Introduction The premotor phase of Parkinson’s Disease (PD) has heterogenous clinical manifestations and progression. Preliminary research has attempted to characterise subgroups in early PD, however, there is no definitive consensus on subgroup classification. Standardised subgroup classification is of high clinical and research importance as there may be different underlying pathological mechanisms that necessitate different pharmacological and non-pharmacological interventions. This study aims to investigate whether there are differences in dopamine active transport (DAT) imaging between two hypothesised subgroups: idiopathic rapid eye movement sleep behaviour disorder (iRBD) patients and hyposmic patients compared to healthy controls.
Method The Parkinson’s Progression Markers Initiative dataset was used to identify and gather data on iRBD patients (polysomnography confirmed), hyposmic patients (University of Pennsylvania smell identification test positive) and healthy controls. Patients were excluded if they were positive for genetic variants of PD or below the age of 50.
Results 394 patients were included: Healthy controls 205; iRBD 107; Hyposmic 82. DAT scans were analysed by brain region and the mean value of dopamine uptake in both hemispheres was calculated for each patient. Means were aggregated to find the population mean and standard deviation. A mean asymmetry index was also calculated, assessing for asymmetry between hemispheres. Analysis of the variance with Tukey’s honestly significant difference was used to assess for differences between groups (table 1).
Asymmetry Index = 100 × (Difference between two hemispheres)/(Mean of two hemispheres)
Discussion For all assessed areas, the iRBD and hyposmic groups had significantly lower dopamine uptake and greater asymmetry than healthy controls. However, there was no significant difference between iRBD and hyposmic groups. These results suggest that the iRBD and hyposmic categories may have similar characteristics of reduced and asymmetric DAT availability compared to controls. Further research is required to consider differences between iRBD and hyposmic phenotypes in other clinical domains.
Data used in the preparation of this article were obtained (26/01/2023) from the Parkinson’s Progression Markers Initiative (PPMI) database www.ppmi-info.org/access-data-specimens/download-data
RRID: SCR 006431. For up-to-date information on the study visit: https://www.ppmi-info.org/
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