Novel Proximal 14q Deletion: Clinical and Diffusion Tensor Imaging Tractography Findings in a Patient with Lissencephaly, Agenesis of the Corpus Callosum, and Septo-Optic Dysplasia

Discussion

Previous reports of proximal 14q deletions have been made, but it is an extremely rare occurrence. However, none have the specific chromosome deletion or the combination of CNS malformations as in our case. SOD has not been previously described, to our knowledge; and lissencephaly has been described only once in prior reports of cases with proximal 14q deletion.¹ Also, the MR images of the reported case with lissencephaly were not presented, but the description of the imaging findings significantly differed from those in our patient.¹ DTI tractography was used in the evaluation of our patient (Fig 2). This allowed better visualization of the aberrant white matter connections, which are often found in congenital CNS malformations and are not visualized with conventional MR imaging. DTI tractography allowed us to better evaluate the morphologic changes associated with agenesis of the corpus callosum. With DTI tractography, we visualized fibers from the hemispheric cortex failing to cross the midline and forming thick bundles running anteroposteriorly to become Probst bundles.²

Agenesis of the corpus callosum results in failure of association fibers to decussate to the contralateral hemisphere due to lack of induction by the massa commissuralis. The Probst bundle is formed from the development of association fibers, which do not pass through the callosal precursor and then continue to grow caudally along the medial surface of the ipsilateral cerebral hemisphere.³ Tractography in our patient demonstrated the Probst bundles as having prominent connections to the temporal lobes. Also, fibers were visualized extending into the anterior internal capsules on the left greater than on the right (Fig 2).This degree of temporal lobe and internal capsule connections has not been previously reported, though variability of Probst connections has been noted.^2–7 The Probst connections that we have visualized may be an indication of the variability that can exist with their formation. However, tractography of the Probst bundles may also be demonstrating white matter connections of the cingulate gyri to the temporal lobes secondary to the cingulate bundles not being separated from the Probst bundles. Many attempts were made by tractography to separate the cingulate bundle from Probst bundle without success. Furthermore, our tractography findings may indicate spurious fiber tract connections resulting from performing fiber tractography in a 4-month-old with incompletely myelinated white matter. It would be interesting to see if the appearance of the Probst bundles in this subject changes with age.

DTI tractography also allowed us to determine the true nature of the suspected heterotopias found running adjacent to the lateral ventricles as actually being corticospinal tract fibers (Fig 2D). The isointense-to–gray matter T2 signal intensity associated with the corticospinal tract makes it similar in signal intensity to the gray matter; hence, the heterotopic appearance is hypothesized to be due to myelination of the corticospinal tract.

In conclusion, this is a case report of previously undescribed and rarely described findings associated with a proximal 14q deletion that included SOD and lissencephaly.¹ Conventional MR imaging allowed excellent delineation of agenesis of the corpus callosum with the associated findings and the presence of diffuse lissencephaly. However, DTI tractography played a significant role in the evaluation by delineating the extent of potential dysmorphic connections of the Probst bundles and clarifying that apparent areas of periventricular heterotopias were in fact the corticospinal tracts.