Essential tremor – is there white matter pathology?

Essential tremor is the most frequent movement disorder. Neuropathologic studies of the disease are rare, and there is conflicting evidence for the involvement of white matter structures in the current literature. In this study, we recruited a set of ET patients with a matched control group. We acquired DTI data optimised for high signal-to-noise ratio and low susceptibility artefacts to test whether there is white matter pathology in ET as detectable by non-invasive diffusion MRI. We aim to study 40 subjects in total. Here, we report results on a subgroup studied so far.

10 patients with ET (mean age 67.7 +- 4.2yrs) and 9 control subjects (61.8 +- 8.8yrs) received diffusion MR imaging (Siemens Trio 3T, 60 directions, b=1000mm s-2, 70 axial slices, 2×2x2mm³, SENSE 2, TE 95ms, 3 repeats). FMRIB's software library (FSL) was used for data anlysis. After motion- and eddy current-correction, we averaged the three runs and fitted the diffusion tensor model. TBSS analysis of FA images registered maps non-linearly to a common target, and a tract skeleton for FA>0.2 was derived. Individual FA were projected onto this skeleton using maximum FA values found perpendicularly to the planes defined by the skeleton. We computed non-parametric statistics using permutation-based testing and threshold-free cluster formation, with age as a regressor of no interest.

In the group studied so far, we failed to detect significant reduction of FA in patients with ET. However, there was a trend around p=0.1 for reduced FA in white matter underlying right somatosensory and left premotor cortex.

While we failed to reach statistical significance in this subgroup, results are encouraging for analysis of the whole collective. If our findings hold true in a larger study collective, they would be suggestive of involvement of the cerebral motor loops in the generation of ET, an interpretation compatible with effective targetting of ET through thalamic deep brain stimulation.