Spina bia is a defect in the vertebrae, typically the dorsal arches, with exposure of the meninges and spinal cord. The birth prevalence approximates 1 in 2000 (Cragan, 2009; Dolk, 2010). Most cases are open spina bia-the defect includes the skin and sot tissues. Herniation of a meningeal sac containing neural elements is termed a myelomeningocele (Fig. 10-1i1). When only a meningeal sac is present, the defect is a meningocele. Although the sac may be easier to image in the sagittal plane, transverse images more readily demonstrate separation or splaying of the laterl processes.
Detection of spina bifida is aided by two characteristic cranial findings (Nicolaides, 1986). Scalloping of the frontal bones is termed the lemon sign, and anterior curvature of the cerebellum with efacement of the cisterna magna is the banana sign (Fig. 10-12). These indings are manifestations of the Chiari II malformation, also called the Arnold-Chiari maormation. This develops when downward displacement of the spinal cord pulls a portion of the cerebellum through the foramen magnum and into the upper cervical canal. Ventriculomegay is another frequent associated sonographic finding, particularly after midgestation. More than 80 percent of infants with open spina bifida require ventriculoperitoneal shunt placement. A small BPD is often present as well. Children with spina biida require multidisciplinary care to address problems related to the defect, therapeutic shunting, and deicits in swallowing, bladder and bowel function, and ambulation. Fetal myelomeningocele surgery is discussed
Chapter 16 (p. 319).
Characterized by distention of the cerebral ventricles by cerebrospinal luid (CSF), this finding is a nonspecific marker of abnormal brain development (Pilu, 2011). The atrium normally measures between 5 and 10 mm from 15 weeks' gestation until term (see Fig. 10-6). Mild ventriculomegaly is diagnosed when the atrial width measures 10 to 15 mm (Fig. 10-13), and overt or severe ventriculomegaly when it exceeds 15 mm. The larger the atrium, the greater the likelihood of an abnormal outcome (Gaglioti, 2009; J06, 2008). CSF is produced within the ventricles by the choroid plexus, which is composed of loose connective tissue surrounding an epithelium-lined capillary core. he choroid plexus often appears to dangle within the ventricle when severe ventriculomegaly is present.
Ventriculomegaly may be caused by various genetic and environmental insults. It may be due to other central nervous system (CNS) abnormalities-such as Dandy-Walker malformation or holoprosencephaly, to an obstructive process-such as aqueductal stenosis, or to a destructive process-such as porencephaly or an intracranial teratoma. Initial evaluation includes a targeted examination of fetal anatomy, testing for congenital infections such as cytomegalovirus and toxoplasmosis, and chromosomal microarray analysis, which is described in Chapter 13 (p. 271). Fetal MR imaging should be considered to assess for associated abnormalities that may not be detectable sonographically.
FIGURE 10-12 Cranial findings in myelomeningocele, A. Image of a fetal head at the level of the lateral ventricles demonstrates inward bowing or scalloping of the frontal bones (arrows)-the lemon sign. B. Image of a fetal head at the level of the posterior fossa shows anterior curvature of the cerebellum (arrows) with effacement of the cisterna magna-the banana sign.