Classification of Scans Without Evidence of Dopamine Deficit (SWEDD) According to the Olfactory Function

Background: Scans without evidence of dopaminergic deficit (SWEDD), a heterogeneous disorder, refers to patients with normal dopamine transporter scan presumed to have Parkinson's disease (PD). Although clinically hard to discriminate, it is important to differentiate SWEDD from PD in the early stage of disease in practice. Olfactory dysfunction is a sensitive biomarker of neurodegeneration. A considerable number of SWEDD patients show abnormal olfaction. We investigated the olfaction and non-motor symptoms in SWEDD, PD, and healthy controls. A subgroup analysis of SWEDD by olfactory function was also conducted. Methods: This study enrolled 40 SWEDD, 28 PD, and 26 healthy controls. The Korean version of Sniffin's stick test II (KVSS II) and the Korean version of non-motor symptoms scale for PD (K-NMSS) were used to evaluate olfaction and non-motor symptoms. Patients with SWEDD were categorized into normal olfaction (SWEDD-N) and olfactory dysfunction (SWEDD-D) subgroups. Results: Overall KVSS II scores of SWEDD group was distinctive when compared to those of PD or controls. K-NMSS scores were increased in PD compared to those in SWEDD or controls. In subgroup analysis, KVSS II scores in SWEDD-D were similar to those in PD. K-NMSS scores in SWEDD-D were also significantly higher than those in SWEDD-N but lower than those in PD. Conclusions: Olfactory dysfunction could be a biomarker to differentiate SWEDD from PD. Non-motor symptoms differed between the two SWEDD subgroups classified by olfaction. These findings suggest that the underlying pathophysiology of SWEDD with normal olfaction and olfactory dysfunction might be different.