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KEY POINTS
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Key Points

  • Accounts for approximately one-half of all cases of neural tube defects.

  • Incidence is approximately 0.3/1000 births. Maternal risk factors include Hispanic ethnicity, pregestational diabetes, obesity, and hyperthermia.

  • First trimester sonographic findings include a reduced crown-rump length, and the “Mickey Mouse” sign in the coronal view. Second trimester findings include an absent upper cranial vault and no cerebral tissue above the level of the orbits.

  • Differential diagnosis includes amniotic bands, ruptured encephalocele, and iniencephaly.

  • Approximately a quarter of affected pregnancies are complicated by polyhydramnios.

  • Consider prenatal karyotype if associated anomalies present. If anencephaly is isolated, consider postnatal karyotype.

  • Condition is uniformly fatal postnatally. Only 7% of fetuses die in utero.

  • Preconceptual folic acid (4 mg/day) significantly decreases recurrence if anomaly is isolated and not due to a chromosomal or single-gene disorder.

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CONDITION
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Anencephaly [congenital absence of a major portion of the brain, skull, and scalp (Medical Task Force on Anencephaly, 1990)] is the most severe and single most common prenatally detected neural tube defect (Goldstein and Filly, 1988). Although the cerebral hemispheres can develop in this condition, any exposed brain tissue is subsequently destroyed (see Chapter 13). This produces a hemorrhagic, fibrotic mass of neurons and glia, with no functional cortex. The brainstem and cerebellum may be spared. Despite the severe brain abnormalities, the facial bones and base of the skull are nearly normally formed. The frontal bone, however, is always absent and the brain tissue is always abnormal.

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Anencephaly is sometimes divided into two subcategories. The milder form is known as meroacrania, which describes a small defect in the cranial vault covered by the area cerebrovasculosa. The more severe form is holoacrania, in which the brain is completely absent.

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Van Allen et al. (1993) proposed that multisite neural-tube closure provides the best explanation for neural tube defects in humans. The closure sites are most likely controlled by separate genes expressed during embryogenesis. These authors hypothesized that the majority of neural tube defects could be explained by a failure of fusion of one of the closures or their contiguous neuropores. Anencephaly results from failure of closure site 2 for meroacranium and closures 2 and 4 for holoacranium. Folate deficiency is thought to affect the closures of sites 2 and 4. This hypothesis has been demonstrated in humans with more than one neural tube defect (Pantzar et al., 1993).

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INCIDENCE
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Anencephaly accounts for approximately one-half of all cases of neural tube defects (Chescheir et al., 2003). The incidence of anencephaly in livebirths and stillbirths has been estimated as 0.3 per 1000 by the Centers for Disease Control (Medical Task Force on Anencephaly, 1990). Female fetuses are more commonly affected. The ratio of affected females to males is 3:1 to 4:1 (Naidich et al., 1992). In one study, there ...

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