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Definitions
Arnold-Chiari or Chiari type II malformation: cranial and cerebral anomalies arising as a part of the spectrum of open spina bifida.
Closed spina bifida: a defect of the posterior vertebral arches covered by skin, frequently associated with subcutaneous cystic lesions.
Dandy-Walker continuum: a spectrum of different posterior fossa anomalies that share in common an increased amount of fluid in the posterior fossa and/or an anomaly of position or morphology of the cerebellar vermis.
Holoprosencephaly: a spectrum of cerebral anomalies that share in common an incomplete separation of the cerebral hemispheres.
Open spina bifida: a defect of the posterior vertebral arches exposing the contents of the neural canal, usually associated with a posterior cystic lesion formed by the dysmorphic neural cord and meninges (myelomeningocele).
Ventriculomegaly: enlargement of the lateral cerebral ventricles, usually defined by an internal diameter of the atrium of 10 mm or more.
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Central nervous system (CNS) malformations are some of the most common congenital abnormalities. Neural tube defects are the CNS malformations most frequently encountered at birth and amount to about 1 to 2 cases per 1000 births. The incidence of intracranial abnormalities with an intact neural tube is uncertain as probably most of these escape detection at birth and only become manifest in later life. Long-term follow-up studies suggest, however, that the incidence may be as high as 1 in 100 births.1
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The CNS was probably the first major organ system to be investigated in utero by diagnostic ultrasonography. Since then, the investigation of the fetal neural axis has steadily remained a central issue of antenatal sonography. There are a number of reasons for such an interest. Central nervous system anomalies are frequent and often have a severe prognosis. In many cases they have a genetic background, and as a consequence of this there are a large number of couples at specific risk that demand antenatal diagnosis. Modern high-resolution ultrasound equipment yields a unique potential in evaluating normal and abnormal anatomy of the fetal neural axis from very early stages of development. Yet identification of selected anomalies remains a challenge in many cases.
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In recent years, fetal magnetic resonance imaging (MRI) has emerged as a promising new technique that may add, in selected cases, important information,2,3 although the real advantage over ultrasonography remains less defined.4,5 In this chapter, the sonographic investigation of the fetal brain and the identification of CNS anomalies is reviewed.
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NORMAL SONOGRAPHIC ANATOMY OF THE FETAL CENTRAL NERVOUS SYSTEM
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Transvaginal high-frequency high-resolution probes reveal fine details of the developing cerebrum (Figure 15-1). Starting from 7 weeks, menstrual age, the primary cerebral vescicles can be identified with transvaginal sonography as fluid-filled areas. From 11 weeks' gestation, the brightly echogenic choroid plexuses filling the large lateral ventricles are the most prominent intracranial structures. In the second ...