Anencephaly

Anencephaly is an extreme form of neural tube defect (NTD) in which a baby is born without parts of the brain and skull. Acrania, or absence of the skull vault, leads to exencephaly, or protrusion of the brain. The cerebral hemispheres are exposed to amniotic fluid, which results in destruction of tissues.

The clinical features of anencephaly are:

• absence of forebrain;
• absence of cerebrum;
• absence of bony covering over the back of the head;
• folding of the ears;
• cleft palate;
• congenital heart anomalies; and
• presence of some basic reflexes (however, without the cerebrum there can be no consciousness and the baby cannot survive).

Anencephaly is a complex condition that is likely caused by the interaction of multiple genetic and environmental factors. Some of these genes have been identified and include:

• MTHFR, which provides instructions for making a protein that is involved in processing folate (also called vitamin B9). While a deficiency in this vitamin is an established risk factor for NTDs, there are many others that can contribute.
• MARCKSL1 (also known as MacMARCKS), which is active during neural tube development.
• FOXN1, a gene that is important for the proper development of the central nervous system.

However, no genes appear to play a major role in causing the condition and the exact cause remains unknown.

Most cases of anencephaly are sporadic, which means they occur in people with no history of the condition 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 probability of having another affected child is increased compared with the probability in the general population. After one child with anencephaly, the recurrence risk is 5%, rising to 10% if the parents have had two affected children. This is similar to other NTDs.

Anencephaly is usually diagnosed during routine pregnancy scans and can typically be identified at the first trimester or dating scan. Prognosis is very poor, and termination of pregnancy should be discussed. Almost all babies born with anencephaly will die shortly after birth.

If the family chooses to continue the pregnancy, routine ongoing fetal medicine appointments are unnecessary; however, these pregnancies are often complicated by significant polyhydramnios at later gestations, which may warrant further review.

Delivery planning should be made based on maternal risks, and vaginal delivery is not contraindicated. Early induction of labour may be considered in order to facilitate a vaginal birth.

There is no known cure or standard treatment for anencephaly.

Case reports of babies with anencephaly becoming organ donors after delivery have been examined, and this is an option that could be discussed in a sensitive manner with parents.

For clinicians

• Fetal Medicine Foundation: Acrania
• Medline Plus (National Library of Medicine): Anencephaly
• NHS England: National Genomic Test Directory

For patients

• Gov.uk: Anencephaly: Information for parents
• Shine: Anencephaly