Anencephaly is the most common major fetal anomaly and is also the most common neural tube defect, occurring in approximately 1/1000 births. Females are affected more frequently than males with the frequency of 4 to 1. Risk factors for anencephaly are multifactorial and include a family history of a previous neural tube defect.
Anencephaly is an anomaly characterized by absence of the cerebral hemispheres and cranial vault. It is thought to result from failure of the rostral neuropore to close by 24 days of fetal life (34 menstrual days). Although the cerebral hemispheres and cranium are absent, the brain stem, portions of the midbrain, and portions of the cranium that form the cartilage at the base of the calvarium are present. The area of the face is usually normal but appears unusually prominent because of lack of the remainder of the head. Initially, hyperechoic tissue may be identified superior to the orbits. It was identified in one series in 45% of cases and is termed angiomatous stroma (area cerebrovasculaosa). This on occasion may be quite sizeable. It may appear as either solid or mixed solid and cystic. Because of this, there are occasions where a much rarer anomaly termed exencephaly (acrania) may be confused with this entity.
Exencephaly is characterized by complete or partial absence of the calvarium with complete but abnormal development of brain tissue. This brain tissue may "wear away" so that it looks so abnormal it may appear as the angiomatous stoma. If tissue is identified, the importance of distinguishing these two entities is that exencephaly is a sporadic abnormality without additional risk factors. However, in both cases, there is a uniformly fatal outcome to the fetus.
The differential diagnosis would also include an amniotic band syndrome. While an amniotic band syndrome involving the head is a relatively rare occurrence, when it occurs, it almost always causes markedly asymmetric distortion. The symmetry noted in anencephaly and exencephaly would therefore virtually rule out this entity. In addition, amniotic bands commonly affect other parts of the fetus, particularly the extremities.
Associated with anencepahy is a spina bifida which occurs in 17% of cases, a cleft lip or palate in 2% of cases, and club foot in 1.7% of cases. Omphaloceles have been also described but are very infrequent. This case had no other abnormalities.
It is has been stated that when a significant central nervous system defect is present, hydramnios (polyhydramnios) is also detected. In this case, and noted in others, is the finding that amniotic fluid may be normal early in gestation. Therefore, the lack of hydramnios does not change the diagnosis. Alpha fetoprotein levels are also elevated in the maternal and amniotic fluid.
The diagnosis can be routinely made from 14 gestational weeks. It is important to accurately make it because the outcome of these fetuses is uniformly fatal within the first hours or days of life. Even if left undetected until term, only 32% of these fetuses will even result in live births. For this reason, termination of pregnancy should be offered at the time when diagnosis is made.
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