Structural integrity of midbrain nuclei in tremor-dominant and non-tremor Parkinson’s disease

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Background The reason for clinical variability between tremor-dominant and non-tremor Parkinson’s Disease (PD) patients is still unclear. Post-mortem evidence suggests that some of this variability may be explained by differences in neurodegeneration patterns in the substantia nigra (SN) and retro-rubral area (RRA). The aim of this study is to in vivo relate patterns of neurodegeneration in the SN and RRA to PD subgroups and resting tremor. Methods Using high-resolution diffusion tensor imaging scans of 71 subjects (38 tremor-dominant, 10 non-tremor and 23 healthy controls), we test whether fractional anisotropy (FA) values in the SN and RRA differ between PD subgroups and healthy controls. Circular regions of interest were manually drawn by two raters in sub-regions of the SN, in the RRA, and in the cerebral peduncles as control areas. Results FA values for the different type of regions [region of interest (SN posterior and RRA) or control region (cerebral peduncles)] did not differ between PD patients and healthy controls (p = 0.400). This was the same between tremor-dominant, non-tremor and healthy controls (p = 0.306). When solely looking at the SN and RRA, there were no non-specific FA decreases in PD patients compared to healthy controls in both the SN (p = 0.090) and the RRA (p = 0.174). No correlation was found between resting tremor scores and FA values for the RRA (r = 0.064, p = 0.648). Conclusions Our findings question whether FA values can be used as a consistent proxy for structural integrity. Other promising measures, like free-water, may provide more reliable measures of neurodegeneration patterns in PD patients.
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