SWEDD refers to individuals diagnosed with Parkinson’s Disease (PD) who display normal presynaptic dopaminergic imaging. The clinical characteristics of SWEDD are still uncertain, raising questions about whether these patients share similar features with PD or differ in their phenotypes and genotypes.

This study aims to compare the clinical features of patients with Early Onset Parkinson's Disease (EOPD) with dopamine deficiency to those with scans without evidence of dopamine deficiency (SWEDD).

Among our EOPD patients, we compared motor and non-motor characteristics of EOPD individuals based on their DAT scan (Ioflupane [123I] SPECT) results, distinguishing between those with evidence of dopamine deficiency and those without.

10/63 EOPD cases (15.9%) had SWEDD whereas 53 exhibited a positive DAT scan result. Both groups showed comparable demographic characteristics, with no significant differences in age, age at first symptom presentation, and disease duration. Importantly, no significant differences were observed in motor symptoms and phenotypes explored trough Hoehn and Yahr stages at baseline and follow-up, as well as MDS-UPDRS Part II and III scores. In contrast, significant differences emerged in non-motor symptoms: the EOPD-SWEDD group had a higher prevalence of non-motor symptoms as the first presentation, with 60% of patients presenting with non-motor symptoms initially, compared to only 23% in the EOPD-dopamine deficiency group (p = 0.026). Additionally, SWEDD patients showed a greater prevalence of hyposmia (60% vs. 17%, p = 0.009) and reported significantly higher scores on the MDS-UPDRS Part I (Non-Motor Aspects of Experiences of Daily Living) compared to their counterparts with dopamine deficiency.

EOPD patients with SWEDD have similar motor features to those with dopamine deficiency but are more prone to present with non-motor symptoms.