Anencephaly
Anencephaly is a serious neural tube defect that prevents the normal development of the brain and skull, typically resulting in death before or shortly after birth.
Overview
Anencephaly occurs when the neural tube—a layer of cells that develops into the brain and spinal cord—fails to close during the first few weeks of embryonic development. Because the tube remains open, the developing nervous system is exposed to amniotic fluid, causing the tissue to degenerate. Consequently, infants with anencephaly are missing large parts of the cerebrum and cerebellum, regions necessary for thinking, hearing, vision, emotion, and movement. The bones of the skull are also missing or incompletely formed.
Frequency
Anencephaly affects approximately 1 in 1,000 pregnancies, though many of these end in miscarriage. The prevalence in newborns is estimated at 1 in 10,000 infants in the United States.
Causes
Anencephaly is a complex condition caused by the interaction of multiple genetic and environmental factors. While changes in dozens of genes, such as MTHFR (involved in processing folate or vitamin B9), have been studied, no single gene plays a major role in causing the condition.
Environmental factors, particularly folate deficiency, play a significant role. Taking folic acid supplements before and during early pregnancy significantly reduces the risk of neural tube defects. Other potential maternal risk factors include:
- Diabetes mellitus
- Obesity
- Exposure to high heat (e.g., fever, hot tubs, or saunas) in early pregnancy
- Use of certain anti-seizure medications
Inheritance
Most cases are sporadic, occurring in families with no history of the disorder. While a small percentage of cases run in families, there is no clear pattern of inheritance. Parents who have had a child with anencephaly have an increased risk of having another affected child compared to the general population.