Acalvaria: a unique congenital anomaly.

Acalvaria is a rare malformation usually regarded as a postneurulation defect. It consists of absence of the calvarial bones, dura mater and associated muscles in the presence of a normal skull base and normal facial bones. The condition is frequently confused by prenatal ultrasonography with anencephaly or an encephalocele. Whereas the cerebral hemispheres are absent in anencephaly, the cranial contents in acalvaria are generally complete, though some neuropathological abnormality is often present. The presumed pathogenesis of acalvaria is faulty migration of the membranous neurocranium with normal placement of the embryonic ectoderm, resulting in absence of the calvaria but an intact layer of skin over the brain parenchyma. We describe 2 cases of acalvaria, one misdiagnosed ultrasonographically as an occipital encephalocele prenatally. The brain in one fetus demonstrated semilobar holoprosencephaly and micropolygyria, but in the other, was structurally and histologically normal with the exception of hydrocephalus.