Differentiation and Evaluation of Disease Progression in Essential Tremor Utilizing MRI Biomarkers
Essential tremor (ET) is one of the most common movement disorders, characterized by kinetic tremor in the upper extremities with additional cranial tremor often present in the neck or jaw. While it is well established that ET is primarily a cerebellar disorder, recent investigations have shown more widespread pathological effects throughout the brain. Furthermore, the neurodegenerative nature of ET is still disputed and requires additional investigation. Additionally, the link between ET and Parkinson’s disease (PD) is of special interest, as it can be challenging to clinically differentiate these diseases.
While post-mortem studies have helped to further the pathological understanding of these diseases, non-invasive in-vivo techniques allow for more accurate diagnosis in the clinic. With a more accurate diagnosis comes a more targeted treatment, and hopefully an improved remediation of the disease. My thesis seeks to further investigate the neurodegenerative hypothesis of ET as well as explore magnetic resonance imaging (MRI) biomarkers for potential differences in ET and PD.
These aims will be accomplished in three steps. First, gray matter volume loss in the cerebellum was investigated using voxel-based morphometry and the Spatially Unbiased Infra-Tentorial Template (SUIT) atlas on a lobule level. High resolution 3D T1-weighted MRI images were acquired on 47 ET cases and 36 controls. The cerebellum was segmented into 34 lobules using the SUIT atlas. Percent gray matter was calculated as the ratio of lobule gray matter volume divided by total lobule volume. No significant differences were identified between ET cases and controls in any of the 34 lobules. However, nine lobules had significantly decreased percent gray matter in ET cases with head or jaw tremor (n = 27) compared to controls. Also, 11 lobules had significantly decreased percent gray matter in ET cases with voice tremor (n = 22) compared to controls. This result confirms, with increased regional accuracy, gray matter volume loss in the cerebellum of ET cases.
Second, gray matter volume loss beyond the cerebellum, in the cerebrum, was investigated using voxel-based morphometry. High resolution 3D T1-weighted MRI images were acquired on 47 ET cases and 36 controls for processing in SPM12. The processing steps of SPM12 were updated to include a higher resolution atlas and set of tissue probability maps to optimize the segmentation and normalization of each subject image. After segmentation, normalization, and smoothing, a voxel-wise statistical analysis was performed to identify clusters of gray matter volume in ET cases compared to controls. ET cases showed decreased gray matter volume in the bilateral superior temporal region and the anterior and posterior cingulate cortex. These results, in combination with previous work provide support of wide-spread neurodegeneration in ET using optimized methodology.
Third, we applied T2* mapping to determine relative iron concentrations in the substantia nigra (SN) and globus pallidus (GP) in ET and PD cases. Three separate studies were independently investigated to validate the reproducibility and detectability of group differences using T2* mapping. The first study (ET study) acquired T2* maps on 21 ET cases and 12 matched controls, the second study (PD study 1) acquired T2* maps on 10 PD cases and 7 controls, and the third study (PD study 2) acquired T2* maps on 21 PD cases and 17 controls. Regions of interest (ROIs) were manually placed in the SN and GP for each subject and group differences were calculated independently for each study using a linear regression model with age and sex as covariates. A significant decrease in T2* was found in PD study 1 and PD study 2 in the right SN in PD cases compared to their respective controls, indicating increased iron deposition. No significant difference was found in the ET group compared to their respective controls in the SN. No significant differences were found in any of the three studies in the GP. These results provide evidence for a difference in brain iron regulation in the pathology of ET and PD.
Together, these thesis aims provide additional evidence in support of the neurodegenerative hypothesis of ET using updated methodology and present a quantitative imaging difference between groups of ET and PD cases.