Teaching Case

Sinusitis, incidental lipoma

On a CT scan obtained for paranasal sinusitis, an intracranial lipoma with partially calcified rims, was incidentally discovered. An MR imaging study was later performed.

The corpus callosum normally develops in an anterior to posterior direction, starting from the comissural plate of the lamina terminalis. The genu of the corpus callosum forms first followed by posterior growth to form the body and splenium. This process is completed by approximately 20 weeks gestational age. The rostrum, however, the most anterior part, forms latest of all. The corpus callosum may be totally absent due to agenesis of the comissural plate or due to persistence of the meninx primitiva in the comissural plate. When the earlier-formed segments (genu, body) are present, and the later-formed segments (splenium-rostrum) are absent, the condition reflects callosal dysgenesis or partial absence. A completely formed but hypoplastic corpus callosum represents another type of callosal dysgenesis associated with some types of intrauterine insults such as ischemia, infections, and diffuse neuronal migrational disorders. Pericallosal lipomas represent the most common type of intracranial lipomas. Their occurrence has been attributed to maldifferentiation of the meninx primitiva which embryologically fills the potential subarachnoid cisterns. Meninx primitiva is normally resorbed leaving behind subarachnoid cisterns, however, its incomplete resorbtion may result in a lipoma. Pericallosal lipomas present as two main types: an anteriorly located bulky tubulonodular variety, and a posteriorly located ribbonlike curvilinear variety. The former type can extend to the choroid plexuses (examples of both types are shown in the figures). These frequently interfere with callosal development resulting in varying degrees of callosal dysgenesis.