Anencephaly is the most severe form of cranial neural tube defect (NTD) and is characterised by absence of cortical tissue (although brainstem and cerebellum may be variably present) as well as 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 recognised female predilection with a F:M of ~4:1.
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 antral end of neural tube which is expected to occur at approximately 24 days of fetal life.
As with many other malformations, a number of associated abnormalities are recognised:
- other neural tube defects: spina bifida (especially cervical)
- congenital heart defects
- cleft lip/palate
- diaphragmatic hernia(s)
- spinal dysraphism
- skeletal anomalies: e.g. clubfeet
- gastrointestinal abnormalities: e.g. omphalocoele
- urinary tract abnormalities: hydronephrosis most common
- maternal serum alpha feto protein (MSAFP) levels are highly elevated (x 2.5 MoM): of all the neural tube defects, anencephaly usually gives the highest elevation in MSAFP 7
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 approximating 100% at 14 weeks
- no parenchymal tissue is seen above the orbits and calvarium is absent: parts of the occipital bone and mid brain 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 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/brain and bulging orbits.
- may show evidence of polyhydramnios: due to impaired swallowing
Treatment and prognosis
Not surprisingly anencephaly is incompatible with life. Folic acid therapy may reduce risk of recurrence. There is a slight risk (~2.5%) in recurrence of a neural tube defect in future pregnancies.
Anencephaly should not be confused with hydranencephaly in which the cranial vault is present and absence of cerebral tissue is due to antenatal vascular insult.
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