Anencephaly

Definition

• Anencephaly is a developmental disorder in which the skull vault is absent and most or all of the brain tissue is lacking; the brainstem, cerebellum and diencephalon are usually present.
• It belongs to the neural tube defects and results from failure of closure of the rostral end of the neural tube; failure of the caudal end causes spina bifida.

Classification

• A distinction is made between meroacrania (foramen magnum not involved), holoacrania (defect extending beyond the foramen magnum) and holoacrania with rachischisis when spina bifida coexists.

Aetiopathogenesis

Aetiology and risk factors

• The neural tube forms in the fourth week of development; the cranial neuropore closes around day 25, the caudal neuropore about two days later.
• If the rostral neuropore remains open, exencephaly develops first; without a protective calvarium the brain tissue degenerates while the brainstem is preserved (acrania–exencephaly–anencephaly sequence).
• The aetiology is multifactorial; one of the most important nutritional factors is folate deficiency, together with antiepileptic agents and excess vitamin A intake via folate antagonism; valproate is considered the most teratogenic.
• Further maternal factors are diabetes, obesity, exposure to toxins, a positive family history of neural tube defects and high fever or hyperthermia (sauna, hot tub) in early pregnancy.

Clinical features

Clinical features and complications

• Anencephaly is incompatible with life: mortality is 100 per cent, in utero or within hours to days after birth.
• Additional structural anomalies are present in 12 to 25 per cent.

Diagnosis

Typical imaging findings

• Ultrasound: In the sagittal plane the trunk is normally developed, but above the orbits the echogenic outline of the vault is missing. Brain tissue appears irregular and without a surrounding bony shell.
• Ultrasound: Above the fetal facial plane the bony cranial vault is completely absent. Brain tissue lies unprotected on the skull base and floats in the amniotic fluid; the head contour ends abruptly above the orbits.

• On first-trimester ultrasound anencephaly is now detected in virtually all cases; transvaginal imaging shows acrania and a variable degree of brain degeneration.
• Gestational age derived from crown–rump length is on average about two weeks less than expected in affected fetuses.
• In anencephaly the cerebral hemispheres are replaced by masses of connective and vascular tissue.
• Raised maternal serum alpha-fetoprotein suggests open spina bifida, anencephaly or an abdominal wall defect.

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Further reading (selection)

1. StatPearls: Embryology, Anencephaly (NCBI Bookshelf)
2. Rom J Morphol Embryol 2020: The etiopathogenic and morphological spectrum of anencephaly – a comprehensive review (PMC, open access)
3. MSD Manual Professional Edition: Noninvasive Prenatal Fetal Screening Tests

Cross-references