Midline telencephalic dysgenesis: report of three cases

The classification of cortical dysplasias through molecular genetics

Recent genetic insight into the mechanisms of human brain malformation have allowed one to consider a classification of these disorders by the genetic disruption. In this article an attempt is made to classify human cortical dysplasias by the known genetic disruptions or insults that lead to them. The discussion of malformation is within the context of the embryologic processes that have thought to have gone awry. Human disorders of segmentation, cell proliferation, telencephalic cleavage, differentiation, and neuronal migration are discussed. As this is a rapidly changing area, the reader is encouraged to check online databases for updates on the genetic insights that have been gained since the publication of this article.

[Congenital malformations of the brain. 2: Malformations of the corpus callosum and holoprocencephalies]

The corpus callosum is formed between the 7th and the 20th gestational week. If this process is disrupted, partial or complete callosal agenesis may ensue. As large parts of the supra- and infratentorial brain are created during this critical period, associated anomalies need always to be searched for when callosal agenesis is present. Associations with neuro-genetic syndromes also exist. The corpus callosum is generally formed from front to back ("front-to-back rule"). Therefore, a partial callosal agenesis usually involves the posterior portion of the corpus callosum, while a secondary lesion of the corpus callosum does not follow this rule. Holoprosencephalies are a notable exception to this rule, as the frontal part of the corpus callosum is absent in spite of their classification as congenital malformations. They represent a disturbance of the differentiation and cleavage of the prosencephalon with a disruption of the separation of the cerebral hemispheres. Holoprosencephalies can be due to genetic causes, but also to intrauterine infections or other teratogenic causes. The holoprosencephalies are subdivided into alobar, semilobar and lobar holoprosencephalies. This article aims to describe the most important features of callosal agenesis and holoprosencephalies highlighting the respective imaging characteristics.

Brain development and the genetics of brain development

The progress made in the understanding of the genetics of human brain malformations has lead to insight into the formation of brain and into mechanisms of disease affecting brain. It bears upon neurologists and geneticists to recognize the patterns of diseases of brain formation, to properly diagnose such disorders, to assess the recurrence risk of these malformations, and to guide families with appropriate expectations for outcomes. This article may serve as a guide to neurologists in their approach to these disorders. Because this area is one of rapid progress, the clinician is advised to seek more current information that may be available through on-line databases and other sources.

Single forebrain ventricle without prosencephaly: agenesis of the corpus callosum with dehiscent fornices

Coalescence of the cerebral ventricles with formation of a single forebrain ventricle is described in an unusual case of agenesis of the corpus callosum with dehiscent fornices and severe hydrocephalus. The cerebral hemispheres were fully cleaved. The detached fornices were widely separated from the thalami. The membrana tectoria was retroverted over the midbrain and cerebellum, where it joined the fornices and merged with a curved membranous ependymal dome which, at a great distance, circumvented the thalami as it extended toward the anterior commissure. Other anomalies included arhinencephaly, multiple subependymal heterotopias, and Dandy-Walker malformation. Similar malformations have been described as interhemispheric cysts, as (holo)prosencephaly, or as "midline telencephalic dysgenesis." The basic features of prosencephaly and agenesis of the corpus callosum are reviewed and compared to the present case.