In normal brain development the prosencephalon (forebrain) gives rise to the telencephalon and diencephalon which develop into the cerebral hemispheres and thalami respectively. Disruption of this process may lead to severe underdevelopment of the cerebrum with an intact cranial vault; this condition is called aprosencephaly. Atelencephaly is considered to be a less severe form, whereby the thalami are preserved. Both result in extreme microcephaly.


The condition is rarely diagnosed antenatally. The calvarium is intact, and extreme microcephaly is evident. No forebrain derivatives are present. Oculo-facial anomalies (almost always midline) may include cyclops with no proboscis and extreme nasal hypoplasia. Adrenal hypoplasia, congenital heart disease, abnormal genitalia and limb malformations have all been reported in association with aprosencephaly.

Differential Diagnosis

Anencephaly is characterised by loss of the bony calvarium as well as absence of most or all cerebral tissue. In hydranencephaly there is virtual absence of the cerebral hemispheres, but a normal skull in which there are fluid filled sacs replacing the brain. The cerebellum is normal and the falx is usually present. The facial abnormalities are similar to those seen in alobar holoprosencephaly but the presence of a sickle-shaped monoventricle will confirm diagnosis of the latter.

Sonographic Features

Severe microcephaly with intact calvarium.

Underdevelopment of brain especially in anterior cranium.

Facial anomalies including cyclops (no proboscis) and extreme nasal hypoplasia.

Associated Syndromes

  • Chromosomal
  • XK-aprosencephaly


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