Anencephaly and the anterior neuropore
Anencephaly is a defect of the embryonic tube that becomes the brain and spinal cord (the neural tube): its head-end opening, the anterior neuropore, fails to close.
Remember
When the anterior neuropore at the head end stays open the result is anencephaly, whereas an open posterior neuropore at the tail end gives spina bifida (an open defect of the spine).
Memory aid
If the cranial neuropore stays open the cranium is missing, and if the caudal neuropore stays open the result is spina bifida.
The head-end opening goes with anencephaly and the tail-end opening goes with spina bifida.
In the third and fourth weeks after fertilisation, during neurulation, the neural plate, a flat sheet of tissue on the embryo's back, folds up into the neural tube. The tube closes from the middle towards both ends, so for a time it has an opening at each end. These openings are the neuropores: the anterior (cranial) one at the head end and the posterior (caudal) one at the tail end.
Anencephaly is the result when the anterior neuropore fails to close. Because the head end stays open, the brain and the cranial vault (the skull vault) above it do not form properly, and brain tissue is left exposed. Anencephaly is typically incompatible with life, and miscarriage or stillbirth commonly follow.
Because the defect is open, alpha-fetoprotein (a protein that normally stays within the foetus) leaks out, so the level in the mother's blood rises on screening. This test is not specific, because defects in other systems can also raise it. A raised alpha-fetoprotein does not by itself tell you which end of the tube is affected, so the ultrasound finding supplies the location.
Closure of the tube is completed at around 4 weeks after fertilisation, when many women do not yet know they are pregnant. Neural tube defects are less common when women take folic acid for 3 months before conception and through the first trimester (early pregnancy).
Key facts
- Cause
- failure of the anterior neuropore, the head-end opening of the neural tube, to close.
- On scan
- the cranial vault is absent and brain tissue is exposed.
- Outcome
- typically incompatible with life, with miscarriage or stillbirth commonly following.
- Screening
- a raised alpha-fetoprotein in the mother's blood supports an open defect, but it is not specific.
- The other end
- failure of the posterior (caudal) neuropore to close causes spina bifida.
- Prevention
- folic acid started 3 months before conception and continued through the first trimester lowers the risk of these defects.
Exam traps
- Failure of the posterior neuropore, the tail-end opening, does not explain a missing cranial vault, because its failure gives spina bifida, an open defect of the spine.
- Abnormal migration of neural crest cells does not cause an open neural tube, because these cells migrate away from the tube to form many tissues, and their failure causes syndromes.
- Degeneration after the anterior neuropore has closed normally is not anencephaly, because anencephaly is a primary failure of closure, which is why tissue is exposed.
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