Anencephaly

*Anencephaly facts medical author: Melissa Conrad Stöppler, MD

• Anencephaly is an example of a neural tube defect, a condition that results from an error in the first weeks of embryonic development.
• In anencephaly, the bones of the skull and brain do not develop properly. Babies with anencephaly are missing large areas of the brain and have an incomplete skull.
• Anencephaly affects about 1 out of every 1,000 pregnancies, but most cases end up as miscarriages. About 1 out of every 10,000 babies in the U.S. is born with anencephaly.
• In most cases, anencephaly is sporadic, meaning it does not run in families.
• Anencephaly is not compatible with life. Most babies with anencephaly are stillborn or die within days or hours of birth.
• The exact cause of anencephaly is unknown, but it is likely the result of an interaction among several genetic and environmental factors.

Anencephaly is a condition that prevents the normal development of the brain and the bones of the skull. This condition results when a structure called the neural tube fails to close during the first few weeks of embryonic development. The neural tube is a layer of cells that ultimately develops into the brain and spinal cord. Because anencephaly is caused by abnormalities of the neural tube, it is classified as a neural tube defect (NTD).

If the neural tube fails to close properly, the developing brain and spinal cord are exposed to the amniotic fluid that surrounds the fetus. This exposure causes the nervous system tissue to break down (degenerate). As a result, people with anencephaly are missing large parts of the brain called the cerebrum and cerebellum. These brain regions are necessary for thinking, hearing, vision, emotion, and coordinating movement. The bones of the skull are also missing or incompletely formed.

Almost all babies with anencephaly die before birth or within a few hours or days after birth.

Anencephaly is one of the most common types of neural tube defect, affecting about 1 in 1,000 pregnancies. However, most of these pregnancies end in miscarriage, so the prevalence of this condition in newborns is much lower. An estimated 1 in 10,000 infants in the United States is born with anencephaly.

Anencephaly is a complex condition that is likely caused by the interaction of multiple genetic and environmental factors. Some of these factors have been identified, but many remain unknown.

Changes in dozens of genes may influence the risk of anencephaly. The best-studied of these genes is MTHFR, which provides instructions for making a protein that is involved in processing the B-vitamin folate (also called folic acid or vitamin B9). Changes in other genes related to folate processing and genes involved in the development of the neural tube have also been studied as potential risk factors for anencephaly. However, none of these genes appear to play a major role in causing the condition.

Researchers have also examined environmental factors that could contribute to the risk of anencephaly. A shortage (deficiency) of folate appears to play a significant role. Studies have shown that women who take supplements containing this vitamin before they get pregnant and very early in their pregnancy are significantly less likely to have a baby with anencephaly or a related neural tube defect. Other possible risk factors for anencephaly include diabetes mellitus, obesity, exposure to high heat (such as a fever or use of a hot tub or sauna) in early pregnancy, and the use of certain anti-seizure medications during pregnancy. However, it is unclear how these factors may influence the risk of anencephaly.

Most cases of anencephaly are sporadic, which means they occur in people with no history of the disorder in their family. A small percentage of cases have been reported to run in families; however, the condition does not have a clear pattern of inheritance. For parents who have had a child with anencephaly, the risk of having another affected child is increased compared to the risk in the general population.