Agenesis of the corpus callosum may occur as an isolated event or as part of numerous developmental and dysmorphic syndromes. Although affected patients may exhibit few obvious neurologic functional deficits, the absence of the corpus callosum, a structure responsible for interhemispheric connections, carries consequences with respect to information processing and language functions. In this article, the author discusses attempts to classify and characterize agenesis of the corpus callosum based on morphologic and molecular findings. Findings from large scale analyses are also described.
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• In patients with primary or isolated agenesis of the corpus callosum, the intelligence quotient (IQ) may be normal, but impaired processing between the hemispheres can be demonstrated by detailed psychometric testing and may manifest as deficits in learning and recall; associations with autism spectrum disorder have been studied.
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• In cases where agenesis of the corpus callosum occurs as part of a syndrome or larger malformative cluster, the clinical scenario is usually dominated by the primary condition.
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• Hypoplasia of the corpus callosum exists when all the components of the callosum are present, but the structure is thinner than normal. This represents a global defect of brain development, whereas agenesis of the corpus callosum may be an isolated abnormality.
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• Agenesis of the corpus callosum can be diagnosed or suspected in the antenatal period based on axial view ultrasound findings; further MRI studies may be able to further define anatomic abnormalities.
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• Many genetic alterations have been etiologically linked to agenesis of the corpus callosum, both as isolated developmental events as well as in the context of syndromic clusters.
Historical note and terminology
The corpus callosum, the largest fiber tract in the brain, connects to the 2 cerebral hemispheres in order to facilitate the integration of motor and sensory information from the 2 sides of the body. Agenesis of the corpus callosum has been recognized pathologically for a long period of time, and descriptions of the various clinical manifestations continue to accumulate (41).
The recognition and diagnosis of agenesis of the corpus callosum in the living patient, however, was not possible with any degree of certainty until the development of pneumoencephalography in the early part of the 20th century (16). Newer neuroimaging techniques used to demonstrate agenesis of the corpus callosum include cranial ultrasonography, CT, and MRI. With the advent of even newer MRI techniques such as diffusion tensor imaging and tractography (58), the recognition of various forms of partial agenesis and hypoplasia of the corpus callosum has also become possible. With additional experience and refinement of radiographic techniques, agenesis of the corpus callosum can now generally be confidently made, or at least highly suspected on fetal sonography, especially when interpreted in conjunction with MRI (45; 56).
The term "agenesis of the corpus callosum" is as old as the early pathologic descriptions. The use of the term "partial agenesis of the corpus callosum" is relatively new to the clinical literature, and the delineation of the diagnostic criteria for "hypoplasia" of the corpus callosum has only recently been accomplished concurrent with the use of computer-assisted analysis of MRI images (50).