[Anatomic MRI study of commissural agenesis and dysplasia of the Telencephalon (Agenesis of the corpus callosum and related anomalies). Clinical correlations and morphogenetic interpretation].

A series of 78 patients presenting with agenesis of the cerebral commissures and properly investigated with MR imaging, was reviewed and analyzed morphologically. Results were compared with descriptive data from the literature, and with the developmental models proposed. From this, a model of a-commissural brain is described: the lamina terminalis would be homologous to a telencephalic anterior medullary velum of which the commissure would be the anterior commissure, and the lamina of white matter described as the Probst's and the fornical bundles, would be homologous to a posterior medullary velum having become a medial medullary velum due to the division of the prosecephalon into two cerebral hemispheres, and of which the commissure would be the (posterior) calloso-hippocampal commissure. Also, the comparison with the model establishes that in the actual malformations, defects of the cingulum and of at least some of the intralobaroccipital association bundles are observed beside the commissural defect. Such a model would reclassify these disorders, distinguishing the "simple" commissural defects, complete or segmental, global or dissociated, without or with a ventricular expansion, from more complex forms with multicystic defects, adding major dysplastic lesions of the dura mater, leptomeninges and parenchyma, to the commissural defects. Paradoxically, the latter group seems to be clinically less severe than the "simple agenesis" group, of which prognosis (including the neurologic and intellectual disorders as well as the associated pathologies) is generally very poor; this should be seriously considered since the antenatal diagnosis of these malformations is made routinely with ultrasonography and MRI.