Ectopic peripontine arcuate fibres, a novel finding in pontine tegmental cap dysplasia

Abstract

BackgroundPontine Tegmental Cap Dysplasia (PTCD) is a recently described hindbrain malformation presenting hypoplasia of the ventral pons, and a “pontine tegmental cap”. Previous DTI studies identified ectopic transversely oriented nerve fibres in the cap, and absence of transverse fibre bundles in the ventral pons, characterizing PTCD as an embryonic axon guidance defect. A new case with relatively mild symptoms was investigated to identify fibre tracts in the tegmental cap by tracking their connections. In the process a new bilateral ectopic fibre tract was found. MethodsRoutine T1- and T2 weighted images and Diffusion Tensor Imaging (DTI) data were obtained on a 3 T MR scanner. Fractional Anisotropy maps colour coded for orientation were generated. High Angular Resolution Diffusion Imaging (HARDI) data were used for reconstructing maps denoting multiple fibre orientations (i.e. fibre crossings) per voxel through which accurate fibre tracking was performed. Results1.A pontine tegmental cap was discovered on routine MRI and shown to carry transverse oriented fibres on DTI as in previously reported cases of PTCD.2.A new finding revealed by DTI with colour coding and fibre tracking, was a robust midline bundle, traced back to the lower brainstem, that ended rostrally in two semi-arcuate loops skirting the outer margins of the pons, to connect to the cerebellar hemispheres close to but outside the middle cerebellar peduncles. Part of this fibre tract was detectable on routine MRI. InterpretationPeripontine arcuate fibres were identified, representing a second structural abnormality not previously recorded in PTCD.