The middle interhemispheric variant of holoprosencephaly: magnetic resonance and diffusion tensor imaging findings.

OBJECTIVE: The middle interhemispheric (MIH) variant of holoprosencephaly (HPE) is the incomplete separation of midline cerebral hemispheres with the absence of callosal body. We aimed to describe the additional knowledge of diffusion tensor imaging (DTI) over conventional MRI in the evaluation of patients with MIH variant of HPE.

METHODS: Conventional MRI and DTI data of five patients were retrospectively evaluated. The parenchymal anomalies as well as changes at white matter tracts were systematically reviewed.

RESULTS: Except the callosal body and central cingulum fibres, which were missing in all patients, all other major white matter tracts (superior and inferior longitudinal, superior and inferior fronto-occipital, subcallosal and uncinate fasciculi and anterior commissure) had a normal course, thickness and integrity on diffusion tensor images. The genial and splenial callosal fibres were altered and rarefied on tractography. All patients had a central ventricular notch extending into the non-cleaved heterotopic grey matter involving the body of the corpus callosum, which is very typical for the MIH variant of HPE. The remnant traversing white matter fibres above the non-cleaved heterotopic grey matter and incomplete partition of the interhemispheric fissure were also identified. No Probst bundles were detected. A single common ventricle without the septum pellucidum was noted in all patients. One patient had incomplete partition of the thalami, and two patients had abnormally oriented thalami without any prominent interthalamic connection. Vertically oriented hippocampi were detected in four out of five patients. Three patients had relatively flat and vertically oriented Sylvian fissures and in two patients, fissures were abnormally connected over the vertex.

CONCLUSION: Additional DTI findings can not only clearly reveal the white matter alterations better than conventional MRI but also provide a better understanding of the aetiological changes that cause the MIH variant of HPE.

ADVANCES IN KNOWLEDGE: DTI can provide a better analysis of cerebral white matter connectivity and promotes understanding of the underlying microstructural changes that occur in patients with the MIH variant of HPE.