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Background Reduced diffusion along perivascular spaces in adults with Alzheimer’s-disease-related-dementias has been reported and attributed to reduced glymphatic flow. Objectives To apply quantitative measures of diffusion along, and orthogonal to, perivascular spaces in a cohort of older adults with and without clinical symptoms of alpha-synuclein related neurodegeneration. Methods 181 adults with Parkinson disease (PD) or essential tremor (ET) additionally sub-classified by the presence of cognitive impairment underwent 3 Tesla MRI. Diffusion-tensor-imaging (spatial resolution=2x2x2 mm; b-value=1000 s/mm2; directions=33) measures of diffusion (mm2/s) parallel and orthogonal to perivascular spaces at the level of the medullary veins, and the ratio of these measures (DTI-ALPS), were calculated. Regions were identified by a board-certified neuroradiologist from T1-weighted and T2-weighted MRI. Evaluations of motor impairment and mild cognitive impairment (MCI) were interpreted by a board-certified neurologist and neuropsychologist, respectively. Multiple regression with false discovery rate correction was applied to understand how diffusion metrics related to (i) disease category (PD vs. ET), (ii) cognition (MCI status), and (iii) white matter disease severity from the Fazekas score. Results The DTI-ALPS score was reduced in PD compared to ET participants (p=0.037). No association between DTI-ALPS score and MCI status, but an inverse association between DTI-ALPS and Fazekas score (p=0.002), was observed. DTI-ALPS scores were inversely associated with age (p=0.007). Conclusion Diffusion aberrations near perivascular spaces are evident in patients with alpha-synuclein related neurodegenerative disorders, and are related to age and white matter disease severity.