The axial images demonstrate that that there is a very parallel orientation of the lateral ventricles. This appearance is sometimes described as a “racing car” appearance (imagine the anterior horns and the posterior horns resembling the wheels of a Formula 1 car from above). The occipital horns are dilated, an isolated appearance that is termed colpocephaly. The anterior horns of the ventricles are slit like and better seen on the axial images demonstrate lateral displacement and appear curved, an appearance sometimes described as a “bull’s horn” or “steer-horn” like. These appearances are highly typical of the diagnosis of agenesis or dysgenesis of the corpus callosum.
The diagnosis can be confirmed on the sagittal reconstructions demonstrating that there is no corpus callosum running over the third ventricle but rather gyri extend right down to the third ventricle. The gyri are vertically oriented because the cingulate gyrus is also missing in corpus callosal agenesis. Similar appearances are confirmed on the axial images. The third ventricle is often elevated, although is only minimal in this case. The “bull’s horn” appearance is due to displacement by additional longitudinally oriented fibres termed the “bundles of Probst” which act a replacement for the absence of the corpus callosum.
Corpus callosal agenesis is frequently an incidental finding as per this case. Subtle neurological effects including seizures may be present and hypertelorism has been described in some children. However, the clinical presentation is largely defined by the additional associated abnormalities that can occur in the same cases. These include associations with some trisomy syndromes (8,13,18), Apert’s, Gorlin’s, Fetal alcohol syndrome, Zellweger’s etc.
Embryologically the corpus callosum development starts from the genu and extends posteriorly to the splenium. The rostrum develops last so if you can identify this effectively agenesis is excluded. Myelination of the corpus callosum occurs in the opposite direction from the splenium to the genu. The extent of agenesis may, therefore, define the timing of the embryological insult.
When reviewing such cases look for additional abnormalities that may be present such as corpus callosal cysts, intracranial lipomas, Dandy-Walker and Chiari II malformations occurring in about 10% of cases each. Polymicrogyria and pachygyria are more infrequent.
The findings are equally detectable on CT and MRI, the latter better at identifying associated atrophy of the limbic system and fornices. Prenatal ultrasound diagnosis is also described based on the “racing car” sign.