Anencephaly is one of a group of malformations of the central nervous system collectively called neural tube defects. Anencephaly is readily apparent at birth because of the absence of the skull and scalp and with exposure of the underlying brain. The condition is also called acrania (absence of the skull) and acephaly (absence of the head). In its most severe form, the entire skull and scalp are missing. In some cases, termed "meroacrania" or "meroanencephaly," a portion of the skull may be present. In most instances, anencephaly occurs as an isolated birth defect with the other organs and tissues of the body forming correctly. In approximately 10% of cases, other malformations coexist with anencephaly.
As an isolated defect, anencephaly appears to be caused by a combination of genetic factors and environmental influences that predispose to faulty formation of the nervous system. The specific genes and environmental insults that contribute to this multifactorial causation are not completely understood. It is known that nutritional insufficiency, specifically folic acid insufficiency, is one predisposing environmental factor and that mutations of genes involved in folic acid metabolism are genetic risk factors. The recurrence risk after the birth of an infant with anencephaly is 3-5%. The recurrence may be anencephaly or another neural tube defect, such as spina bifida.
Anencephaly occurs in all races and ethnic groups. The prevalence rates range from less than one in 10,000 births (European countries) to more than 10 per 10,000 births (Mexico, China).
Signs and symptoms
Anencephaly is readily apparent at birth because of exposure of all or part of the brain. Not only is the brain malformed, but it is also damaged because of the absence of the overlying protective encasement. In about 10% of cases of anencephaly, other malformations are also present.
Anencephaly is diagnosed by observation. Prenatal diagnosis may be made by ultrasound examination after 12 to 14 weeks' gestation. Prenatal diagnosis of anencephaly can also be detected through maternal serum alpha-fetoprotein screening. The level of alpha-fetoprotein in the maternal blood is elevated because of the leakage of this fetal protein into the amniotic fluid.
Treatment and management
No treatment is indicated for anencephaly. Affected infants are stillborn or die within the first few days of life. The risk for occurrence or recurrence of anencephaly may be reduced by half or more by the intake of folic acid during the months immediately before and after conception. Natural folic acid, a B vitamin, may be found in many foods (green leafy vegetables, legumes, orange juice, liver). Synthetic folic acid may be obtained in vitamin preparations and in certain fortified breakfast cereals. In the United States, all enriched cereal grain flours have been fortified with folic acid.
Anencephaly is uniformly fatal at birth or soon thereafter.
Czeizel, A. E., and I. Dudas. "Prevention of the first occurrence of neural tube defects by preconceptional vitamin supplementation." New England Journal of Medicine 327 (1992): 1832-1835.
Medical Research Council Vitamin Study Research Group. "Prevention of neural tube defects: results of the Medical Research Council vitamin study." Lancet 338 (1991): 131-137.
Sells, C. J., and J. G. Hall, Guest Editors. "Neural Tube Defects." Mental Retardation and Developmental Disabilities Research Reviews. 4, no. 4, Wiley-Liss, 1998.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. email@example.com. <http://www.modimes.org>.
National Birth Defects Prevention Network. Atlanta, GA (770) 488-3550. <http://www.nbdpn.org>.
Roger E. Stevenson, MD