Undecussated superior cerebellar peduncles and absence of the dorsal transverse pontine fibers: a new axonal guidance disorder?

Axonal guidance disorders are a newly recognized group of diseases of the human central nervous system. These disorders are characterized by white matter tracts with abnormal course and failure to cross the midline or presence of ectopic white matter tracts. Diffusion tensor imaging (DTI) and fiber tractography are suitable neuroimaging tools to detect morphological abnormalities in the course, decussation, and location of white matter tracts. We report on a 6.5-year-old child with significant global developmental delay. Axial color-coded fractional anisotropy (FA)-maps revealed absence of (1) the midline "focal red dot" at the level of the pontomesencephalic junction representing absence of decussation of the superior cerebellar peduncles and (2) the dorsal component of the transverse pontine fibers. These findings are highly suggestive of an axonal guidance disorders. The complete neuroimaging phenotype of this child does not match well-known diseases with similar DTI findings. We show how DTI reveals important information of microstructural brain malformations that may go undetected or remains underestimated and consequently DTI may suggest the possible pathomechanism. We conclude that this child may be suffering from a not yet described subtype of an axonal guidance disorder.