Definition of Anencephaly

Reviewed on 10/18/2021

Anencephaly is a neural tube defect (NTD) that occurs when the cephalic (head) end of the neural tube fails to close, usually between the 23rd and 26th days of pregnancy, resulting in the absence of a major portion of the brain, skull, and scalp. Infants with this disorder are born without a forebrain, the largest part of the brain consisting mainly of the cerebrum, which is responsible for thinking and coordination. The remaining brain tissue is often exposed; it is not covered by bone or skin. Infants born with anencephaly are usually blind, deaf, and unconscious. Although some individuals with anencephaly may be born with a rudimentary brainstem, the lack of a functioning cerebrum permanently rules out the possibility of ever gaining consciousness. Reflex actions such as respiration (breathing) and responses to sound or touch may occur. The disorder is one of the most common disorders of the fetal central nervous system. Approximately 1,000 to 2,000 babies are born with anencephaly each year in the U.S. The disorder affects females more often than males. There is no cure or effective treatment for anencephaly and the prognosis (outlook) for affected individuals is very poor. Most infants with anencephaly do not survive infancy. If the infant is not stillborn, then he or she will usually die within a few hours or days after birth. Anencephaly can often be diagnosed before birth through screening of the mother's blood for the level of alpha-fetoprotein (AFP) and an ultrasound examination. The addition of ample folic acid to the diet of women of child-bearing age significantly reduces the incidence of neural tube defects.


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Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.

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