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  1. Anomalies of ventral induction of the fetal brain include a group of conditions that share in common an anomalous process of cleavage of the brain and formation of midline structures, encompassing a wide spectrum of severity. They are frequently associated with other malformations and genetic conditions.

  2. Holoprosencephaly is one of the most severe disorders of ventral induction; it features incomplete separation of the cerebral hemispheres and has a very severe prognosis in most cases.

  3. Agenesis of the corpus callosum is either complete or limited to the posterior portion; the anatomy is variable, but enlargement of the lateral ventricles and the absence of the cavity of the septum pellucidum are frequently present. The prognosis is largely influenced by the the association with other anomalies; isolated agenesis of the corpus callosum may be associated with a normal intellectual development, although the experience is limited.

  4. Agenesis of the septum pellucidum is frequently a part of other, often severe malformations, including holoprosenephaly, gross hydrocephaly, and schizencephaly. Isolated agenesis of the septum pellucidum may be a normal variant, although it may be the only antenatal finding of septo-optic dysplasia, a condtiion that is usually associated with visual impairment and endocrine disfunction.

The ontogenesis of the cerebral midline is frequently referred to as the process of ventral induction. It takes place between the fifth week after conception and midgestation.1 Anomalies occurring during this period lead to the development of a wide range of malformations with a severity that is closely related with the time of occurrence. Beause ventral induction is closely related to facial development, many fetuses and children with prosencephalic disorders suffer from facial anomalies. A categorization of these anomalies is presented in Table 6–1. Disorders of prosencephalic formation, aprosencephaly and atelencephaly, will not be discussed in the following sections, as they are the extreme end of the spectrum of severity, and their occurrence is extremely rare. Aprosencephaly is the complete lack of development of the telencephalon and diencephalon, and in atelencephaly, the diencephalon is present but usually abnormal.2 In all prenatal reports, a different brain malformation was diagnosed in utero, and the definitive diagnosis was reached only after pathologic examination.3 Disorders of prosencephalic cleavage, holoprosencephaly and holotelencephaly, follow in the degree of severity. Disorders of prosencephalic midline development, agenesis of the corpus callosum, agenesis of septum pellucidum, and septo-optic dysplasia, usually have less severe presentations, but affected subjects may suffer from neurodevelopmental retardation and endocrinologic and visual disorders.





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