Olfaction in patients with Parkinson’s disease: a new threshold test analysis through turning points trajectories

Olfactory defcit is a widely documented non-motor symptom in Parkinson’s disease (PD). Abnormal turning points trajectories through olfactory threshold testing have been recently reported in patients with olfactory dysfunction, who seem to adapt faster to olfactory stimuli, but data on PD patients are lacking. The aim of this study is to perform olfactory threshold test and explore the turning points trajectories in PD patients in comparison to normal controls. We recruited 59 PD patients without dementia, and no conditions that could infuence evaluation of olfaction and cognition. Sixty healthy subjects served as controls. Patients and controls underwent a comprehensive olfactory evaluation with the Snifn’ Sticks extended test assessing threshold, discrimination and identifcation and a full neuropsychological evaluation. Besides, threshold test data were analyzed examining all the turning points trajectories. PD patients showed a diferent olfactory threshold test pattern, i.e., faster olfactory adaptation, than controls with no efect of age. Normosmic PD patients showed diferent olfactory threshold test pattern, i.e., better threshold score, than normosmic controls. Visuospatial dysfunction was the only factor that signifcantly infuenced this pattern. Olfactory threshold trajectories suggested a possible adaptation phenomenon in PD patients. Our data ofered some new insights on normosmic PD patients, which appear to be a subset with a specifc psychophysical profle. The analysis of the turning points trajectories, through an olfactory threshold test, could ofer additional information on olfactory function in PD patients. Future larger studies should confrm these preliminary findings.