Last revised by Joshua Yap on 8 May 2023

Anencephaly is the most severe form of cranial neural tube defect and is characterized by an absence of cortical tissue (although the brainstem and cerebellum may be variably present) as well as an absence of the cranial vault. The morphological spectrum within anencephaly ranges from holocrania (most severe form) to merocrania (mildest form) 2

Incidence is around 1:1000. There is a recognized female predilection with a F:M ratio of ~4:1.

As with many other malformations, a number of associated abnormalities are recognized:

In affluent nations, the diagnosis is usually made antenatally. At birth, the diagnosis is made due to obvious malformation of the cranial vault. 

It results from a failure of closure of the antral end of the neural tube which is expected to occur at approximately day 24 of embryonal life. 

Anencephaly may be sonographically detectable as early as 11 weeks. Ultrasound can be a non-invasive, cost-effective, and fast method to detect anencephaly and has an accuracy of approximately 100% at 14 weeks. Sonographic features of anencephaly include:

  • no parenchymal tissue is seen above the orbits and the calvarium is absent: parts of the occipital bone and midbrain may be present 

  • if a small amount of neural tissue is present, it is then termed exencephaly

    • this may be seen at an earlier stage

  • less than expected value for the crown-rump length (CRL)

  • a "frog eye" or "Mickey Mouse" appearance may be seen when seen in the coronal plane due to absent cranial bone and/or brain and bulging orbits

  • may show evidence of polyhydramnios: due to impaired swallowing

Not surprisingly anencephaly is incompatible with life. Folic acid therapy may reduce the risk of recurrence. There is a slight risk (~2.5%) of recurrence of a neural tube defect in future pregnancies.

Anencephaly should not be confused with hydranencephaly in which the cranial vault is present and the absence of cerebral tissue is due to an antenatal vascular insult.

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