| Discussion: Acrania, also known as exencephaly, is a lethal anomaly that manifests as absence of the cranial bones with the presence of complete, although abnormal, development of the cerebral hemispheres.
This anomaly occurs at the beginning of the 4th gestational week when the mesenchymal tissue fails to migrate and does not allow bone formation over the cerebral tissue.
The prevalence of acrania is rare, with only a few cases reported. In addition, acrania usually progresses to anencephaly as the brain slowly degenerates as a result of exposure to amniotic fluid, which is due to polyhydramnios. The lack of calvarium allows differentiation of the cerebral hemispheres within the amniotic fluid giving the fetal head a bilobed appearance - which is best identified in the first trimester and has been described as a "Mickey Mouse" appearance.
Acrania may be associated with other anomalies, including spinal defects, cleft lip and palate, talipes, cardiac defects, and omphalocele. Acrania has also been associated with amniotic band syndrome. These fetuses are usually hyperactive. Patients with this condition may have elevated maternal serum alpha-fetoprotein. The prognosis is lethal, 50% are stillborn and the other 50% die in neonatal period. |
ACRANIA(EXENCEPHALY)
