hese defects include anencephaly, myelomeningocele (also called spina bifida), cephalocele, and other rare spinal fusion (or schisis) abnormalities. hey result from incomplete closure of the neural tube by the embryonic age of 26 to 28 days. heir birth prevalence is 0.9 in 1000 in the United States and most of Europe and 1.3 in 1000 in the United Kingdom (Cragan, 2009; Dolk, 2010). Many neural-tube defects can be prevented with folic acid supplementation. When isolated, neural-tube defect inheritance is multifactorial, and the recurrence risk without periconceptional folic acid supplementation is 3 to 5 percent (Chap. 13, p. 270).
Screening for neural-tube defects with maternal serum alpha-fetoprotein (MSAFP) has been ofered routinely as part of prenatal care since the 1980s (Chap. 14, p. 283). Women currently have the option of neural-tube defect screening with MSAFP, sonography, or both (American College of Obstetricians and Gynecologists, 2016). Serum screening is generally performed between 15 and 20 weeks' gestation. And, if using an upper threshold of 2.5 multiples of the median (MoM), the anticipated detection rate is at least 90 percent for fetal anencephaly and 80 percent for myelomeningocele. Targeted sonography is the preferred diagnostic test, and in addition to characterizing the neural-tube defect, it may identiy other abnormalities or conditions that also result in MSAFP elevation (Table 14-6, p. 283).
Anencephay is characterized by absence of the cranium and telencephalic structures above the level of the skull base and orbits (Fig. 10-9). Acrania is absence of the cranium with protrusion of disorganized brain tissue. Both are uniformly lethal and are generally considered together, with anencephaly as the final stage of acrania (Bronshtein, 1991). These anomalies are often diagnosed in the late first trimester, and with adequate visualization, virtually all cases may be diagnosed in the second trimester. Inability to image the BPD raises suspicion. The face often appears triangular, and sagittal images readily demonstrate absence of the ossified cranium. Hydramnios from impaired fetal swallowing is common in the third trimester.
Cphaocee is the herniation of meninges through a cranial defect, tpically located in the midline occipital region (Fig. 10-10).
FIGURE 10-10 Encephalocele. This transverse image depicts a large defect in the occipital region of the cranium (arrows) through which meninges and brain tissue have herniated.
FIGURE 10-1 1 Myelomeningocele, In this sagittal image of a lumbosacral myelomeningocele, the arrowheads indicate nerve roots within the anechoic herniated sac. The overlying skin is visible above the level of the spinal defect but abruptly stops at the defect (arrow),
When brain tissue herniates through the skull defect, the anomaly is termed an encephaocee. Herniation of the cerebellum and other posterior fossa structures constitutes a Chiari II maomaion. Associated hydrocephalus and microcephaly are common, and survivors have a high incidence of neurological deficits and intellectual disability. Cephalocele is an important feature of the autosomal recessive Meckel-Guber syndrome, which includes cystic renal dysplasia and polydactyly. A cephalocele not located in the occipital midline rises suspicion for amnionic-band sequence (Chap. 6, p. 116).