<unk>  <unk>  is a rare birth defect in which there is a complete or partial absence of the<unk><unk> . It occurs when the development of the<unk><unk> , the band of white matter connecting the two hemispheres in the brain, in the embryo is disrupted. The result of this is that the fibers that would otherwise form the<unk><unk>  are instead longitudinally oriented along the ipsilateral ventricular wall and form structures called Probst bundles. In addition to agenesis, other degrees of callosal defects exist, including hypoplasia (underdevelopment or thinness), hypogenesis (partial agenesis) or dysgenesis (malformation).<unk>  is found in many syndromes and can often present alongside hypoplasia of the cerebellar vermis. When this is the case, there can also be an enlarged fourth ventricle or hydrocephalus; this is called Dandy–Walker malformation.
Agenesis of the corpus callosum
Germinal matrix hemorrhage
Congenital bilateral perisylvian syndrome
Cerebellar agenesis
Gray matter heterotopia
Bilateral frontoparietal polymicrogyria
Cerebellar hypoplasia
Pontocerebellar hypoplasia