Joseph R Siebert PhD (Dr. Siebert of the University of Washington has no relevant financial relationships to disclose.)
Originally released October 10, 1994; last updated May 7, 2020; expires May 7, 2023

This article includes discussion of anencephaly, anencephalus, cranial dysraphia, cranioschisis, exencephaly, holoacrania, and meroacrania. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.


Anencephaly is a severe and uniformly lethal malformation, resulting from incomplete closure of the anterior neural tube, in which fetuses or infants present with absent calvarial and cerebral structures. In this article, the author discusses the etiology, pathogenesis, genetic contribution, and epidemiology of anencephaly as well as approaches toward the diagnosis and prevention of this and other neural tube defects.

Key points


• Anencephaly represents one of the major forms of neural tube defect, along with encephalocele and spina bifida.


• The prenatal administration of folic acid has proven to be of considerable value in preventing neural tube defects; other vitamins may exert a preventative effect as well.


• A number of women, perhaps as many as 30%, do not benefit from prenatal folic acid, and, for these, the search for risk factors and additional treatment modalities continues.

Historical note and terminology

Anencephaly has probably always existed, although it was not described in recognizable form until the 16th century (Ballantyne 1904; Lemire et al 1978). Numerous classifications and synonyms have led to confusion about the spectrum of anencephaly. Acephalus, acrania, anencephalus, cranioschisis, and cranial or cerebral dysraphia are used commonly but not necessarily correctly. The term "acrania" is, for example, misleading in that it indicates complete absence of the cranium, a condition that is morphologically much more severe than anencephaly. Atelencephaly and aprosencephaly differ from anencephaly in that the cranial vault is covered with skin, cerebral damage probably occurring by means of an encephaloclastic process after closure of the neural tube (Siebert et al 1986). The degree to which misdiagnosis influences epidemiologic and other studies is unknown.

Anencephaly belongs to a group of congenital malformations known collectively as "neural tube defects" (which also include encephalocele and myelomeningocele). The common neuroanatomical feature in anencephaly is an open defect in the calvaria and skin. The defect varies in size but is often so large that the predominant feature is a degenerated and hemorrhagic mass of tissue lying on an essentially exposed basicranium. One classification that provides a framework for all types of anencephaly is craniorachischisis (head and spine are open), holoacrania (cranial defect extends through the foramen magnum), and meroacrania (partial cranial defect not extending to foramen magnum).

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