Anencephaly

Anencephaly is the most severe form of cranial neural tube defect (NTD) and is characterised 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. 

Epidemiology

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

Clinical presentation

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

Pathology

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. 

Associations

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. omphalocele
• urinary tract abnormalities: hydronephrosis most common

Markers

• maternal serum alpha-fetoprotein (MSAFP) levels are highly elevated (x2.5 MoM (multiples of the median)): of all the neural tube defects, anencephaly usually gives the highest elevation in MSAFP ⁷

Radiographic features

Antenatal ultrasound

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 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 the risk of recurrence. There is a slight risk (~2.5%) in recurrence of a neural tube defect in future pregnancies.

Differential diagnosis

• amniotic band syndrome: if the band passes through the head
• severe microcephaly

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.

See also

• exencephaly
• acrania