Agenesis/dysgenesis of the corpus callosum can occur either as a primary congenital defect or secondary to an insult during embryological development early in life such as an infection, hemorrhage, or other malforming defect of the brain. The condition can be manifested shortly after birth, or it can remain asymptomatic for the life of the individual and be found subsequently on postmortem exam. Due to this large variation in presentation, this defect presents itself along a wide spectrum, making it difficult to diagnose unless imaging of the brain is preformed.

   Occurrence of corpus callosal defects is said to occur in roughly 0.7-5.3% of the US population, with a suggested similar worldwide occurrence . The condition is noted to be hereditary in some cases, being passed down either as an X-linked recessive or autosomal dominant depending on the associated disease. It has also been known to occur often with one of the many different trisomies, most often to include 8, 13, 18, and 21 . Conditions which often coincide with this defect include Chiari II malformations, encephalocele, schizencephaly, lissencephaly, and many syndromes such as Aicardi’s, CRASH and fetal alcohol syndrome.
   
   The congenital form of callosal agenesis usually occurs due to a defect in embryological development during the 7-20th gestational week . Normal development of the corpus callosum begins with formation from the commissural plate of the telencephalon . The genu and the body are formed first, followed in time by the development of the rostrum and splenium. From this, white matter axons develop which form projections into the bilateral cerebral cortices. An embryological defect in any of the stages can cause either complete or partial agenesis of the callosum, if occurring later in development a defect in the posterior section is generally observed. Finally, if the genu and body are dysgenic, one can assume that a secondary cause is manifesting such as infection or hemorrhage due to the embryologic sequence of formation previously mentioned.

   Presentation usually occurs secondary to an associated congenital defect, mental retardation, seizures, or as an incidental finding on an MRI. Diagnosis is best performed by MRI, which demonstrates much finer detail of the lesion and any surrounding neuronal disturbances. Findings often noted in addition to an agenic/dysgenic corpus callosum include a high riding third ventricle, enlarged occipital horns of the lateral ventricles (colpocephaly), longitudinal straightening of the lateral ventricles, and parallel white matter bands along the midline where the corpus would normally develop termed “Probst bundles.â€� In addition, the absence of the cingulate gyrus results in the sulci of the inner aspect of the cerebrum to be found to radiate outwards from the third ventricle instead of their normal appearance surrounding the corpus. Imaging is usually performed by midline sagittal MRI in order to fully demonstrate the extent of callosal dysgenesis, coronal views to best visualize Probst bundles if present, and axial views which demonstrate colpocephaly. T1, T2, FLAIR, and proton density enhanced scans can all be used in conjunction with each other to best visualize the anatomy of the lesion and its surrounding parts.

Therapy for this condition depends completely upon the clinical presentation of the disease, along with any associated syndromes. Due to the wide range of presenting factors, some individuals may go their entire life unaware of their lesion, while others may present at birth. Due to this fact and the varying degree of callosal absence, treatment must be directed at the specific clinical symptoms and not towards the lesion itself.

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