Acrania is a rare lethal congenital anomaly characterised by an absence of the calvarium.
The estimated incidence is at ~1:1000 pregnancies 4.
The condition is thought to result from abnormal migration of mesenchymal tissue, which normally covers the cerebral hemispheres. The calvarial bones are partially or completely absent with relative (albeit abnormal) preservation of development of the cerebral hemispheres. The cerebral hemispheres are surrounded by a thin membrane.
As the ossification of the fetal cranium begins and accelerates after 9 weeks, antenatal ultrasound allows diagnosis from 11 weeks onwards. It is important to look specifically for frontal bone ossification in the axial and coronal planes.
Acrania is lethal and can progress to anencephaly through the acrania-anencephaly sequence 8.
Imaging differential considerations include conditions such as:
- meroacrania 4: preservation of occipital bone (sometimes considered as part of acrania spectrum)
- anencephaly/exencephaly: has absent/deficient brain tissue as well
- congenital hypophosphatasia
- severe osteogenesis imperfecta: additional findings such as bowing, shortening, or fractures of long bones often aid in distinction from acrania
- calvarial defect(s) due to an amniotic band
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