Objective:
To explore differences in white matter microstructural integrity in tremor dominant and non-tremor dominant Parkinson’s disease (PD) using tract based spatial statistics (TBSS).

Background:
PD can phenotypically be classified into tremor dominant, and non-tremor dominant PD (NTDPD). Several studies have suggested that tremor dominant PD (TDPD) has a relatively benign course in comparison to NTDPD, and displays a slower rate of progression of motor symptoms, and less cognitive impairment. Hence, it is plausible to expect differences in white matter microstructural integrity between NTDPD and TDPD.

Method:
Fourteen patients of TDPD, and 14 patients of NTDPD underwent a 3T MRI with a 32-channel head coil, and multi-shell diffusion weighted images were acquired. Pre-processing, i.e. eddy correction, generation of the B0 brain mask and fitting the diffusion tensor model was performed using FMRIB’s diffusion toolbox. Following this TBSS was performed using FMRIB’s software library, and the randomize command was run at 5000 permutations. The non-Fractional anisotropy (FA) TBSS script was used to generate maps of mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD). Statistical significance was set at 0.01 for uncorrected maps, and an arbitrary minimum cluster percentage of 30% was set for a tract to be considered significant.

Results:
There were no differences in FA. NTDPD showed higher MD in bilateral anterior thalamic radiation. RD was also higher in NTDPD, involving bilateral anterior thalamic radiations (ATR), superior longitudinal fasciculus, right inferior longitudinal fasciculus, and forceps major and minor. TDPD did not show significantly high RD or MD. Higher AD was observed in TDPD involving bilateral ATR, inferior fronto-occipital fasciculus, right corticospinal tract, left uncinate fasciculus, cingulate gyrus, and forceps minor. No significant abnormality in AD was observed in NTDPD.

Conclusion:
NTDPD showed significant abnormalities in MD and RD, the latter of which is modulated by abnormalities in myelin, whereas the abnormal AD observed in TDPD is more specific to axonal degeneration. These results suggest possible differences in the pathology observed at a microstructural level in phenotypes of PD and may contribute to the different rate of progression and symptomatology observed in TDPD and NTDPD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/white-matter-microstructural-abnormalities-in-tremor-dominant-and-non-tremor-dominant-parkinsons-disease/