Anencephaly, the single most common open neural tube defect, is characterized by the absence of the cerebral hemispheres and the lack of bony calvarium above the orbits (Fig. 10-37A-B and Fig. 10-37C ). The fetus has a normal midbrain and posterior fossa, but lacks normal development of the cerebral hemisphere. Approximately one third of cases have a variable amount of angiomatous stroma that may mimic rudimentary brain.(52) Anencephaly may be accompanied by polyhydramnios, which is thought to be secondary to severe brain dysfunction resulting in ineffective fetal swallowing.
Acrania is a developmental abnormality characterized by partial or complete absence of the cranial vault. Exencephaly is acrania with protrusion of brain tissue into the amniotic cavity. Exencephaly is different from but similar to anencephaly.(53) As in anencephaly, the fetal cranium is absent; however, unlike in anencephaly, brain tissue is always present (Fig. 10-38 ). In exencephaly, the brain tissue appears heterogenous and disorganized. The hypothesis of a continuum of findings between exencephaly and anencephaly is supported by the observation of residual brain tissue (angiomatous stroma) in about one third of cases of anencephaly(52) (see Fig. 10-37A-B, Fig. 10-37C, and Fig. 10-38 ).