Hippocampal Commissure and Agenesis of the Corpus Callosum

The hippocampal commissure (commissura fornicis) is part of the fornix and crosses the midline to connect to the contralateral hippocampus inferior to the caudal body and rostral splenium of the corpus callosum. The normal hippocampal commissure small and difficult to visualize on MRI.

Because the crossing fibers of the hippocampal commissure are crucial in the formation of a normal hippocampus, a normal hippocampus can be taken as proof of the presence of a hippocampal commissure.

The hippocampal commissure may be enlarged in some patients with agenesis of the corpus callosum and can be mistaken for its splenium on sagittal views. The differentiating feature is that the enlarged hippocampal commissure connects the fornices on coronal views.

When there is question of callosal agenesis, look for a normal hippocampus. A normal hippocampus implies the presence of the hippocampal commissure, which may mimic the corpus callosum. Therefore, secondary signs of callosal agenesis should be sought, for example, absent cavum septum pellucidum, an elevated third ventricle extending into the interhemispheric fissure, and radial arrangement of the medial cerebral sulci perpendicular to the expected course of the corpus callosum.

On the other hand, the hippocampal commissure may be absent in patients with callosal agenesis; however, it is though that the two are separate forms of cerebral malformation and no definite association has been identified.

References

• Hannay HJ, Dennis M, Kramer L, Blaser S, Fletcher JM. Partial agenesis of the corpus callosum in spina bifida meningomyelocele and potential compensatory mechanisms. J Clin Exp Neuropsychol. 2009 Feb;31(2):180-94.
• Küker W, Mayrhofer H, Mader I, Nägele T, Krägeloh-Mann I. Malformations of the midline commissures: MRI findings in different forms of callosal dysgenesis. Eur Radiol. 2003 Mar;13(3):598-604. Epub 2002 Nov 1.