Acrania, a neural tube defect, is a condition characterized by the partial or complete absence of the fetal skull. The face and base of the skull are present, but due to the malformation of the skull, the brain is exposed. Acrania is considered the early gestation precursor to anencephaly. The progression from exposed brain tissue that appears normal (acrania) to no recognizable brain tissue (anencephaly) is called the fetal acrania-anencephaly sequence.
Acrania can be detected in the first trimester via ultrasound due to the visible lack of skull formation, echogenic amniotic fluid, and exposed brain tissue. Acrania is also detectable through maternal serum screening in the second trimester. Acrania has an estimated incidence rate of 3 in 10,000 second trimester pregnancies. Acrania is considered uniformly fatal in the perinatal period.
Acrania is not typically associated with aneuploidy, or chromosomal abnormalities, but it can be associated with other anomalies such as amniotic band syndrome, body-stalk anomaly, cleft lip and/or palate, congenital diaphragmatic hernia, congenital heart defects, omphalocele, other neural tube defects, and pentalogy of Cantrell.
The recurrence risk depends on the underlying cause. In cases of isolated acrania, or acrania with no known cause, the recurrence risk ranges from 2% to 5%. Parental genetic testing is recommended prior to subsequent pregnancies.
Getting adequate amounts of folate and folic acid can prevent neural tube defects like acrania. All women of childbearing age should get 400 micrograms (mcg) of folic acid daily, prior to and throughout pregnancy. For women who have experienced a prior pregnancy affected by neural tube defects, a high daily dose of 4 milligrams (mg) of folic acid is recommended, staring at least four weeks prior to conception.