Bilateral effects of unilateral cerebellar lesions as detected by voxel based morphometry and diffusion imaging

Abstract

Over the last decades, the importance of cerebellar processing for cortical functions has been acknowledged and consensus was reached on the strict functional and structural cortico-cerebellar interrelations. From an anatomical point of view strictly contralateral interconnections link the cerebellum to the cerebral cortex mainly through the middle and superior cerebellar peduncle. Diffusion MRI (dMRI) based tractography has already been applied to address cortico-cerebellar-cortical loops in healthy subjects and to detect diffusivity alteration patterns in patients with neurodegenerative pathologies of the cerebellum. In the present study we used dMRI-based tractography to determine the degree and pattern of pathological changes of cerebellar white matter microstructure in patients with focal cerebellar lesions. Diffusion imaging and high-resolution volumes were obtained in patients with left cerebellar lesions and in normal controls. Middle cerebellar peduncles and superior cerebellar peduncles were reconstructed by multi fiber diffusion tractography. From each tract, measures of microscopic damage were assessed, and despite the presence of unilateral lesions, bilateral diffusivity differences in white matter tracts were found comparing patients with normal controls. Consistently, bilateral alterations were also evidenced in specific brain regions linked to the cerebellum and involved in higher-level functions. This could be in line with the evidence that in the presence of unilateral cerebellar lesions, different cognitive functions can be affected and they are not strictly linked to the side of the cerebellar lesion.

Fig 1. Anatomical localization of cerebellar ROIs for tractography of MCP and SCP.

Cerebellar ROIs manually drawn on the FA map images for MCP (A) and SCP (B) tracking. For MCP, the coronal seed ROIs (in red) are illustrated. In the axial slice, cerebellar seed and waypoint ROIs are indicated by the white arrows. For the left SCP the seed region (coronal slice) and the endpoint ROI (axial slice) are illustrated (white arrows). Note that for the right SCP the same ROIs were used swapping right and left hemispheres (ROIs not shown).

Fig 2. Lesion reconstruction and distribution in patients.

Each individual lesion is presented and superimposed on coronal (= y), sagittal (= x) and axial (= z) slices of the SUIT atlas template (Diedrichsen et al., 2009) after spatial normalization. Patients codes as in Tables 1 and 2. The bottom left of the figure shows the SUIT atlas.

Fig 3. DTI-based tractography of middle and superior cerebellar peduncles.

DTI-based tractography of the average tract of MCP (red), L-SCP (blue) and R-SCP (green) with voxels belonging to at least 50% of the subjects.

Fig 4. Voxel-wise analysis of white matter tracts.

Regions showing altered Radial Diffusivity (A), Mean Diffusivity (B) and Fractional anisotropy (C) in patients compared to controls. The regions in the middle cerebellar peduncle are shown in light blue; the regions in superior cerebellar peduncle are shown in red. Axial diffusivity was not significantly affected (data not shown).

Fig 5. Between groups voxel-based comparison of cerebral GM density.

Regions showing patterns of reduced regional GM in patients compared to HS in both contralateral and ipsilateral cerebral cortex.