentially will replace routine AFAFP assays for the detection of neural tube defects. Prenat Diagn. 2007;27:29-33.
Harris MJ, Juriloff DM. Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects. Birth Defects Res A Clin Mol Teratol. 2007;79:187-210. Review.
JOURNAL ARTICLES
Green NS. Folic acid supplementation and prevention of birth defects. J Nutr. 2002;132(8 suppl):2356S-2360S.
O'Rahilly R, Muller F. The two sites of fusion of the neural folds and the two neuropores in the human embryo. Teratology. 2002;65:162-70.
Honein MA, Paulozzi LJ, Mathews TJ, et al. Impact of folic acid fortification of the US food supply on the occurrence of neural tube defects. JAMA. 2001;285:2981-86.
Cragan JD, Roberts HE, Edmonds LD, et al. Surveillance for anencephaly and spina bifida and prenatal diagnosis – United States, 1985-1994. MMWR. 1995;44(SS-4):1-13.
Boulet SL, Yang Q, Mai C, et al. Trends in the postfortification prevalence of spina bifida and anencephaly in the United States. Birth Defects Res A Clin Mol Teratol. 2008;82:527-32.
Farley TL. A reproductive history of mothers with spina bifida offspring--a new look at old issues. Cerebrospinal Fluid Res. 2006:3:10.
Boyd PA, Devigan C, Khoshnood B, et al.Survey of prenatal screening policies in Europe for structural malformations and chromosome anomalies, and their impact on detection and termination rates for neural tube defects and Down's syndrome. BJOG. 2008;115:689-96.
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