Agenesis of the corpus callosum

• corpus callosum represents the major commissural tract connecting the two cerebral hemispheres
  • connects mainly the homotopic areas of the brain, and the secondary areas more than the primary areas
    • the first fibres appear in utero around the 12th week, as the splenium and the body develop first, followed by the genu and the rostrum, the number of fibres being complete by the 20th week of gestation. The post-natal period of development of the CC size concerns mainly the splenium
    • corpus callosum reaches a size comparable to the adult one by 2 years of age, but it is one of the last systems to complete myelination, a process which starts at the fourth month of pregnancy but continues into adulthood
    • the role of the corpus callosum has been asserted for transfer of information during bimanual learning
• agenesis of the corpus callosum is among the most common brain malformations observed in humans
  • incidence varies as a function of both diagnostic techniques and sample populations
    • in the general population its estimated prevalence is 3-7 per 1000, while in children with developmental disabilities it is 2-3 per 100
  • agenesis of the corpus callosum may occur as an isolated malformation or as a component of more complex malformation syndromes
  • has been associated with several consistent chromosomal rearrangements in more than 20 autosomal and X-linked malformation syndromes (1)
    • extreme genetic heterogeneity may be due to the embryologic processes underlying the formation of corpus callosum. Indeed, several mechanisms might be responsible for callosal agenesis (1)
      • the same genetic defects responsible for callosal agenesis can determine even more diffuse malformations of the central nervous system or of other organs and apparatus
        • severity of the clinical picture can thus vary significantly
        • in fetuses with prenatally diagnosed agenesis of corpus callosum, about one third of cases are isolated and two-thirds are complicated by associated structural defects and/or an underlying chromosomal abnormality (2)
        • a study of prenatal agenesis of the corpus callosum
          • the authors concluded that "..even when it is an isolated finding, the prognosis of fetal agenesis of the corpus callosum is still guarded, with at least one-third of cases in this series displaying long-term neurodevelopmental delay. This was most notable in the cases with severe ventriculomegaly (>15 mm), but three-quarters had a normal outcome in the absence of severe ventriculomegaly.. "(2)

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Reference:

• 1) Dobyns WB. 1996. Absence makes the search grow longer. Am J Hum Genet 58: 7-16
• 2) Fratellis N et al. Outcome of prenatally diagnosed agenesis of the corpus callosum.Prenat Diagn. 2007 Jun;27(6):512-7