Acrania is a rare lethal congenital anomaly characterized by an absence of the calvaria.
The estimated incidence is at ~1:1000 pregnancies .
The condition is thought to result from abnormal migration of mesenchymal tissue, which normally covers the cerebral hemispheres. The calvarial bones are partially or entirely absent with relative (albeit abnormal) preservation of development of the cerebral hemispheres. A thin membrane surrounds the cerebral hemispheres.
As the ossification of the fetal cranium begins and accelerates after nine 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.
Treatment and prognosis
Acrania is lethal and can progress to anencephaly through the acrania-anencephaly sequence .
Imaging differential considerations include conditions such as:
- meroacrania: 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