Diffusion tensor imaging and olfactory identification testing in early-stage Parkinson's disease

Evidence from imaging, clinical studies, and pathology suggests that Parkinson’s disease is preceded by a prodromal stage that predates clinical diagnosis by several years but there is no established method for detecting this stage. Olfactory impairment, which is common in Parkinson’s disease and often predates clinical diagnosis, may be a useful biomarker for early Parkinson’s. Evidence is emerging that diffusion imaging parameters might be altered in olfactory tract and substantia nigra in the early stages of clinical Parkinson’s disease, possibly reflecting pathological changes. However, no study has examined olfaction and diffusion imaging in olfactory tract and substantia nigra in the same group of patients. The present study compared newly diagnosed Parkinson’s disease patients with a matched control group using both olfactory testing and diffusion tensor imaging of the substantia nigra and anterior olfactory structures. Fourteen patients with stage 1–2 Hoehn & Yahr Parkinson’s disease were matched to a control group by age and sex. All subjects then completed the University of Pennsylvania Smell Identification Test, as well as a series of MRI scans designed to examine diffusion characteristics of the olfactory tract and the substantia nigra. Olfactory testing revealed significant impairment in the patient group. Diffusion tensor imaging revealed significant group differences in both the substantia nigra and anterior olfactory region, with fractional anisotropy of the olfactory region clearly distinguishing the Parkinson’s subjects from controls. This study suggests that there may be value in combining behavioral (olfaction) and MRI testing to identify early Parkinson’s disease. Since loss of olfaction often precedes the motor symptoms in Parkinson’s disease, the important question raised is “will the combination of olfactory testing and MRI (DTI) testing identify pre-motor Parkinson’s disease?”