Interhemispheric lipoma with variable callosal dysgenesis: relationship between embryology, morphology, and symptomatology.

Eight interhemispheric lipomas (five tubulonodular lipomas and three curvilinear lipomas) were examined by magnetic resonance imaging (MRI). The purpose was to further investigate the relationship between the morphology of the different subtypes and the clinical presentation. The imaging findings were reviewed in light of a recent theory on the development of the corpus callosum. Interhemispheric lipomas should be considered as one entity with a variable expression depending on the severity and/or the time of the insult. Curvilinear lipomas can be either small or extensive and are usually not symptomatic. Tubulonodular lipomas can be either predominantly anterior or posterior in location. The anterior subtype appears to be a more severe form of tubulonodular lipoma. The associated structural abnormalities are most likely responsible for the symptoms, rather than the lipoma itself. Magnetic resonance imaging allows a more precise timing of the insult, resulting in the development of a lipoma. The knowledge of the embryology between the 6th and the 20th week is important to explain these abnormalities. Until now it has been accepted that the corpus callosum develops in an orderly fashion. A recent theory has demonstrated that this is not necessarily true, and that fibers can cross the midline at any place irrespective of the normal development. This theory explains the sometimes amorphous appearance of the remnant of the corpus callosum if a lipoma is present.