Agenesis of the corpus callosum

The development of the corpus callosum, which occurs between the 12th and 20th weeks of gestation, begins with the genu, then posteriorly along the body to the splenium, with the rostrum being the last to form. Interestingly myelination of the CC occurs in the opposite direction, from the splenium forwards.

It is important to be aware of this order as the presence of a normal rostrum implies normal CC development, and any abnormality noted elsewhere must be due to a later insult.

Usually agenesis is sporadic, however it is seen in association with trisomy 18 and trisomy 13. Associated abnormalities include:

• intracranial lipoma - 10%
• Dandy-Walker syndrome - 11%
• Chiari malformations - 7%
• holoprosencephaly
• hydrocephalus - 30%
• interhemispheric cysts

As a result of this failure to form, the white matter tracts which usually cross the midline, instead are oriented vertically, separating the lateral ventricles widely, in a "racing car sign" configuration. These bundles of white matter are known as Probst bundles.

The anterior commissure is usually present and often enlarged. The hippocampal formations are usually hypoplastic, with resultant dilatation of the temporal horns of the lateral ventricles.